Dissertations / Theses on the topic 'Antiferromagnetism'
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Robinson, Matthew D. "An Information Theoretic Study of the Ising Antiferromagnet with Quenched Dilution on a Triangular Lattice." Fogler Library, University of Maine, 2003. http://www.library.umaine.edu/theses/pdf/RobinsonMD2003.pdf.
Full textNeves, Herbert Rodrigo. "Nanopartículas antiferromagnéticas de MnO para aplicações em biomedicina como agentes de contraste." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/75/75134/tde-17042012-165853/.
Full textNanomaterials have been widely studied as a result of their interesting physical and chemical properties, which offer a large number of possibilities for applications in biomedicine mainly in cancer therapy and the development of strategies for non-invasive diagnosis. The superparamagnetic iron oxide nanoparticles (SPION) is the main studied material as contrast agent for magnetic resonance imaging (MRI) due to its ability to reduce the transverse relaxation time (T2) in different tissues and lower toxicity than Gd3+ and Mn2+ complexes currently used. However, this SPIONs accumulation can be confused with signals from calcification, bleeding or metal deposits, and the high magnetic susceptibility distorts the background image because its ferromagnetic behavior. Some aspects are desirable to replace SPIONs, such as nanoparticulate form for simple surface modification and labeling with targeting agents, and positive longitudinal T1 relaxation