Дисертації з теми "Exchang bias"
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1
Guo, Zongxia. "Electrical and optical manipulation of exchange bias." Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0204.
Анотація:
L'expansion rapide en termes d'échelle et de complexité des dispositifs électroniques pour les applications d'apprentissage automatique et d'intelligence artificielle actuelles engendre une demande considérable en nouvelles architectures. L'industrie est activement à la recherche de nouvelle génération de mémoires et de stockage capables d'offrir vitesse, densité, et une meilleure efficacité énergétique. Les mémoires non volatile magnétorésistive (MRAM), dont les états magnétiques permettent de stocker l'information suscitent un intérêt grandissant. Jusqu'à présent la direction d'aimantation de matériaux ferromagnétique définissaient l'état magnétique. Cependant, les matériaux antiferromagnétiques ne possédant quasiment pas d'aimantation sont intéressant puisqu'ils ne sont pas sensibles aux champs magnétiques externes. Cette caractéristique permet de minimiser l'influence d'interactions dipolaires entre des dispositifs adjacents, favorisant ainsi une intégration plus dense. De plus les matériaux antiferromagnétiques présentent une dynamique à haute fréquence allant jusqu'à la gamme des térahertz, ce qui ouvre théoriquement la possibilité de vitesses d'écriture plus élevées que les dispositifs ferromagnétiques. Toutefois, des dispositifs à aimantation nulle rendent la manipulation et la détection de l'état mémoire complexe par des méthodes classiques. Dans le cadre de cette thèse, nous avons démontré le renversement du couplage d'échange ou exchange bias dans des héterostructures ferromagnétique/antiferromagnétique au sein de jonctions magnétiques à tunnel à trois terminaux, parvenant ainsi à détecter électriquement une couche antiferromagnétique via une variation de magnétorésistance tunnel de 80 %, soit deux ordres de grandeur plus importantes que les résultats publiés précédemment. Ici, la configuration magnétique de la couche antiferromagnétique (IrMn) impacte la couche libre ferromagnétique de CoFeB. De plus, nous avons pu démontrer un retournement d'aimantation rapide dépendant de la polarité du courant, en 0,8 ns. Nous avons pu identifier deux mécanismes de basculement, à savoir le mode chauffage et le mode induit par le couple spin-orbite, en fonction de la largeur d'impulsion du courant. Le second cas est en accord avec des simulations numériques, suggérant que le couple spin-orbite généré par le Pt induit la précession de l'IrMn, tandis que le couplage d'échange à l'interface IrMn/CoFeB détermine la polarité de basculement de l'IrMn. De plus, dans des structures IrMn/CoGd, nous avons étudier le couplage d'échange perpendiculaire en variant les épaisseurs d'IrMn et les concentrations de CoGd. Nous avons pu démontrer dans une structure optimisée, le retournement du couplage d'échange avec une seule impulsion laser femtoseconde. Par ailleurs, nous avons analysé l'impact de la fluence laser et du nombre pulse laser sur l'évolution du couplage d'échange. Grace a des mesures pompe-sonde, nous avons pu démontrer le retournement du couplage d'échange en moins de 100 ps. Ces résultats ont pu être reproduit par des simulations atomistiques prenant en compte la structure des grains des films d'IrMn polycristallins et l'état amorphe des couches d'alliage de CoGd. L'IrMn montre une désaimantation plus rapide que les matériaux ferromagnétiques, chaque grain d'IrMn parvenant à se réaimanter en un état monodomaine en seulement 2 ps. En conclusion le couplage d'échange peut être manipuler grâce à un courant électrique ou une impulsion optique ultra brève sans champ magnétique appliqué et pour des dispositifs ayant une bonne stabilité thermique, ce qui en fait une solution intéressantes pour diverses applicationsThe rapid growth in scale and complexity of neural network architectures in today's machine learning and artificial intelligence applications is creating a significant demand for advanced hardware solutions. The semiconductor industry is actively seeking next-generation storage technologies that can offer improved speed, density, power consumption, and scalability. One such technology that shows great promise for high-performance data storage and processing is magnetoresistive random access memory (MRAM), which stores information in the magnetic state of materials. However, with the continuous requirement of high-density and ultrafast scenarios, antiferromagnet as the basic unit of MRAM shows obvious advantages. Antiferromagnetic materials have negligible macroscopic magnetism, making them highly robust to external magnetic fields. This property also allows for the absence of dipole interactions between adjacent bits, enabling higher-density integration. Additionally, antiferromagnetic materials exhibit high-frequency dynamics up to the terahertz range, theoretically enabling faster write speeds than ferromagnetic devices. However, such fully compensated magnetic moments make the magnetization state of the antiferromagnetic material difficult to manipulate and detect by traditional electrical methods. In this thesis, we demonstrate the antiferromagnetic exchange bias switching in three-terminal magnetic tunnel junctions and achieve electrical detection of antiferromagnetism by the tunnelling magnetoresistance with a ratio over 80%, which is two orders larger than previous methods. This is achieved by imprinting the state of antiferromagnet IrMn on the CoFeB free layer. We further realize current polarity-dependent switching, rather than current orientation-dependent switching of IrMn down to 0.8 ns. We identify two switching mechanisms, the heating mode and the spin-orbit torque driven mode, depending on the current pulse width. The latter case is supported by numerical simulations, which suggest that spin-orbit torque generated by Pt induces the precession of IrMn and exchange coupling at the IrMn/CoFeB interface determines the switching polarity of IrMn. Furthermore, to break the ferromagnetic and electrical write speed limit and further explore the antiferromagnetic switching speed, we experimentally realize exchange bias switching by a single femtosecond laser pulse. In the IrMn/CoGd structure, the perpendicular exchange bias is investigated for different IrMn thicknesses and CoGd concentrations. Using the optimized structure, the exchange bias was switched under a single femtosecond laser, and the dependence of the exchange bias variations with different laser fluence and pulse numbers was detailed investigated. The pump-probe time-resolved measurement is used to demonstrate the exchange bias switching time scale of less than 100 ps. The grain structure of polycrystalline IrMn films and the amorphous state of CoGd alloy layers are accurately described using atomistic simulations. The IrMn exhibits a faster demagnetization than ferromagnetic materials and each IrMn grain remagnetizing to a single-domain state in only 2 ps. In addition, the different grains of IrMn exhibit independent and stochastic probabilistic switching in the ultrafast time scale. The electrical and all-optical manipulation of exchange bias system allows ultrafast, field-free and energy-efficient control of antiferromagnet with high ordering temperature and thermal stability, making it highly suited to applications
2
Carpenter, Robert. "Exchange bias in nanostructures." Thesis, University of York, 2015. http://etheses.whiterose.ac.uk/9080/.
Анотація:
This study was concerned with two issues facing the technological application of exchange bias. The first is the ferromagnet (F)/antiferromagnet (AF) interface which was investigated via the effect of the IrMn (111) in-plane texture on the magnetisation reversal of IrMn/CoFe samples. The second is the change in the magnitude of the exchange bias (Hex) and the median blocking temperature (Tbmed)with element size in sub-500nm nanostructures. The evolution of the magnetisation reversal over the annealing/field cooling process in CoFe and CoFe/IrMn thin films was measured for samples deposited on Cu and NiCr seed layers. The samples deposited on the Cu seed layer were found to be polycrystalline but randomly oriented whilst those on a NiCr seed layer had a strong (111) in-plane texture. The training effect is the change in the first point to reversal (Hc1) between the first and second hysteresis loops measured after field cooling. This was found to vary drastically with texture where deltaHc1 was found to be (10+/-2.5)Oe and (60+/-2.5)Oe for the samples deposited on the NiCr and Cu seed layers respectively. This was hypothesised to be due to a distribution of orientation of the easy axis of the interfacial spin clusters. In the case of the sample deposited on the Cu seed layer there is a 3-D random distribution of easy axis orientations similar to that of Stoner-Wohlfarth entities whereas for the sample grown on the NiCr seed layer there is a 2-D random distribution of easy axis. It is the tensioning of the cluster-cluster interactions with the easy axis orientation that is thought to give rise to the training effect and other phenomena in exchange bias. The change in Hex and Tbmed with element size was measured in 4x4mm arrays of nanodots patterned through e-beam lithography and fabricated using the lift-off method. Following an initial decrease of 66% due to the patterning process, Hex decreased from (105+/-5)Oe to (40+/-5)Oe with element size from 425 to 80nm. However whilst an initial decrease in Tbmed from (448+/-5)K to ~405K occured due to the patterning process no significant change is seen with element size. This implies that the change in Hex is due to a modification of the interface i.e. non-planar deposition as a result of depositing the film through a mask.
3
Liu, Frank Ph D. Massachusetts Institute of Technology. "Exchange bias in patterned nanostructures." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/103268.
Анотація:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 119-127).
Exchange bias between a ferromagnet (FM) and antiferromagnet (AFM), which is utilized to pin the magnetization of a FM into a fixed direction in space, is essential in commonly used electronic components such as magnetic recording heads and magnetic memory cells, as well as novel magnetic logic and memory devices. However, the exchange bias effect has been optimized in materials and used in devices for decades without a good scientific understanding, both due to lack of nanoscale research and conflicted results from differences in fabrication and feature size. In this thesis, we present a special fabrication method that produces exchange bias reliably and consistently. We also show the results of both experimental and simulated investigation of the properties of exchange biased nanostructures such as domain formation, magnetostatic interactions, and response to field-driven switching. -A fabrication method for creating locally exchange biased nanostructures is first developed. By etching back a predeposited FM film, and regrowing a thin FM layer and then the AFM film, this hybrid method combines the benefits of a clean interface produced using subtractive methods and the scalability produced using additive methods. Its consistency is analyzed through vibrating sample magnetometry (VSM) and scanning electron microscopy (SEM). Next, the fabrication method is applied to an array of nanodots with varying ion beam etch durations and dot diameters, demonstrating a reduced exchange bias for small diameters, and no significant change in exchange bias unless the ion beam etch duration exceeded 30s. Based on the consistency of this method, new device-like patterns were fabricated both experimentally and by modeling, in which a grating of AFM stripes was exchange biased with a continuous FM film. Competing magnetic interactions were found in the modeling, and produced extraordinary hysteresis loop shapes in the experimental samples. Next, a grating of AFM stripes was exchange biased with a 900 offset grating of FM stripes using the same fabrication method, which simulates an array of individual magnetic devices. A different set of competing magnetic interactions was found, and the feature sizes of the FM and AFM components were demonstrated to tune these interactions and thus the switching behavior of such devices. Exchange bias of materials with perpendicular magnetic anisotropy (PMA) was attempted by exchange coupling a PMA FM material with an in-plane FM material, which in turn exchange couples with the AFM material. However, the magnitude of the exchange bias was found to be negligible when compared to the coercivity of the PMA material.
by Frank Liu.
Ph. D.
4
Zheng, Rongkun. "Exchange bias in magnetic nanoparticles /." View abstract or full-text, 2004. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202004%20ZHENGR.
Анотація:
Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2004.Includes bibliographical references (leaves 103-116). Also available in electronic version. Access restricted to campus users.
5
Rosa, Diego Saldanha da. "Estudo de exchange bias via magnetorresistência anisotrópica." Universidade Federal de Santa Maria, 2013. http://repositorio.ufsm.br/handle/1/9237.
Анотація:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorAnisotropic magnetoresistance (AMR) corresponds to the change of R in an ferromagnetic material with the angle between electric current and magnetization. Sensors using this effect are suited to detect both angular and linear displacements. In this work, structural, magnetic and electric characterization were performed in order to study the exchange interaction between antiferromagnetic IrMn and ferromagnetic NiFe, in a bilayer and a multilayer. Simulations of the AMR measurements were performed and showed good agreement with the experimental data. Different anisotropy field values were observed. The difference between the anisotropy field and the exchange field values is responsible for the different AMR data sets extracted from each sample. The model takes into account the, anisotropy (uniaxial), Zeeman, and exchange-bias (unidirectional) energies was used to explain the observed behavior.
Magnetorresistência anisotrópica (AMR) consiste na variação da resistência de um material ferromagnético em função do ângulo entre a corrente elétrica e a magnetização do material, o que faz com que sensores que utilizam este efeito sejam promissores para medidas de posição tanto angulares quanto lineares. Neste trabalho, caracterização estrutural, magnética e elétrica foram realizadas para estudar a interação de troca entre camadas antiferromagnética de IrMn e ferromagnética de NiFe em uma bicamada e uma multicamada. Simulações das medidas de AMR foram realizadas e boa concordância entre os dados experimentais e os simulados foi obtida. Diferentes valores de campos de anisotropias foram observados. A diferença entre o campo de anisotropia unidirecional e o campo de exchange é responsável pela diferença entre as medidas de AMR obtidas. Um modelo que considera as energias de anisotropia (uniaxial), Zeeman e de exchangebias (unidirecional) foi usado para explicar o comportamento observado.
6
Aley, Nicholas Paul. "Structure and anisotrophy in exchange bias systems." Thesis, University of York, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.533445.
7
Polenciuc, Ioan. "A racetrack memory based on exchange bias." Thesis, University of York, 2016. http://etheses.whiterose.ac.uk/17517/.
Анотація:
This thesis describes preliminary studies for a new type of computer memory, racetrack memory. Racetrack memory was initially proposed by scientists at IBM. Data in racetrack memory is stored in domains within ferromagnetic nanowires which are separated by domain walls. The data is moved in the wires by moving the domain walls. Control over the movement of domain walls was initially attempted via use of notches cut into the wires, but these were not only expensive and difficult to fabricate but also proved to be unreliable. The method for pinning domain walls described in this thesis uses antiferromagnetic wires grown perpendicular to ferromagnetic wires so that exchange bias is induced at the crossing points. Exchange bias occurs when an antiferromagnet is in contact with a ferromagnet. When the structure is cooled in an applied field from near the Néel temperature of the antiferromagnet, the hysteresis loop shifts along the field axis resulting in pinning of the ferromagnetic layer. Multiple ferromagnetic materials were considered for the ferromagnetic layer. Initially unpinned ferromagnetic films were grown and characterised. Exchange biased films were then grown in configurations where the antiferromagnetic layer was either under or above the ferromagnetic layer but showed no major differences in the exchange bias. Ferromagnetic wires were patterned on Si substrates using e-beam and photolithography. Coercivity of the wires was measured along the length of the wires. Exchange biased wires in both top and bottom pin configurations were fabricated afterwards using the same methods and characterised using the same technique as the unbiased wires. The comparison between the biased and unbiased wires showed that domain walls can be pinned in nanowires using exchange bias. The top bias configuration showed a maximum value for pinning of about 55 Oe which is comparable to that initially reported in notched systems.
8
Guhr, Ildico. "Exchange-Bias-Effekt in magnetischen Filmen auf Partikelmonolagen." Aachen Shaker, 2008. http://d-nb.info/988801426/04.
9
Lage, Enno [Verfasser]. "Magnetoelektrische Dünnschichtkomposite mit integriertem Exchange Bias / Enno Lage." Kiel : Universitätsbibliothek Kiel, 2014. http://d-nb.info/1049929101/34.
10
Kaeswurm, Barbara. "Magnetic and electrical studies of exchange bias systems." Thesis, University of York, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534929.
11
Nicolodi, Sabrina. "Exchange Bias em filmes de IrMn/Cu/Co." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2007. http://hdl.handle.net/10183/10353.
Анотація:
Medidas de magnetização (MAG) e de ressonância ferromagnética (FMR) foram usadas para estudar a interação de troca entre a camada antiferromagnética (AFM) de IrMn e a ferromagnética (FM) de Co no sistema IrMn/Cu/Co em função da espessura do Cu. Parâmetros importantes extraídos destas medidas foram comparados com os respectivos valores obtidos dos ajustes dos dados experimentais através de um modelo fenomenológico. Foi observada uma concordância muito boa entre os dados experimentais e os simulados numericamente. Os valores dos campos de anisotropia e de interação, obtidos pelas duas técnicas, MAG e FMR, foram comparados. Observou-se que, apesar das variações angulares experimentais dos campos de Exchange Bias coincidirem para todas as amostras, a intensidade de acoplamento J e os campos de anisotropia das camadas de Co (obtidos via simulação numérica) HU , são diferentes. Para todas as espessuras de Cu, vimos que J FMR > J MAG e FMR U H < MAG U H . Outro resultado importante é que J diminui exponencialmente com o aumento da espessura do espaçador, resultado de uma interação de curto alcance mediada por pinholes. Todas estas características foram explicadas através de um modelo que considera camadas magnéticas policristalinas com distribuições de eixos fáceis independentes, tomando em conta a anisotropia rodável. Conhecer o papel dos grãos AFM com diferentes tamanhos e diferentes estabilidades magnéticas também é essencial para entender as propriedades deste sistema. Os fatos dos HEB observados serem iguais (diferentemente dos resultados até agora conhecidos) e as interações assim como as anisotropias uniaxiais serem diferentes foram atribuídos ao fato de que as duas técnicas detectam processos diferentes. A medição estática, MAG, prioriza a amostra como um todo, enquanto a FMR (a qual depende da freqüência de oscilação da magnetização) detecta somente os momentos magnéticos cujos tempos de relaxação são menores do que o tempo de excitação da microonda. Quando não existe mais contato entre as camadas de IrMn e Co (e consequentemente, a interação), as diferenças entre as medidas não são mais observadas.Magnetization (MAG) and ferromagnetic resonance (FMR) measurements were used to study the exchange interaction between the IrMn (antiferromagnetic, AFM) and Co (ferromagnetic, FM) layers in an IrMn/Cu/Co system as a function of the Cu spacer thickness. Important parameters, extracted from these measurements, were compared with the respective values obtained from the experimental data fittings through a phenomenological model, and a very good agreement between experiment and model was observed. The comparison between the anisotropy and interaction fields obtained from both techniques, MAG and FMR, showed that although the experimental angular variations of the exchange-bias fields, FMR eb H and MAG eb H coincide, the coupling strengths, J and the Co layers' anisotropy fields, U H , obtained via numerical simulations, are different. For all Cu thicknesses, it was estimated that J FMR > J MAG e FMR U H < MAG . U H Another important result is that J decreases exponentially with the spacer thickness and is a short-range interaction mediated by pinholes. All these characteristics were explained in the framework of a model considering polycrystalline magnetic layers with independent easy axis distributions and taking into account the rotatable anisotropy. The role of antiferromagnetic grains at the interface with different sizes and different magnetic stability is essential for understanding the behavior of this exchange-biased system. The facts that the observed FMR eb H and MAG eb H are equal (differently from all results known till now) and that the interactions as well as the uniaxial anisotropies are different were attributed to the different processes detected by the techniques. While the static MAG measurement ‘senses’ the sample as a whole, the FMR (which depends on the frequency of the oscillation of the magnetization) detects only the magnetic moments with relaxation times less than the period of the microwave excitation. There are no differences observed between the measurements when there is no contact (and consequently, the interaction) between the IrMn and Co layers.
12
Morales, Erie Hector. "Exchange bias in the Mnx̲Pt₁-x̲/Co system." Morgantown, W. Va. : [West Virginia University Libraries], 2003. http://etd.wvu.edu/templates/showETD.cfm?recnum=3206.
Анотація:
Thesis (M.S.)--West Virginia University, 2003.Title from document title page. On t.p. "x̲" is subscript. Document formatted into pages; contains viii, 34 p. : ill. Vita. Includes abstract. Includes bibliographical references (p. 30-31).
13
Biswas, Sutapa. "Magnetism and Associated Exchange Bias Effects in Mn2Ni1+xGa1-x Heusler Alloys and Selected Fe Doped Derivatives." Miami University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=miami1596051931485739.
14
Dias, Diego FÃlix. "SÃntese e caracterizacÃes de nanopartÃculas de FeCo/(Fe,Co)304 com acoplamento magnÃtico tipo exchange spring sintetizadas pelo mÃtodo sol-gel Proteico." Universidade Federal do CearÃ, 2015. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=16021.
Анотація:
Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgicoNanopartÃculas de FeCo recobertas com (Fe,Co)3O4 (Magnetita dopadas com cobalto) na estrutura casca caroÃo (Core-Shell), foram sintetizadas pela rota quÃmica conhecida como Sol-Gel Proteica. Os materiais sintetizados foram caracterizados por Termogravimetria (TG), DifraÃÃo de Raios-X (DRX), Magnetometria de Amostra Vibrante (VSM), Espectroscopia MÃssbauer, Microscopia EletrÃnica de Varredura (MEV) e Microscopia EletrÃnica de TransmissÃo (MET). Os resultados mostram que o aumento da temperatura de re-oxidaÃÃo influi diretamente no tamanho da casca (Shell) bem como o aumento da razÃo entre a magnetizaÃÃo remanente pela magnetizaÃÃo de saturaÃÃo. O efeito de Exchange Spring e Exchange Bias foram observados nas curvas de histerese. A estrutura Core-Shell foi formada de maneira nÃo homogÃnea, ou seja, nem todas as partÃculas foram recobertas pela magnetita.
FeCo Nanoparticles coated with (Fe,Co)3O4 (Magnetite doped with cobalt) were synthesized by the chemical route known as Sol-Gel Protein. The synthesized materials were characterized by thermogravimetry (TG), X-Ray Diffraction (XRD), Vibrating Sample Magnetometry (VSM), MÃssbauer spectroscopy, Scanning Electronic Miscroscopy (SEM) and Transmission Electronic Microscopy (TEM). The results show that the increase in temperature directly influences the size of the shell (Shell) and increased the ratio of remanent magnetization to the saturation magnetization. The effect of Exchange Spring and Exchange Bias were observed in the hysteresis curves. The Core-Shell structure was formed a non-homogeneous manner, ie not all the magnetite particles were coated.
15
Meyl, Markus [Verfasser]. "Abhängigkeit des Exchange-Bias Feldes und des Koerzitivfeldes von der mittels Röntgendiffraktometrie gemessenen Kornvolumenverteilung des Antiferromagneten in Exchange-Bias Schichtsystemen / Markus Meyl." Kassel : Universitätsbibliothek Kassel, 2020. http://d-nb.info/1216666385/34.
16
Poerschke, Julia Grasel. "Multicamadas de NiFe/FeMn com potencial de aplicação como sensores a altas frequências." Universidade Federal de Santa Maria, 2016. http://repositorio.ufsm.br/handle/1/9259.
Анотація:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorIn this work we investigated the magnetization process and the magnetoimpedance in samples exhibiting exchange bias. Ni81Fe19=Fe50Mn50=T a bilayers and multilayers where grown onto glass substrates. The exchange bias was induced during growth by a 2 kOe static magnetic field. Structural, magnetic and impedance characterization was carried out. In the hysteresis curves from all samples a shift was observed. This shifting was also observed in the magnetoimpedance curves, and it had the same magnitude observed in the magnetic hysteresis curve. This displacement caused the impedance versus magnetic field curve to be approximately linear in the region near to H = 0. It is possible to use these systems as a magnetic field sensor with a linear response. The sensitivity, defined as the rate of change of the real part of the impedance with the magnetic field, reached values above 200 m =Oe at frequencies below 1 GHz.
Neste trabalho são investigados os processos de magnetização e a magnetoimpedância em filmes com exchange bias (EB). São estudadas amostras de Ni81Fe19=Fe50Mn50=T a depositadas como bi e multi-camadas, sob substratos de vidro. O Exchange Bias foi induzido durante o crescimento das amostras através da aplicação de um campo magnético estático de 2 kOe. A caracterização estrutural das amostras foi feita através de medidas de difração de raios-x a altos ângulos, utilizando uma configuração θ 2θ. A caracterização magnética estática foi feita através de medidas de magnetização obtidas com um magnetômetro de gradiente de campo alternado. As componentes indutivas da impedância foram medidas em função da freqüência (100 MHz 3 GHz) e do campo estático (H=±200Oe). Em todas as amostras foi observado o deslocamento da curva de histerese magnética devido a ocorrência do fenômenos de Exchange Bias. Este deslocamento também é observado nas curvas de magnetoimpedância e tem mesma magnetude do observado nas curvas de histerese magnética dentro da precisão experimental que temos na medida do campo magnético. Este deslocamento faz com que a variação da impedância com o campo mangnético aplicado seja aproximadamente linear na região próxima a H = 0. Tornando o fenômeno de exchange bias um possivel aliado para a obtenção de assimetrias nas curvas de magnetoimpedância para o possível uso destes sistemas como sensores de campo magnético com resposta linear. Definindo a sensibilidade como a taxa de variação da parte real da impedância com o campo magnético, foram obtidos valores, em módulo, acima de 200m =Oe em frequências abaixo de 1 GHz.
17
Hochstrat, Andreas. "Extrinsische Kontrolle des Exchange-Bias in Metall-Isolator-Heteroschichtstrukturen." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=985281693.
18
Bah, Mohamed Alpha. "Coercivity and Exchange Bias Study of Polycrystalline Hollow Nanoparticles." Thesis, University of Delaware, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10283261.
Анотація:
Magnetic nanoparticles (NPs) have the potential to be useful in a variety of applications such as biomedical instruments, catalysis, sensing, recording information, etc. These nanoparticles exhibit remarkably different properties compared to their bulk counter parts. Synthesis of magnetic NPs with the right morphology, phase, size and surface functionality, as well as their usage for specific applications are challenging in terms of efficiency and safety. Morphology wise, there have been numerous reports on magnetic nanoparticles where morphologies such as core/shell, hollow, solid, etc., have been explored. It has been shown that morphology affects the magnetic response. Achieving the right crystal structure with required morphology and the magnetic behavior of the nanoparticle phases determines the magnetic response of the structure. For example, in the case of core/shell NPs various ferromagnetic (FM), ferrimagnetic (FiM), and antiferromagnetic (AFM) core and shell combinations have been reported. In these cases, interesting and strikingly different features, such as unusually high spin glass transition temperature, large exchange bias, finite size effects, magnetic proximity effects, unusual trend of blocking temperature as function of average crystal size, etc., have been reported.
More specifically, the morphology of core/shell nanoparticles provides added degrees of freedom compared to conventional solid magnetic nanoparticles, including variations in the size, phase and material of the core and shell of the particle, etc. which helps enhance their magnetic properties. Similar to traditional core/shell nanoparticles, inverted core/shell having a FiM or FM order above the Curie temperature (TC) of the shell has been reported where the Néel temperature (TN) is comparable with the bulk value and there is nonmonotonic dependence of the coercive field (HC) and exchange bias (HEB) on the core diameter.
In addition to the core/shell morphology, nanoparticles with hollow morphology are also of interest to the scientific community. For such cases, surface spin glass transition enhancements have been reported due to the presence of the additional inner surface. CoFe2O4, NiFe 2O4 and γ-Fe2O3 hollow nanoparticles exhibit strikingly contrasting magnetic behavior compared to bulk and conventional solid particles; similar behavior was also observed in core/shell nanoparticles. Structurally, hollow polycrystalline nanoparticles are composed of multiple crystallographic domains. This random orientation of the crystallographic domains also causes randomization of the local anisotropy axes. Hence the overall effect of this morphology on the magnetic properties is exhibited through the high coercivity, relatively high temperature magnetic irreversibility, lack of magnetic saturation, high blocking temperature, etc.
Over the years, extensive work on core/shell nanoparticles have been carried out to understand their exchange bias phenomenon and the effect on coercivity. Recently, focus has been given to hollow polycrystalline nanoparticles for the reason mentioned above. This thesis investigates the root cause for the above-mentioned effects on the coercivity and exchange bias. Since hollow nanoparticles with polycrystalline structure have shown to exhibit different and improved magnetic behavior compared to bulk and other conventional solid particles, they will be the focus of our investigation. First, extensive field and temperature dependent magnetic study on polycrystalline hollow nickel ferrite (NiFe2O4) have revealed the effect of the presence of inner surface in a single oxide nanoparticle. Second, the effect of having multiple oxides with different magnetic properties (i.e. FM and AFM) in a single nanoparticle, while maintaining a hollow morphology was investigated by studying polycrystalline hollow γ-Mn2O3 and MnO nanoparticles. Studies on various conventional solid manganese oxide nanoparticles have already been reported. Therefore, focus was only made on the fabrication and magnetic study of hollow polycrystalline manganese oxide, with a comparison of the results to those from solid nanoparticles already available in literature. A conclusion was drawn to the importance of the coupling of different magnetic phases (i.e. FM and AFM, FiM and AFM, or SG and AFM), in contrast to just having one single oxide in the hollow nanoparticles. Finally, the importance of this coupling as compared to the increase of surface-to-volume ratio was evaluated in CoO/Co3O4/CoFe2O4 polycrystalline hollow nanoparticles by varying the AFM phase (CoO/Co 3O4) in the nanoparticles and observing how the magnetic properties varied. This system helped address the effect of the coupling between different magnetic phases, super-exchange interaction, and proximity effect.
19
Elphick, Kelvin. "Heat assisted magnetic recording media based on exchange bias." Thesis, University of York, 2016. http://etheses.whiterose.ac.uk/13940/.
Анотація:
A study of a new paradigm for a heat assisted magnetic recording (HAMR) media based on the use of exchange bias is presented. Exchange bias occurs when an antiferromagnetic (AF) layer such as IrMn is grown in contact with a ferromagnetic (F) layer and the system if field cooled resulting in a hysteresis loop shifted along the field axis. In this concept the temperature dependent anisotropy is provided by IrMn. Therefore the information is stored in the AF layer and the recording layer is actually part of the read/write structure when field cooled. The F layer when magnetised, serves to align the F layer in the direction required to store the information and then provides a read out signal indicating in which direction the AF layer is oriented. Hence in a complex way the “recording layer” is actually part of the read/write head. The key to achieve a structure of this kind is the control of the orientation of the Mn ions of the IrMn such that they are aligned perpendicular to the plane of the film. In this way a perpendicular exchange bias required for information storage in the AF layer has been achieved. Over 300 samples have been prepared and evaluated to determine the optimised structure. A segregated sample CoCrPt-SiO2 was sputtered using a pressed powder target in a HiTUS deposition system. Dual Ru seed layers of 8nm and 12nm were deposited using 3mTorr and 30mTorr process pressure, respectively. The median grain size of D_m = (6.2 ± 0.2)nm was achieved using these sputtering conditions. High resolution cross section TEM imaging has been used to show that the CoCrPt grains remains segregated by SiO2 after the deposition of IrMn. The key feature of this media is that the recorded information is impossible to be erased by a demagnetising field. In order to achieve this requirement the hysteresis loop has to be completely shifted to a negative field. The highest shift or exchange bias of H_ex = (240 ± 5)Oe was measured at room temperature. This was achieved by depositing an ultrathin (0.8nm) Co interlayer above a Pt seed layer.
20
Akmaldinov, Kamil. "Ferromagnetic/antiferromagnetic exchange bias nanostructures for ultimate spintronic devices." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GRENY009/document.
Анотація:
Les applications d’électronique de spin telles les mémoires à accès aléatoire (MRAM), les capteurs (e.g.les têtes de lecture des disques durs d’ordinateurs) et les éléments de logique magnétique utilisent les interactionsd’échange ferromagnétique/antiferromagnétique (F/AF) dans le but de définir une direction de référence pour lespin des électrons de conduction. Les MRAM à écriture assistée thermiquement (TA-MRAM) utilisent mêmedeux bicouches F/AF : une pour le stockage de l’information et l’autre comme référence. Ces dernièresapplications technologiques impliquent des étapes de nanofabrication des couches minces continues pour formerune matrice de cellules mémoires individuelles. La qualification industrielle du produit final impose de sérieusescontraintes sur la largeur des distributions des propriétés magnétiques - y compris d’échange F/AF - de cellulemémoire à cellule mémoire. Des phases verres de spin réparties de manière aléatoire sur la couche continue, àl’interface F/AF ou dans le coeur de l’AF pourraient contribuer de manière significative à ces distributionsd’échange F/AF dans les dispositifs, après nanofabrication ; comme supposé il y a de cela quelques années. Lebut de cette thèse est d’étudier factuellement le possible lien entre verre de spin répartis dans des couches mincesF/AF et dispersions de propriétés d’échange de cellule mémoire à cellule mémoire dans les dispositifs TAMRAMcorrespondants. Avant cela, l’origine de ces régions verre de spin a été étudiée et une attention plusparticulière a été portée au rôle joué par les diffusions de Mn. Ces dernières ont été directement observées,comprises et l’utilisation de barrières complexes pour les réduire et par là même pour diminuer la quantité dephases verre de spin a été mise en oeuvre avec succès. En guise d’alternative pour varier la quantité de verres despin, l’utilisation d’AFs composites a été également étudiée dans le cadre de cette thèse. Ce type d’AF permet dumême coup de varier la stabilité thermique des grains AF et de répondre à un autre problème identifié pour lesTA-MRAM qui consiste à trouver des matériaux AF avec des propriétés de rétention et d’écriture intermédiairespar rapport aux matériaux actuellement utilisés. Finalement, ces AFs composites ont été utilisés comme moyende varier la quantité de verres de spin dans des dispositifs TA-MRAM réels et de prouver le lien direct avec ladispersion de propriétés de cellule mémoire à cellule mémoireSpintronics applications such as magnetic random access memories (MRAM), sensors (e.g.. hard diskdrive read head) and logic devices use ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) interactionsto set the reference direction required for the spin of conduction electrons. Thermally-assisted (TA-) MRAMapplications even use two F/AF exchange biased bilayers: one for reference and one for storage. Suchtechnological applications involve patterning full sheet wafers into matrix of individual bit-cells. Industrialproducts qualification imposes stringent requirements on the distributions of the magnetic properties from cell tocell, including those related to EB. It was supposed few years ago that randomly spread spin-glass like phases atthe F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB propertiesin devices after processing. This thesis aimed at factually studying the link between spin-glasses spread overF/AF thin films and bit-cell dispersions of EB in corresponding TA-MRAM. Prior to that the origin of the spinglasslike regions and more specifically the role of Mn-diffusion was consolidated. Mn-diffusion was directlyobserved, understood and the use of complex barriers to reduce such diffusion and consequently to minimize theamount of spin-glass was successfully studied. Mixing AFs as another way to tune the amount of spin-glass likephases was also evidenced in the framework of this thesis. All at once, this last solution also tuned the AF grainsthermal stability and this solved another issue related to TA-MRAM, i.e. finding AF-materials with intermediateretention and write properties compared to the AFs presently used. Finally, those mixed antiferromagnets werethe mean chosen to tune the amount of spin-glass like phases in real TA-MRAM devices and to factually provetheir link with bit-cell distributions of EB properties
21
Oliveira, Artur Harres de. "Curvas de remanência de sistemas que apresentam exchange bias." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2014. http://hdl.handle.net/10183/109024.
Анотація:
As propriedades magnéticas de filmes finos que apresentam exchange bias são analisadas através do método de curvas de remanência. Trata-se de uma técnica comumente empregada no estudo de interações magnéticas em sistemas que apresentam laços de magnetização simétricos. Como o deslocamento em campo da curva de histerese é uma das principais manifestações do exchange bias, os valores das magnetizações remanentes podem ser distintos em módulo, ou até possuir o mesmo sinal, para os dois ramos da magnetização. Isto impede a aplicação direta do método. Uma adaptação é proposta aqui, e testada em uma variedade de sistemas que contêm interfaces entre materiais ferromagnéticos e antiferromagnéticos. Ela envolve a redefinição do sistema de coordenadas, e acaba gerando um aumento no número de gráficos distintos que podem ser construídos, evidenciando a assimetria intrínseca que existe entre os dois ramos da magnetização. Três tipos distintos de sistemas foram estudados, todos na forma de filmes finos depositados por desbastamento iônico. São eles: bicamadas de NiCu/IrMn, nas quais a temperatura de Curie do ferromagneto é menor que a temperatura de Néel do antiferromagneto; tricamadas de Co/Cu(tCu)/IrMn, construídas com espessuras variadas do espaçador de cobre com intuito de reduzir a interação efetiva entre o cobalto e o irídio-manganês; e amostras de CoFe/IrMn que possuem diferentes espessuras de material antiferromagnético. Curvas δ M(H) e gráficos de Henkel construídos para cada uma das amostras são apresentados, e os desvios do comportamento esperado para sistemas não-interagentes investigado, traçando um paralelo com parâmetros como o campo de exchange bias e o campo coercivo. Durante a interpretação dos dados, simulações computacionais foram implementadas na tentativa de separar as contribuições dos diferentes mecanismos que influenciam os processos de reversão da magnetização.Magnetic properties of EB thin films are analyzed using the remanence plots method. This procedure is commonly employed to probe magnetic interactions in systems presenting symmetrical hysteresis curves. Since the field shift of the magnetization loops is one of the exchange bias manifestations, distinct remanence values might be observed for the two magnetization branches. This prevents the direct application of the method. An adaptation is proposed here, and tested in a variety of systems containing ferromagnetic/antiferromagnetic interfaces. It involves a redefinition of the coordinate system and gives rise to a number of distinct plots, revealing the intrinsic asymmetry between the two magnetization branches of exchange bias samples. Three different systems were studied, all in the form of thin films deposited via sputtering. These are: NiCu/IrMn bilayers, where the Curie temperature of the ferromagnet is lower than the Néel temperature of the antiferromagnet; Co/Cu(tCu)/IrMn trilayers, where the copper spacer thickness was varied in order to diminish the cobalt/iridium-manganese effective coupling; and CoFe/IrMn samples with different antiferromagnetic layer thickness. Henkel and δM(H) plots constructed for each sample are presented, and their deviations from the non-interacting behavior investigated, tracing a parallel with exchange bias parameters such as Heb and Hc. Computational simulations were employed aiming to discriminate contributions coming from different mechanisms that might influence the magnetization reversal.
22
Barnsley, Lester Clinton. "Exchange Bias in Manganese Alloys with Mixed Magnetic Behaviour." Thesis, Griffith University, 2012. http://hdl.handle.net/10072/367858.
Анотація:
Exchange bias has been a well known property of materials consisting of a ferromagnet
interfaced with an antiferromagnet for many years, and the anisotropy resulting
from the interaction has recently been nominated as a possible means for overcoming
the superparamagnetic limit in high-density magnetic information storage media.
However, a generalized theoretical description of the phenomenon remains elusive.
More recently, analogues between the exchange bias eect and the behaviours of spin
glass materials have become increasingly recognized. In this thesis, the analogues
between exchange bias and spin glass systems are explored in a set of concentrated
spin glass samples that exhibit a strong sensitivity to short range order of magnetic
entities.
A number of models were developed to analyze and account for the measurements of
magnetization made on these samples. The rst was a simple, empirical model, based
on an approximation of non-interacting, monodisperse, superparamagnetic clusters
against a paramagnetic background to t single branches of minor hysteresis loops.
In this model, the parameter to describe the coercive eld was added ad hoc. Another,
more sophisticated, model was based on a modied Stoner-Wohlfarth expression for
magnetic clusters and applied to the data using Monte Carlo simulations. The best
ts resulted from this model when the cluster easy axis orientations were allowed to
have a Gaussian distribution.Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Biomolecular and Physical Sciences
Science, Environment, Engineering and Technology
Full Text
23
Siqueira, Junara Villanova de. "Exchange bias em multicamadas de NiFe/IrMn/Ta: um estudo através da magnetorresistência anisotrópica." Universidade Federal de Santa Maria, 2015. http://repositorio.ufsm.br/handle/1/9252.
Анотація:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorIn this work a study of the exchange interaction between ferromagnetic (NiFe) and antiferromagnetic (IrMn) layers was done through structural, magnetic and electric characterization. NiFe/IrMn/Ta films were grown with different number of repetitions of this basic structure aiming to evaluate possible changes on the magnetic anisotropies presented by the samples. It was implanted in the Laboratório de Magnetismo e Materiais Magnéticos a system in order to measure the Anisotropic Magnetorresistance (AMR) as function of the applied field angle. The AMR consists in a change of the eletric resistance of a ferromagnetic material as function of the angle between the electric current and the magnetization of the material and, by this way, sensible to changes in the anisotropy presented by the samples. It is presented a simple model to calculate the AMR as function of the angle field and, by comparing with the experimental curves, to obtain the magnetic parameters who describe the system. In the model the equilibrium direction of the magnetization is obtained from the minimization of the free magnetic energy, which is given by the Zeeman, uniaxial, unidirectional and manetostatic energies. The AMR curves present an assymetry around 180 degrees when measured at fields below the bias one. According to the fittings, it was found that such assymetry can be due by a misalignment between the anisotropy axis (uniaxial and unidirectional) or by a misalignment between the measuring current and the easy magnetic axis of the samples. It was not observed, as expected, an expressive increase of the anisotropy dispersion of the uniaxial anisotropy with the increase of the number of trilayers. The same was observed with the bias and coercive fields.
Neste trabalho, o estudo da interação de troca entre as camadas ferromagnética (NiFe) e antiferromagnética (IrMn) em multicamadas foi realizado através da caracterização estrutural, magnética e de transporte elétrico. Foram crescidos filmes de NiFe/IrMn/Ta com diferentes números de repetições dessa estrutura básica com o objetivo de avaliar as possíveis modificações nas anisotropias apresentadas pelas amostras. Foi implantado no Laboratório de Magnetismo e Materiais Magnéticos (LMMM) um sistema para medidas de Magnetorresistência Anisotrópica (AMR) em função do ângulo de aplicação do campo. A AMR consiste na variação da resistência elétrica e a magnetização do material e, portanto, sensível as modificações de anisotropia nas amostras. É apresentado um modelo simples para calcular as curvas de AMR em função do ângulo do campo e, na comparação com as curvas experimentais obter os parâmetros magnéticos que descrevem o sistema. No modelo, a direção de equilíbrio da magnetização é obtida a partir da minimização da energia livre magnética, que por sua vez é dada pela soma da energia Zeeman, uniaxial, unidirecional e magnetostática. As curvas de AMR apresentam uma assimetria em torno de 180 graus quando medidas em valores de campo menores que o campo de Bias. De acordo com os ajustes, foi verificado que esta assimetria pode ser causada tanto por um desalinhamento entre os eixos de anisotropias (uniaxial ou unidirecional) como por um desalinhamento entre o eixo da corrente de medição e o eixo de fácil magnetização da amostra. Não foi observado, conforme esperado, um aumento expressivo na dispersão da anisotropia uniaxial com o aumento do número de repetições das tricamadas. O mesmo ocorrendo com os valores dos campos de Bias e coercivo.
24
Silva, Ricardo Barreto da. "Processos de Magnetização e Magnetoimpedância em Filmes com Exchange Bias." Universidade Federal de Santa Maria, 2006. http://repositorio.ufsm.br/handle/1/3886.
Анотація:
In this work, the magnetization process and the magnetoimpedance effect in thin films with exchange bias were studied. We have investigated NiO/NiFe and NiFe/FeMn/Ta multilayer produced by magnetron sputtering on glass and Si(100) substrates, respectively. The exchange anisotropy in the NiO/NiFe samples were induced by a field cooling process, with a static magnetic field (2 kOe), from a temperature Tc > T > TN to the room temperature. In the NiFe/FeMn samples, the exchange anisotropy was induced during the deposition by application of a static magnetic field (460 Oe). The structural characterization was made by high angle x-ray diffraction in a q-2q setup and Cu-Ka radiation. The static magnetic characterization was performed by a VSM. The real and imaginary parts of the impedance were measured as a function of the current frequency (100kHz 1.8 GHz) and of the magnetic field intensity (± 300 Oe). The NiO multilayer was insensitive to the magnetoimpedance. The magnetoimpedance ratio was as large as 70 % in the NiFe/FeMn/Ta multilayer. On the other hand, the center of symmetry of the MI curves are shift of the position around H = 0 to H = Heb. The angular dependencies of the exchange bias field were calculated to prove the Mauri model to the exhange bias. The dispersion relation obtained by MI vs. f curves was fitting combined the Smit-Beljers approach to the FMR and mode s Mauri to the exchange bias. This fitting allowed that intrinsic parameters, such as the anisotropies of the system, were obtained. In summary, the MI and EB effects were grouped to origin a new magnetic system with singular magnetic characteristics. The MI curves are shift with respect to H = 0 and it allowed that the EB effect could be studied through the MI measurement.Neste trabalho são investigados os processos de magnetização e a magnetoimpedância (MI) em filmes com exchange bias (EB). São estudados dois conjuntos de amostras: Ni50O50/Ni81Fe19 e Ni81Fe19/Fe50Mn50/Ta produzidas como bi e multi-camadas. As amostras foram produzidas por magnetron sputtering sobre substrato de vidro e Si (100), respectivamente. No primeiro caso a anisotropia de troca foi induzida após a deposição através de um processo de resfriamento em campo desde T > TN (onde TN é a temperatura de Néel do antiferromagneto) até a temperatura ambiente. No segundo caso o EB foi induzido durante o crescimento das amostras através da aplicação de um campo estático de 460 Oe. A caracterização estrutural das amostras foi feita via medidas de difração de raios-x XRD) a altos ângulos, utilizando uma configuração q-2q e radiação Cu-Ka. A caracterização magnética estática foi feita através de medidas de magnetização obtidas com um magnetômetro de amostra vibrante (VSM) que permite a aplicação de um campo em diferentes orientações em relação a uma direção de referência. As componentes resistivas e indutivas da impedância foram medidas em função da freqüência (100 kHz 1.8 GHz) e do campo estático (H = ± 300 Oe). As impedâncias das multicamadas de NiO foram praticamente insensíveis as variações de campo e de freqüência, devido principalmente as altas anisotropias e relativamente baixas espessuras destas estruturas. Variações de impedância de até 70 % foram obtidas com as multicamadas de iFe/FeMn/Ta. Além disso, as curvas de MI obtidas são inteiramente deslocadas da posição de simetria usual em torno de H = 0, para o caso de uma amostra com anisotropia uniaxial, para H = Heb (onde Heb é campo de bias obtido das curvas de magnetização estática pela medida do deslocamento do centro da curva de histerese). A dependência angular do campo de bias, em relação ao ângulo entre o campo estático (aplicado no plano do filme) e a direção preferencial da anisotropia (paralela à direção do campo de resfriamento) foi obtida. Os dados experimentais foram ajustados usando um modelo de Stoner-Wolfarth modificado pela inclusão de um temo de troca e de um termo associado à formação de uma parede de domínios no material antiferromagnético na expressão para a densidade de energia livre magnética. A relação de dispersão (fr vs. H) foi experimentalmente obtida dos espectros em freqüência da MI, os quais apresentam nítidas características associadas à ressonância ferromagnética (FMR). Do ajuste teórico da relação de dispersão utilizando a fórmula de Smit-Beljers, na qual a freqüência de ressonância é expressa em termo dos ângulos de equilíbrio da magnetização e das derivadas em segunda ordem da energia em relação a estes ângulos, parâmetros intrínsecos das amostras, tais como as anisotropias, foram estimados. Em suma, unindo o EB e a MI em um único sistema foram criadas amostras com características magnéticas singulares. As curvas de MI destas amostras são deslocadas em relação ao eixo dos valores de campo.
25
Pang, Wenjie. "Synthesis and characterization of magnetic thin films--exchange bias systems." University of Western Australia. School of Physics, 2005. http://theses.library.uwa.edu.au/adt-WU2005.0110.
Анотація:
[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes
26
Jackson, Jerome. "Computer simulations of exchange bias : the Role of the antiferromagnet." Thesis, University of York, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534908.
27
Guhr, Ildico [Verfasser]. "Exchange Bias-Effekt in magnetischen Filmen auf Partikelmonolagen / Ildico Guhr." Aachen : Shaker, 2008. http://d-nb.info/1162791195/34.
28
Müglich, Nicolas David [Verfasser]. "Exchange-Bias-Dünnschichtsysteme - Charakterisierung, Modellierung und Anwendung / Nicolas David Müglich." Kassel : Universitätsbibliothek Kassel, 2016. http://d-nb.info/1110994923/34.
29
Wang, Chao. "Exchange bias effects of CrSe/Fe/GaAs thin film structure /." View abstract or full-text, 2008. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202008%20WANG.
30
Du, Yuqing. "Film deposition and microfabrication of magnetic tunnel junctions with an MgO barrier." Thesis, University of Plymouth, 2012. http://hdl.handle.net/10026.1/1174.
Анотація:
Magnetic tunnel junctions (MTJs), which consist of a thin insulation layer sandwiched by two ferromagnetic (FM) layers, are among the key devices of spintronics that have promising technological applications for computer hard disk drives, magnetic random access memory (MRAM) and other future spintronic devices. The work presented here is related to the development of relevant techniques for the preparation and characterization of magnetic films, exchanged biased systems and MTJs. The fabrication and characterization of PtMn/CoFe exchange biased systems and MTJs with Al-O barriers were undertaken when the new Aviza StratIon fxP ion beam deposition tool was developed by the project consortium funded by DTI MNT. After the Nordiko 9550 spintronic deposition tool was installed at Plymouth, the work focused on the development of MTJ multilayer stacks with layer structures of CoFeB/MgO/CoFe/IrMn and IrMn/CoFeB/MgO/CoFeB to achieve coherent tunneling with a crystalline MgO barrier. The film deposition, microfabrication, magnetic field annealing, microstructural and nano-scale characterization, magnetic and magneto-transport measurement for these devices have been systematically studied to achieve smooth interfaces and desired crystallographic textures and magnetic properties of layer stacks. Magnetoresistance (MR) of up to 200% was obtained from MTJs with a layer structure of Ta/CuN/Ta/CoFeB/MgO/CoFe/IrMn/Ta and a CuN bottom electrode. Enhanced exchange anisotropy from the bottom pinned IrMn/CoFeB stacks has been obtained, which demonstrated the possibility of fabricating MTJs with CoFeB as both the top and bottom FM electrodes with strong exchange bias. The origin of the enhanced exchange bias field was studied by employing high resolution transmission electron microscopy (HRTEM) and x-ray magnetic circular dichroism (XMCD) to examine the mmicrostructure properties and element specific magnetic properties of the stacks. Results demonstrate that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins. MTJs with layered structures of IrMn/CoFeB/MgO/CoFeB were prepared based on this exchange bias system. However, further work is required for the optimisation of the (001) crystallographic textures of the CoFeB/MgO/CoFeB stack to achieve coherent tunneling.
31
MUZZI, BEATRICE. "Investigation of synthetic strategies for enhancing the energy product of spinel ferrite nanoparticles." Doctoral thesis, Università di Siena, 2021. http://hdl.handle.net/11365/1126979.
Анотація:
This work was focused on the development of synthesis strategies for enhancing the energy product of spinel ferrite nanoparticles. The final goal was that of finding the best conditions to realize novel materials with improved magnetic properties, which could be used as building blocks for the realization of rare earth free - permanent magnet with higher energy products than commercial ferrites (i.e. strontium ferrite). To this aim, we focused on quasi-zero-dimensional magnetic materials and as possible strategies to enhance their performances we explored the coupling at the nanoscale with magnetic components endowed with complementary properties, and the modification of the intrinsic core properties by solvent mediated post-synthetic treatment.
In the first part of this work, hybrid nanocomposite magnets were designed, where the effective exchange-coupling through the interface between hard and soft magnetic constituents or between AFM and a F(i)M phases is exploited to increase the energy product of the material. In particular, metal/ferrite nano-heterostructures, NHSs, were synthesized by an optimized one-pot thermal decomposition method, which allowed us to tune the composition of the final product by tuning the synthetic parameters. The best NHSs obtained by this approach had energy product of 19 kJm-3, higher than that of the commercial hexagonal ferrite. With the aim of developing a system able to display exchange bias at room temperature, iron oxide AFM|FiM core|shell (CS) NPs doped with two divalent cations (Co(II) and Ni(II)) were also synthetized. The co-doping with cobalt and nickel revealed to be a good compromise for obtaining nanocrystals stable to further oxidation processes, and with increased anisotropy, bias and Néel temperature (260 K) than the standard CS iron oxide NPs.
The second part of the thesis was devoted to the improvement of the magnetic performances of the prepared CS AFM|F(i)M hybrid NPs and of standard cobalt ferrites NPs, by solvent mediated post-synthesis treatments. In the first case a mild oxidation of the Fe0.95O|Fe3O4, Co0.3Fe0.7O|Co0.8Fe2.2O4 and Ni0.17Co0.21Fe0.62O|Ni0.4Co0.3Fe2.3O4 CS NPs was realized by solvent mediated annealing in the presence of air, with the aim of investigating the role of the nature of the divalent ions on the structure and room temperature magnetic behaviour of the oxidized product. The Ni(II) and Co(II) amount during the oxidation process was found to play a crucial role in the formation of crystalline antiphase boundaries, leading to a mosaic texturing of the spinel structure in the NPs, characterized by enhanced exchange bias and high field magnetization with respect to the standard AFM|F(i)M NPs. The second investigated approach was the solvent mediated annealing treatment of standard cobalt ferrites NPs. This approach has proven to be an excellent strategy for increasing of the 48 % the BHmax value of the material thanks to the reduction of the lattice microstrain generated during the crystallites’ growth. The presented results provide indications on the validity of the proposed approaches. A further investigation is required to obtain materials characterized by exchange effect at room temperatures through a synthesis that can be carried out on a large scale.
32
Upadhyay, Sneha R. "Investigation of Possible Exchange Bias in L10 MnGa/ θ-MnN Bilayers". Ohio University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1544106639347172.
33
Shi, Hongtao. "Exchange bias and its angular dependence in Fex̲Zn₁-x̲F₂/Co bilayers." Morgantown, W. Va. : [West Virginia University Libraries], 2002. http://etd.wvu.edu/templates/showETD.cfm?recnum=2732.
Анотація:
Thesis (Ph. D.)--West Virginia University, 2002.Title from document title page. On t.p. "x̲" is subscript. Document formatted into pages; contains ix, 93 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 89-92).
34
Cramp, Nicholas Charles. "Characterisation of interface spin clusters in polycrystalline exchange bias thin films." Thesis, University of York, 2013. http://etheses.whiterose.ac.uk/6578/.
Анотація:
A study of exchange bias systems has been made focusing on the characteristics of the interfacial spin clusters found in-between the bilayers. The materials studied were primarily IrMn (10nm) / CoFe (2nm) thin films of varying grain sizes. They were selected as the bulk properties have already been well characterised and they exhibit the effects being looked for. A combination of vibrating sample magnetometry and electron microscopy has been used to investigate the nature of the interfacial spin clusters. Measurements were selected which specifically highlight the properties of the clusters. The first set of measurements consisted of increasing the setting field. The second, reducing the measurement temperature of median blocking curve measurements. The third involved taking hysteresis loops at liquid Helium temperatures. The first set of measurements clearly showed an increase in the exchange bias that was due to the interfacial spin clusters. This increase was shown to correlate with the spin cluster size of the samples and showed quasi-superparamagnetic behaviour. The second set of measurements showed a temperature dependent variation in coercivity that implied the coercivity was due to the interfacial spin clusters. The final set of measurements showed a change in behaviour of the spin clusters whereby they started to act as a spin glass, which again was cluster size dependent. All of these results could be explained by virtue of an energy barrier distribution of the spin clusters whereby they could take on one of three different states, thermally unstable, partially stable, or fully stable. Taken as a whole, the results and conclusions give a good insight into the nature of the interfacial spin clusters and their effects on the exchange bias and coercivity of IrMn / CoFe polycrystalline thin films.
35
Rigue, Josué Neroti. "Estudo da anisotropia unidirecional e da histerese rotacional em sistemas com exchange bias." Universidade Federal de Santa Maria, 2014. http://repositorio.ufsm.br/handle/1/3924.
Анотація:
Conselho Nacional de Desenvolvimento Científico e TecnológicoThe exchange bias (EB) phenomen occurs due to the coupling between ferromagnetic and antiferromagnetic material and the main characteristics are the rise of unidirectional anisotropy and the rotational hysteresis in torque curves. In this work we have investigated how the unidirectional anisotropy and the rotational hysteresis are influenced by the change of some characteristics in thin films samples. Among these are the stacking of layers, the roughness at the interface between the two materials, the difference in the ferromagnetic layer thickness and the difference between the antiferromagnetic materials. The study was made by using magnetic torque measurements which were interpreted using a granular model for the EB. The parameters obtained from the fitting of the model to the torque curves has also permitted to reproduce data obtained by different magnetic techniques, especially in samples where the unidirectional anisotropy is greater than the uniaxial anisotropy. In NiFe=IrMn samples, the unidirectional anisotropy is favored by stacking layers, by increasing of the interfacial roughness and by the decrease of the ferromagnetic layer thickness, while the rotational hysteresis is substantially increased as the interfacial roughness increases. For the NiFe=FeMn sample the highest values of unidirectional anisotropy and rotational hysteresis were found, as well as a dependence of these parameters with the measuring magnetic field.
O fenômeno de "exchange bias" (EB) ocorre quando um material ferromagnético está acoplado a um material antiferromagnético e apresenta como características principais o surgimento de uma anisotropia unidirecional e a histerese rotacional em curvas de torque. Nesse trabalho foi investigado como a anisotropia unidirecional e a histerese rotacional são influenciadas pela mudança de algumas características em amostras na forma de filme finos. Entre essas características estão o empilhamento de camadas, a rugosidade na interface entre os dois materiais, a diferença na espessura da camada ferromagnética e a diferença entre os materiais antiferromagnéticos. O estudo foi realizado através de medidas de magnetometria de torque nas amostras, as quais foram interpretadas usando um modelo granular para o EB. Os parâmetros resultantes do ajuste do modelo às curvas de torque permitiram também reproduzir dados obtidos por outras técnicas de caracterização magnética, sobretudo em amostras que apresentam a anisotropia unidirecional maior do que a anisotropia uniaxial. Em amostras de NiFe=IrMn, a predominância da anisotropia unidirecional é favorecida pelo empilhamento de camadas, pelo aumento da rugosidade interfacial e pela diminuição da espessura da camada ferromagnética, enquanto a histerese rotacional apresenta aumento significativo somente com o aumento da rugosidade. Em uma amostra de NiFe=FeMn foram encontrados valores maiores tanto para a anisotropia unidirecional quanto para a histerese rotacional, bem como uma dependência desses com o campo de medida.
36
Silva, Oreci Escobar da. "Estudo do exchange bias em filmes finos de NiFe/FeMn (bicamadas) E NiFe/IrMn (multicamadas)." Universidade Federal de Santa Maria, 2016. http://repositorio.ufsm.br/handle/1/9257.
Анотація:
Fundação de Amparo a Pesquisa no Estado do Rio Grande do SulThe Exchange Bias (EB) phenomenon consists of an interfacial exchange coupling between a ferromagnetic (FM) material and an antiferromagnetic (AFM) one and has as the main characteristics the rise of a unidirectional anisotropy and a rotational hysteresis (HR) in torque curves. In this work we have investigated the behavior of the magnetic behavior of systems presenting EB through magnetization curves, anisotropic magnetorresistance (AMR)and torque magnetometry. We have grown films, via magnetron sputtering, of NiFe/FeMn (bilayers) with different thickness of the FM layer and film structures of NiFe/IrMn/Ta with different repetitions of the basic s tructure. From the X-ray diffraction and reflectivity we have determined the preferential crystallographic growth direction, the thickness and the grain size of the studied samples. It was implemented at the Magnetism and Magnetic Materials Laboratory (LMMM) an apparatus to measure the AMR in two configurations: resistance as a function of the applied field angle and resistance as function of the field applied in a particular angle direction. To adjust the magnetization and AMR data we have used a phenomenological model for EB considering four energy terms: Zeeman, uniaxial, unidirectional and magnetostatic. The AMR curves have shown an asymmetry around 180º when measured at field smaller than the Bias one. According to the used model used, such asymmetry can be caused either by a misalignment between the anisotropy axes (uniaxial and unidirectional) or by a misalignment between the measurement current direction easy magnetization axis of the sample. It was not observed in the multilayers an increase on the uniaxial anisotropy dispersion with increasing on the trilayers number, as expected. In order to fit average torque curves, we have used an equation with two harmonic terms. The resulting parameters allow us to study the field evolution of the harmonic amplitudes Kθ e K2θ. In the multilayer system as higher total energy is, higher is the energy losses by HR. In bilayers system, the HR energy losses still increases even above the saturation field. Such behavior was associated to the configuration of interfacial spins and suggests the need of an additional contribution to total energy of the magnetic system.
O fenômeno de Exchange Bias (EB) consiste de um acoplamento de troca interfacial entre um material ferromagnético (FM) e um material antiferromagnético (AFM) e tem como características principais o surgimento de uma anisotropia unidirecional e a histerese rotacional (HR) em curvas de torque. Neste trabalho investigamos o comportamento de sistemas magnéticos com EB através de curvas de magnetização, magnetorresistência anisotrópica (AMR) e magnetometria de torque. Foram crescidos filmes via magnetron sputtering, de NiFe/FeMn (bicamadas) variando a espessura da camada de NiFe e filmes de NiFe/IrMn/Ta com diferentes números de repetições dessa estrutura básica (multicamadas). A partir da difração e refletividade de raios-X determinamos a direção preferencial de crescimento cristalográfico, espessura e o tamanho de grão das amostras estudadas. Foi implementado no Laboratório de Magnetismo e Materiais Magnéticos (LMMM) um aparato experimental para medidas de AMR: resistência em função do ângulo do campo aplicado e resistência em função do campo aplicado para um determinado ângulo escolhido. Para o ajuste aos dados das curvas de magnetização e AMR é apresentado um modelo fenomenológico para EB considerando quatro termos de energia: Zeeman, uniaxial, unidirecional e magnetostático. As curvas de AMR apresentam uma assimetria em torno de 180º quando medidas em valores de campo menores que o campo de Bias. De acordo com o modelo utilizado, tal assimetria pode ser causada por um desalinhamento entre os eixos de anisotropia (uniaxial e unidirecional) ou por um desalinhamento entre a direção da corrente de medida e o eixo fácil de magnetização da amostra. Não foi observado um aumento na dispersão da anisotropia uniaxial com o aumento das repetições das tricamadas, conforme esperado. Para o ajuste das curvas de torque médio utilizamos uma equação com dois termos harmônicos. Os parâmetros resultantes do ajuste das curvas de torque permitiram estudar a evolução das componentes harmônicas Kθ e K2θ. Nas multicamadas quanto maior a energia associada ao torque médio maiores são as perdas de energia por HR. Já nas bicamadas, as perdas de energia por HR aumentam mesmo acima do campo de saturação. Tal comportamento foi associado à configuração de spins interfaciais e sugere a necessidade de uma contribuição adicional para a energia total do sistema magnético.
37
Gomes, Matheus Gamino. "Efeito hall extraordinário em multicamadas granulares de SiO2/Co/CoO com exchange bias." Universidade Federal de Santa Maria, 2012. http://repositorio.ufsm.br/handle/1/3920.
Анотація:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorGranular magnetic systems can be composed by magnetic particles or clusters with size of some nanometers. These magnetic nanoparticles present different magnetic order phases, as superparamagnetic, and they can be embedded in both, metallic or insulating matrix. These systems present several phenomena such as the giant magnetoresistance (GMR), extraordinary Hall effect (EHE), tunnel magnetoresistance (TMR) and Coulomb blockade. That phenomena use to disappear when a small termal fluctuation is high enough to reverse the magnetization of the clusters leading the lost the magnetic information in a very short time range. When it occurs, the particles are in superparamagnetic phase. In order to maintain the magnetic information at high values of temperature, or even to suppress the superparamagnetic limit, many works have tried to use an antiferromagnetic matrix, to induce the increase of the energy barrier among the two easy directions of magnetization by the exchange coupling in the grain(FM)/matrix(AFM) interfaces, with the purpose of stabilizing the particles magnetization. In this work, we have produced Co granular samples embedded in SiO2/CoO insulating/antifferomagnetic matrix through a sequential deposition by magnetron sputtering, in order to study the exchange bias of Co grains laterally surrounded by CoO.Were perfomed measurements of transmission eletronic microscopy (TEM), x-ray diffraction (XRD), measures low fields thermomagnetics (ZFC-FC), to obtain answers of the structural and magnétic charater of the samples. The Exchange Bias were investigated by extraordinary Hall effect, meauresments in differents temperature values in a cooling field (FC) of 5kOe. These non conventional measurements, are carried out with field cooling and the applied external field to perfomed the loop hysteresis both with perpendicular direction to the substrate. The effective anistropy, changes the magnitude up to three ordes of magnitude for different thicknesses of CoO in the limit T!0. On the other hand, the HEB linearily decreases, and goes to zero at a certain temperature. This temperature, is approximately, equal to that where the split of ZFC-FC curves occurs. This split is associated with the CoO blocking temperature, i.e. the ordering temperature of CoO. These results are reported from the finite size effect present in thin films. For the specific case of CoO, these effects lead to a nonzero net magnetization coming from uncompensed moments present on the surface that, in some cases, are responsible for the magnetic coupling governed by a local disorder and frustration, a spin-glass-like behavior. The extraodinary Hall effect and giant magnetoresistance were study of thin Fe-rich amorphous films and Fe-rich/Cu multilayers. Were investigated and compared the extraordinary Hall effect in these two types of samples and discussed it in terms of thickness and sample structure. The thicker films exhibited a strong in-plane magnetic anisotropy, and by decreasing film thickness both saturated Hall resistivity and Hall sensitivity increase. Electrical conductance increases and Hall resistivity decreases when the films are sandwiched with Cu.
Sistemas granulares magnéticos podem ser formados por grãos ou aglomerados magnéticos cujo tamanho é de alguns nanômetros. Estes grãos magnéticos apresentam diferentes fases de ordenamento magnético, como o superparamagnetismo, e podem estar envolvidos tanto por matrizes metálicas como matrizes isolantes. Estes sistemas possuem uma riqueza de fenômenos, como a magnetorresitência gigante (GMR), Efeito Hall Extraordinário (EHE), magnetorresistência túnel (TMR), bloqueio de coulomb entre outros. Estes fenômenos muitas vezes desaparecem quando à energia térmica for suficiente para inverter a magnetização dos grãos, levando-os a perder informação magnética num intervalo de tempo muito curto. Quando isto ocorre dizemos que os grãos estão na fase superparamagnética. Para reter a informação magnética a temperatura ambiente, ou até mesmo suprimir o superparamagnetismo, tem-se tentado o uso de uma matriz antiferromagnética (AFM) onde os grãos ficam imersos, e o acoplamento de troca na interface grão (FM)/matriz (AFM) pode induzir um aumento na barreira de energia entre as duas direções de fácil magnetização e com isso estabilizar a magnetização dos grãos. Neste trabalho foram produzidas amostras granulares de Co imersos em matriz isolante/antiferromagnética de SiO2/CoO pela deposição alternada do metal e dos isolantes por magnetron sputtering , com a finalidade de estudar a evolução do exchange bias entre os grãos de Co que estão lateralmente envolvidos por CoO. Foram realizadas medidas de Microscopia Eletrônica de Transmissão (TEM), Difração de raios-X (XRD), medidas termomagnéticas a baixos campos (ZFC-FC), para obter respostas quanto ao caráter estrutural e magnético das amostras. O Exchange bias foi investigado por efeito Hall extraordinário, medidas à diferentes temperaturas sob um campo de resfriamento (FC) de 5kOe. Estas medidas, diferente do convencional, são realizadas com o campo de resfriamento e o campo externo aplicado para realizar a curva de magnetização ambas na direção perpendicular ao substrato. A anisotropia efetiva, varia, na sua magnitude, até três ordens de grandeza para diferentes espessuras de CoO para o limite T !0. Por outro lado, o HEB decresce linearmente, até ir à zero numa dada temperatura. Esta temperatura onde extingue-se o HEB é, aproxiamdamente, a mesma onde ocorre à separação das curvas ZFC-FC. Esta separação nas curvas está associado com a temperatura de bloqueio de CoO, ou seja, temperatura de ordenamento dos grãos de CoO responsáveis pelo acoplamento direto com os grãos de Co. Estes resultados reportados são oriúndos dos efeitos de tamanho de grão, presentes em filmes muitos finos. Para o caso específico do CoO, estes efeitos levam há presença de uma magnetização diferente de zero oriúndos dos momentos não compensados presentes na superfície que, para alguns casos, são responsáveis pelo acoplamento magnético governado por uma desordem local e frustação, um compotamento do tipo spin-glass-like . Efeito Hall extraordinário e magnetorresistência gigante foram estudados em filmes finos amorfos de FINEMET e multicamdas FINEMET/Cu. Foi investigado e comparado o efetio Hall nos dois tipos de amostras, e discutido em termos da espessura e estrutura da amostra. Para os filmes mais espessos foi observado uma forte anisotropia no plano do filme, por outro lado, quando a espessura diminui ambos, a resistividade Hall satura e a sensibilidade Hall aumenta considerável. A condutividade elétrica aumenta e a resistividade Hall diminui para as multicamadas FINEMET/Cu.
38
Khanal, Shankar. "Study of Static and Dynamic Properties of Magnetic Nanostructures." ScholarWorks@UNO, 2017. http://scholarworks.uno.edu/td/2382.
Анотація:
Magnetic materials are one of the most interesting and promising class of materials for technological applications [1]. Among them, patterned ferromagnetic systems have an important role especially in the prospect of high density data storage [2], domain wall logic devices [3] and magnetic memory [4, 5]. Coupled systems of ferromagnetic and antiferromagnetic materials have been implemented to design sensors such as giant magnetoresistance (GMR) [6-8] and tunnel magnetoresistance (TMR) [9, 10]. Ferromagnetic nanoparticles have been used for the drug targeting, cancer therapy, MRI and many more applications [11, 12]. In addition, more recently, significant attention has been paid to explore the dynamic properties of magnetic materials in the GHz range and use them for technological applications such as microwave filters, signal processing, phase shifter, nonreciprocal microwave devices, spin wave guide, high frequency memory, logic elements [13-19]
Boundary conditions, interactions between individual entities, and lateral confinement of magnetic charges generate diverse magnetic properties especially at nanoscale length [20, 21]. The variation of magnetic properties are even quite different when the size of the magnetic structure is smaller or comparable with the magnetic characteristic length such as mean free path of electron, width of domain wall and even the spin diffusion length [22-24].
In this study, we have considered different magnetic systems. Firstly, the multilayer of coupled ferromagnetic and antiferromagnetic system has been considered to evaluate the exchange bias anisotropy. [FeNi/IrMn]n multilayer systems with different thicknesses of ferromagnetic layer were studied. Static and dynamic properties were revealed through magnetometry measurements (VSM) and VNA-FMR techniques respectively. Angular variation of first order reversal curve (AFORC) and ferromagnetic resonance (AFMR) were performed to learn the intrinsic exchange bias distribution. Secondly, patterned magnetic structures were synthesized to understand the magnetization dynamics in confined geometry. Surface modulated thin films with different periodicity, dumbbell-shaped structures with variable size and three dimensional magnonic crystals have been studied using both static and dynamic measurement techniques. Micromagnetic simulations were performed to understand and explain the experimental results.
39
Radu, Florin. "Fundamental aspects of exchange bias effect in AF/F bilayers and multilayers." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=97610802X.
40
Alonso, Leonardo. "Modelo de Preisach e análise FORC aplicados a filmes com exchange-bias." Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-29082008-103327/.
Анотація:
Neste trabalho foram produzidos filmes finos de Si[100]/buffer/NiFe/FeMn/Ta (buffer = Cu; Ta) e válvulas de spin de Si[100]/Cu/NiFe/Cu/NiFe/FeMn/Ta via Sputtering, e seus comportamentos magnéticos foram estudados analisando curvas de inversão de primeira ordem (First Order Reversal Curves, FORCs). Um Modelo de Preisach com Exchange-Bias foi desenvolvido e sua implementação computacional permitiu simular a histerese e as curvas forcas das bicamadas de NiFe/FeMn. O modelo também permitiu reproduzir os comportamentos assimétricos dos dois ramos da curva de histerese destes filmes. A análise dos resultados evidenciou os efeitos de campo médio sobre a camada de Py em função da espessura da camada de FeMn. A aplicação da análise FORC nas válvulas de spin se revelou interessante para estudar o grau de acoplamento entre as camadas livre e pressa, assim como os efeitos de campo médio em cada camada.In this work, thin films of Si[100]/buffer/NiFe/FeMn/Ta (buffer = Cu; Ta) and spin valves of Si[100]/Cu/NiFe/Cu/NiFe/FeMn/Ta were produced via sputtering and their magnetic behavior were studied by First Order Reversal Curves (FORC\'s) analysis. A Preisach Model with Exchange-Bias was developed and applied in order to simulate the hysteresis and the first order reversal curves of the NiFe/FeMn bilayers. In addition, the model allowed to reproduce the asymmetric behaviors present in both branches of the hysteresis curve in these films. The study put in evidence the mean field effects on de Py layer as a function of the thickness of the FeMn layer. The application of the FORC analysis in the spin valves was interesting to study the degree of coupling between the free and the pressed layers, as well as the effect of mean field in each layer.
41
Schletter, Herbert. "Präparation und Charakterisierung nanostrukturierter Magnetwerkstoffe unter besonderer Berücksichtigung des Exchange Bias Effekts." Doctoral thesis, Universitätsbibliothek Chemnitz, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-135202.
Анотація:
Der Einsatz nanostrukturierter Magnetmaterialien als Speicherschichten in Festplatten stellt ein vielversprechendes Konzept zur weiteren Erhöhung der erreichbaren Speicherdichten im Vergleich zu den heute eingesetzten granularen Medien dar. Für die Realisierung dieses Konzeptes ist eine detaillierte Kenntnis der Struktureigenschaften und deren Einfluss auf das magnetische Verhalten der einzusetzenden Schichten erforderlich. Für die vorliegende Arbeit wurden drei verschiedene magnetische Materialien ausgewählt und insbesondere mit elektronenmikroskopischen Methoden in struktureller Hinsicht untersucht. Dazu zählen ferromagnetische (FePt)(100-x)Cu(x) -Schichten, ferromagnetische [Co/Pt]n -Multilagen sowie ferrimagnetische Fe(100-x)Tb(x) -Schichten.
Der Schwerpunkt der Untersuchungen lag dabei auf der Korrelation zwischen strukturellen und magnetischen Eigenschaften sowie im Einfluss der Nanostrukturierung auf das magnetische Verhalten der Schichten. In dieser Hinsicht wurden Aspekte der durch die Struktur bedingten magnetischen Anisotropie in Form von magnetokristalliner und Grenzflächenanisotropie betrachtet. Zudem wurde das Kopplungsverhalten zwischen einzelnen Strukturelementen in nanostrukturierten Schichten untersucht.
Aufbauend auf die Untersuchung der drei genannten Materialien wurden [Co/Pt]n und Fe(100-x)Tb(x) ausgewählt zum Aufbau eines Systems mit zwei magnetischen Komponenten: Fe(80)Tb(20) / [Co/Pt]10. Die Untersuchungen konzentrierten sich dabei auf die Morphologie der Grenzfläche zwischen den beiden Bestandteilen und deren Einfluss auf den Exchange Bias, der in diesem System vorliegt.
42
Franceschin, Giulia. "Magnetic Oxides-based Hetero-Nanostructured Ceramics : from Nanomaterial Engineering to Exchange-bias Coupling." Thesis, Sorbonne Paris Cité, 2017. http://www.theses.fr/2017USPCC281.
Анотація:
Récemment, les recherches scientifiques ont attiré son attention sur les domaines liés à l'énergie en raison de la consommation croissante d'énergie qui a affecté les dernières décennies. Les matériaux magnétiques sont déterminants dans les applications basées sur l'énergie et l'amélioration de leurs performances joue un rôle primordial dans le développement technologique. Le présent travail explore la possibilité de préparer des composites céramiques magnétiques hétéro-nano-structurés à base de constituants d'oxydes. Un oxyde ferromagnétique (F) a été couplé à un antiferromagnétique (AF) à l'échelle nanométrique pour étudier les propriétés magnétiques résultantes, en accordant une importance primordiale au couplage d'échange entre les deux phases. L’établissement de l’effet de biais d’échange à l’interface des phases F-AF est souhaitable pour augmenter l’anisotropie magnéto-cristalline du système et le produit énergétique relatif BHmax. Dans ce but, deux oxydes F différents ont été pris en compte, le Fe3O4 et le CoFe2O4, et trois oxydes différents de la AF, CoO, NiO et l'hématite α-Fe2O3. Des nanoparticules d'oxyde de chaque composant ont été préparées par synthèse de polyol, avec une bonne qualité cristalline et une morphologie uniforme. Ils ont ensuite été utilisés pour préparer des échantillons de céramique de consolidation par la technique SPS. Pour chaque échantillon, un oxyde F a été mélangé à l'un des oxydes AF. Les céramiques résultantes ont été formées selon différents rapports de masse F / AF, variant entre 0,75 / 0,25, 0,5 / 0,5 et 0,25 / 0,75, et selon différentes combinaisons entre les oxydes F et AF considérés. Tous les échantillons ont été frittés à 500 ° C et 100 MPa pendant 5 minutes. Toutes les céramiques ont été étudiées en profondeur, notamment en ce qui concerne leur structure, leur microstructure et leurs propriétés magnétiques. L’analyse HR-TEM réalisée sur des lames raffinées de céramiques Fe3O4-CoO, Fe3O4-NiO et CoFe2O4-NiO, ainsi que des résultats de diffraction XR, a mis en évidence une importante variation de la composition des échantillons après frittage. Une nouvelle phase métallique est formée après frittage dans l'atmosphère fortement réductrice au cours du processus SPS, modifiant ainsi la composition relative des phases F et AF individuelles. L’établissement d’effets de biais d’échange n’a guère été observé à cause de la diffusion des atomes qui affecte l’échantillon. En effet, les nanoparticules AF d'hématite se sont révélées instables dans une large plage de températures et donc inappropriées pour ce type d'application. En particulier, une transformation de phase se produisant à environ 380 ° C a été observée lorsqu'un champ magnétique externe est appliqué. Une telle transition a été étudiée au moyen d’une caractérisation par magnétomètre, d’une analyse HR-TEM et d’une analyse EELS et a révélé qu’elle impliquait la transformation de l’hématite en magnétite. Le mécanisme de cette transformation n’a pas encore été compris et fait l’objet d’une enquête plus approfondieRecently the scientific research led its attention towards energy related fields because of the increasing energy consumption that affected the last few decades. Magnetic materials are determining in energy-based applications and the enhancement of their performances has a primary role on the technological development. The present work explores the possibility to prepare hetero-nano-structured magnetic ceramic composites based on oxide constituents. A ferromagnetic oxide (F) was coupled with an antiferromagnetic one (AF) at a nanometric size scale to study the resulting magnetic properties, above all concerting the exchange coupling between the two different phases. The establishing of the exchange-bias effect at the F-AF phases interface is desirable in order to increase the magneto-crystalline anisotropy of the system and the relative energy product BHmax. At this aim, two different F oxides were took into account, the Fe3O4 and the CoFe2O4, and three different AF oxides, CoO, NiO and hematite α-Fe2O3. Oxide nanoparticles of each component were prepared by polyol synthesis, with a good crystalline quality and uniform morphology. They were then employed to prepare consolidate ceramic samples by SPS technique. For each sample, one F oxide was mixed with one of the AF oxides. The resulting ceramics were formed by different F/AF mass ratio, varying between 0,75/0,25, 0,5/0,5 and 0,25/0,75, and by different combinations between the considered F and AF oxides. All the samples were sintered at 500°C and 100 MPa for 5 minutes. All the ceramics were deeply studied, above all concerning their structure, microstructure and magnetic properties. HR-TEM analysis performed on FIB-refined slides of the Fe3O4-CoO, Fe3O4-NiO and CoFe2O4-NiO ceramic samples, together with XRD results, highlighted an important variation of samples’ composition after sintering. A new metallic phase is formed after sintering cause to the strongly reductive atmosphere during the SPS process, thus modifying the relative composition of the single F and AF phases too. The establishing of exchange-bias effects was hardly observed exactly because of the atoms diffusion that affects the sample. The hematite AF nanoparticles, indeed, were found to be unstable in a wide temperature range and thus unsuitable for this kind of application. In particular, a phase transformation occurring at about 380°C was observed when an external magnetic field is applied. Such a transition was studied by mean of magnetometer characterisation, HR-TEM and EELS analysis and was found to involve hematite transforming into magnetite. The mechanism of such transformation hasn’t been understood yet and is under further investigation
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Silva, George Frederick Tavares da. "MÃtodo de Monte Carlo para o estudo de Sistemas MagnÃticos Bidimensionais." Universidade Federal do CearÃ, 2008. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=2465.
Анотація:
Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgicoEmploying the Monte Carlo Method, studied is two-dimensional magnetic systems. The phase transition that occurs in Ising model was also investigated for the two-dimensional XY model, or also called 2D XY in the literature. There is this model an unusual transition type Kosterlitz-Thouless-Berezinskii, involving vortices of spins which unbinding in a temperature T_{KT}. However, you can see the transition without the presence of vortices for small systems as was done in this work. The implementation of the method cited in the two models will be detailed and then used in the study of magnetic bylayers systems characterized by the existence of planning ferromagnetic (FM) and antiferromagnetic (AFM). They will be then reproduced through simulations, curves of magnetization in relation to the magnetic field, with the goal of find the conditions in which the phenomenon known as Exchange Bias occurs. This is because the existence of an interface FM-AFM, which will be the focus of research.
Empregando o MÃtodo de Monte Carlo, estudamos sistemas magnÃticos bidimensionais. A transiÃÃo de fase, que ocorre no modelo de Ising, foi tambÃm investigada para o modelo XY bidimensional, ou geralmente chamado na literatura de 2D XY. Observa-se neste modelo uma transiÃÃo nÃo-usual do tipo Kosterlitz-Thouless-Berezinskii, envolvendo vÃrtices de spins que se descorrelacionam em uma temperatura T_{KT}. Contudo, pode-se observar a transiÃÃo sem a presenÃa de vÃrtices para sistemas pequenos como foi feito neste trabalho. A implementaÃÃo do mÃtodo nos dois modelos citados serà detalhada e posteriormente utilizada no estudo de sistemas com bicamadas magnÃticas caracterizados pela existÃncia de ordenamento ferromagnÃtico (FM) e antiferromagnÃtico (AFM). SerÃo entÃo reproduzidas, atravÃs das simulaÃÃes, curvas de magnetizaÃÃo em relaÃÃo ao campo magnÃtico com o objetivo de encontrar as condiÃÃes nas quais o fenÃmeno conhecido como exchange bias ocorre. Este deve-se a existÃncia de uma interface FM-AFM, que serà o foco nas investigaÃÃes.
44
Smith, Justin Dewitt. "Studies to improve estimation of the electromagnetic bias in radar altimetry /." Diss., CLICK HERE for online access, 1999. http://contentdm.lib.byu.edu/ETD/image/etd17.pdf.
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Silva, Andr? Stuwart Wayland Torres. "Propriedades magn?ticas de nanopart?culas de ferro em substratos antiferromagn?ticos." Universidade Federal do Rio Grande do Norte, 2007. http://repositorio.ufrn.br:8080/jspui/handle/123456789/16529.
Анотація:
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Previous issue date: 2007-12-07Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
The e?ect of ?nite size on the magnetic properties of ferromagnetic particles systems is a recurrent subject. One of the aspects wide investigated is the superparamagnetic limit where the temperature destroys the magnetic order of ferromagnetic small particles. Above the block temperature the thermal value of the magnetic moment of the particle vanishes, due to thermal ?uctuations. The value of the blocking temperature diminishes when the size of the particle is reduced, re?ecting the reduction of the anisotropy energy barrier between the uniform states along the uniaxial axis. The increasing demand for high density magnetic media has recently attracted great research interest in periodic arrangements of nanometric ferromagnetics particles, approach in the superparamagnetic limit. An interesting conjecture is the possibility of stabilization of the magnetic order of small ferromagnetic particles (F) by interface coupling with antiferromagnetic (AF) substrate. These F/AF systems may also help to elucidate some details of the e?ect of exchange bias, because the e?ect of interface roughness and the paper of domain walls, either in the substrate or the particle, are signi?cantly reduced. We investigate the magnetic phases of small ferromagnetic particles on a antiferromagnetic substrate. We use a self-consistent local ?eld method, incorporating the interface ?eld and the dipole interaction between the spins of the ferromagnetic particle. Our results indicate that increasing the area of the interface favors the formation of the uniform state. Howere above a critical height value appears a state non-uniform is formed where the spins of in the particle s free surface are rotated with respect to the interface spins direction. We discuss the impact of the competition between the dipolar and interface ?eld on the magnetic charge, that controls the ?eld of ?ux leakage of the particle, and on the format of the hysteresis curves. Our results indicate that the liquid magnetic charge is not a monotonically increasing function of the height of the particle. The exchange bias may display anomalous features, induced for the dipolar ?eld of the spins near the F/AF interface
O efeito de tamanho ?nito nas propriedades magn?ticas de sistemas de part?culas ferromagn?ticos ? um tema recorrente. Um dos aspectos largamente investigados ? o limite superparamagn?tico em que a temperatura desestabiliza a ordem magn?tica de part?culas ferromagn?ticas pequenas. Acima da temperatura de bloqueio o valor m?dio t?rmico do momento magn?tico da part?cula se torna nulo, devido a ?utua??es t?rmicas. A temperatura de bloqueio diminui quando o tamanho da part?cula ? reduzido, re?etindo a queda na barreira de energia de anisotropia entre os estados de magnetiza??o uniforme ao longo do eixo uniaxial. A demanda crescente por meios de grava??o magn?tica de alta densidade tem atualmente despertado grande interesse de pesquisa em arranjos peri?dicos de part?culas ferromagn?ticas de dimens?es nanom?tricas, beirando o limite superparamagn?tico. Uma conjectura interessante, levantada experimentalmente, se refere ? possibilidade de estabiliza??o da ordem magn?tica de part?culas ferromagn?ticos (F) via o acoplamento de interface com substratos antiferromagn?ticos (AF) com temperatura de N?el acima da temperatura ambiente. Esses sistemas se prestam igualmente para elucidar alguns detalhes do efeito de Exchange bias , dado que, o efeito de rugosidade de interface e o papel de paredes de dom?nio, seja no substrato ou na part?cula, s?o reduzidos. Investigamos as fases magn?ticas de uma pequena part?cula ferromagn?tica, com diversas geometrias e disposi??es de deposi??o sobre um substrato antiferromagn?tico. Usamos um m?todo de campo local auto-consistente, incorporando o campo de interface e a intera??o dipolar entre os spins da part?cula ferromagn?tica. Nossos resultados indicam que aumentando a ?rea da interface favorece a forma??o do estado uniforme. Por?m se a altura da part?cula ? superior a um valor cr?tico surge um estado n?o-uniforme em que os spins se orientam de modo a minimizar a carga magn?tica super?cial. Discutimos o impacto que a competi??o entre esses efeitos tem na carga magn?tica l?quida, que controla o campo de fuga da part?cula e no formato das curvas de histerese. Nossos resultados indicam que a carga l?quida n?o ? uma fun??o monot?nicamente crescente da altura da part?cula e que o deslocamento da histerese pode sofrer altera??es relevantes, induzidas pelo campo dipolar, fugindo do padr?o esperado normalmente em sistemas F/AF extensos
46
Silva, Franciscarlos Gomes da. "Propriedades de exchange bias e congelamento de spins de superfície em nanopartículas magnéticas." reponame:Repositório Institucional da UnB, 2009. http://repositorio.unb.br/handle/10482/4667.
Анотація:
Dissertação (mestrado)—Universidade de Brasília, Instituto de Física, 2009.Submitted by Allan Wanick Motta (allan_wanick@hotmail.com) on 2010-03-31T15:29:51Z No. of bitstreams: 1 2009_FranciscarlosGomesdaSilva.pdf: 3878222 bytes, checksum: 0b8e3cd7417456ca6f19d72a46db932c (MD5)
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Nós apresentamos aqui medidas de magnetização realizadas a baixas temperaturas em nanocolóides magnéticos baseados em partículas de ferrita de manganês de tamanho 3.3 nm. Este nanocolóide foi obtido por uma estratégia química núcleo/superfície desenvolvida para a síntese de ferrofluidos. A dependência da temperatura das medidas de magnetização nos permitiu separar duas contribuições. Uma associada ao núcleo bem ordenado, com uma dependência da temperatura que segue a lei efetiva de Block e outra relacionada a superfície de spins congelados em uma estrutura desordenada como a de um vidro de spin. Após resfriar a amostra em alto campo observou-se um deslocamento no ciclo de histerese, o que indica a existência de um acoplamento entre o núcleo ferrimagnético ordenado e a superfície de camada desordenada. O deslocamento no ciclo de histerese nos permite avaliar o campo de exchange, determinado pelo deslocamento do campo da origem e está aumentando com o campo aplicado usado no processo de resfriamento. Diferentemente, medidas de magnetização na dependência da temperatura para diferentes magnitudes de campos aplicados indicam uma contribuição adicional dos spins desordenados para a magnetização em baixas temperaturas. Além disso, foi possível determinar para cada magnitude de campo aplicado uma temperatura de congelamento, na qual a superfície é congelada em uma estrutura como a de um vidro de spins. A variação da temperatura de congelamento Tf com o campo aplicado, indica neste caso um aumento no acoplamento núcleo/superfície com o campo aplicado _________________________________________________________________________________________ ABSTRACT
We present here magnetization measurements performed at low temperatures on magnetic nanocolloids based on 3.3 nm sized particles of manganese ferrite. This nanocolloid was obtained by a chemical core/shell strategy developed for the synthesis of ferro uids. Temperature dependence of the magnetization measurements allowed us to separate two contributions. One associated to the well ordered core, with a temperature dependence which follows an eective Bloch law and the other related to surface spins frozen in a disordered spin glass like structure. After cooling the sample in a high eld we observer a shift on the hysteresis loops, which indicates the existence of a coupling between the ferrimagnetic ordered core and the disordered surface layer. The shift on the hysteresis loop allow us to evaluated the exchange bias eld, determined from the eld oset from the origin, and is increasing as the applied eld used in cooling process. Otherwise measurements of temperature dependence of magnetization for different magnitudes of applied field indicate an additional contribution to the magnetization of the disordered spins to low temperatures. Moreover it was possible to determine for each applied field magnitude a freezing temperature, in which the surface is frozen in a spin glass like structure. The variation of the freezing temperature Tf with the applied field, indicates in this case an increasing in the coupling core/shell with the applied field.
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Dias, Thiago. "Controle de fase do Exchange Bias em filmes de Co/IrMn/Cu/Co." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2010. http://hdl.handle.net/10183/26496.
Анотація:
Tratamentos térmicos e irradiações com íons He+ foram utilizados para controle do Exchange Bias, EB, em filmes finos de Co/IrMn/Cu/Co. Enquanto a direção do campo magnético aplicado durante o tratamento foi variada, as irradiações foram realizadas com campo magnético aplicado em uma direção que forma um ângulo de 100º com a direção do EB. Os resultados dos tratamentos térmicos realizados com o campo de tratamento não-paralelo `a direção do EB apresentaram evolução de uma estrutura negativamente/negativamente deslocada para uma negativamente/ positivamente deslocada com subciclos defasados em 180º nas amostras em que o espaçador de Cu não foi suficientemente espesso para que não houvesse contato atômico entre as camadas ferromagnética (Co) e antiferromagnética (IrMn). Mesmo com campo magnético baixo, o tratamento térmico com campo aplicado na direção do EB resultou, para a amostra sem espaçador não-magnético, em curvas de histerese com os subciclos indistinguíveis. Os filmes tratados sob as mesmas condições `a exceção do campo aplicado fora da direção do EB apresentaram as fases magnéticas referentes as duas camadas de Co defasadas. Irradiações iônicas, realizadas sob as condições utilizadas neste trabalho, mostraram-se muito menos efetivas para o controle do efeito. Embora os tratamentos resultassem em um decréscimo do deslocamento do subciclo referente `a camada de Co depositada sob a de IrMn, os espectros não mostraram modificações significativas nos padrões de difração e/ou refletividade. Este resultado demonstra que o supracitado decréscimo não está relacionado com a interdifusão entre as camadas, nem com mudanças cristalográficas durante o tratamento. Este trabalho propõe um modelo intuitivo para interpretação do supracitado decréscimo do deslocamento.Magnetic thermal treatment and irradiation with He+ ions were used to control the Exchange Bias, EB, in thin Co/IrMn/Cu/Co films. While the direction of the annealing field was varied, the irradiation was performed with magnetic field applied always at a direction that forms 100 with the EB direction. Treatments done with annealing field non-parallel to the EB direction allowed the whole hysteresis loop to be tuned from a double negatively/negatively shifted to a double negatively/positively shifted one with the shifts of the subloops in antiphase for the samples where the Cu spacer is not sufficiently thick to completely separate the ferromagnetic (Co) and antiferromagnetic (IrMn) layers. Even using a low magnetic field, thermal treatment with field applied along the EB direction led, for the sample without a non-magnetic spacer, to hysteresis loops with practically undistinguishable subloops. The sample, annealed using the same conditions but field applied off the EB direction, showed the magnetic phases of the two Co layers out-of-phase. Ion irradiation, performed at the conditions used in the present work, showed itself less-effective for such a control of the effect. Although the magnetic annealing led to a decrease of the shift of the subloop corresponding to the Co layer deposited before the IrMn one, the X-ray spectra did not indicate significant modifications of the diffraction and/or reflectivity patterns. This shows that the above decrease is neither related to interdiffusion between layers nor to crystallographic changes caused by the thermal treatments. This work proposes an intuitive model for the interpretation of the aforementioned shift decrease.
48
Biternas, Andreas. "Study of the training effect in exchange bias using the domain state model." Thesis, University of York, 2009. http://etheses.whiterose.ac.uk/847/.
Анотація:
The most common product of the storage industry is the well known hard disk, based on the magnetic recording and reading of data. The research of magnetic materials is enhanced by the establishment of hard disks as a recording media. Given the current trend toward nanostructured materials and complex materials design, the understanding of the origins of magnetic phenomenology at the atomistic level has become necessary. In this thesis, we focus on the aging effects of magnetic materials. The systems that are investigated are bilayer thin films comprised of two materials with different magnetic order. The so called exchange bias systems are comprised of a ferromagnetic layer in contact with an antiferromagetic layer. Exchange bias systems are part of the current read-heads, which utilise the phenomenon of Giant Magnetoresistance discovered by A. Fert and P. Grünberg, who were awarded a Nobel prize in 2007 for this discovery. The most wellknown characteristic of the exchange bias systems is the shift of the hysteresis loop along the horizontal axis. The reduction of this shift with consecutive hysteresis cycles is called the training effect. The current research focused on the dependence of the training effect on various exchange bias system parameters using the well established domain state model. A novel analysis was developed for the study of the antiferromagnet during consecutive hysteresis loops. A special focus was given to the response of training effect on temperature. Several characteristics of the system were varied to investigate the physics of the training effect, such as the antiferromagnetic thickness and dilution with non-magnetic defects. The reversal modes of the ferromagnet were also investigated varying the anisotropies of the system as well the angle of the magnetic field. New characteristics were added to the domain state model increasing the realism of this model. The interface roughness was introduced in the model, as more representative of realistic exchange bias systems. Furthermore, different crystallographic structures such as body-centered cubic and hexagonal-close packed, were investigated as in these structures the coupling between the ferromagnet and the antiferromagnet increases. In these structures, in addition to the interface roughness, the enhance coupling is shown to give rise to complex trends of the exchange bias and the training effect.
49
Gupta, P. "Study of negative magnetization, exchange bias and magnetization switching in rare earth chromites." Thesis(Ph.D.), CSIR-National Chemical Laboratory, Pune, 2015. http://dspace.ncl.res.in:8080/xmlui/handle/20.500.12252/2042.
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Zocca, Andrea. "Produzione e funzionamento di un dispositivo spin valve." Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7699/.
Анотація:
Il fenomeno della magnetoresistenza gigante (GMR) consiste nella marcata variazione della resistenza elettrica di una struttura in forma di film sottile, composta da un’alternanza di strati metallici ferromagnetici (FM) e non magnetici (NM), per effetto di un campo magnetico esterno. Esso è alla base di un gran numero di sensori e dispositivi magnetoelettronici (come ad esempio magnetiche ad accesso casuale, MRAM, ad alta densità) ed ulteriori innovazioni tecnologiche sono in via di elaborazione. Particolarmente rilevanti sono diventate le Spin Valve, dispositivi composti da due strati FM separati da uno spaziatore NM, metallico. Uno dei due film FM (free layer) è magneticamente più soffice rispetto all’altro (reference layer), la cui magnetizzazione è fissata mediante accoppiamento di scambio all’interfaccia con uno strato antiferromagnetico (AFM) adiacente. Tale accoppiamento causa l’insorgenza di una anisotropia magnetica unidirezionale (anisotropia di scambio) per lo strato FM, che si manifesta in uno shift orizzontale del ciclo di isteresi ad esso associato (effetto di exchange bias), solitamente accompagnato anche da un aumento del campo coercitivo.
Questo lavoro di tesi riporta la deposizione e la caratterizzazione magnetica e magnetoresistiva di due valvole spin, una a struttura top (SVT) composta da strati di Si/Cu[5 nm]/Py[5 nm]/Cu[5 nm]/Py[5 nm]/IrMn[10 nm], ed una a struttura bottom (SVB), di composizione Si/Cu[5 nm]/IrMn[10 nm]/Py[5 nm]/Cu[5 nm]/Py[5 nm], allo scopo di verificare il comportamento magnetoresistivo gigante del dispositivo per questa particolare scelta dei materiali. I campioni sono stati depositati mediante DC Magnetron sputtering, e caratterizzati magneticamente mediante magnetometro SQUID; la caratterizzazione resistiva è stata eseguita tramite metodo di van der Pawn.
Vengono infine presentati i risultati sperimentali, in cui si osserva una variazione di magnetoresistenza nei campioni nell’ordine del punto percentuale.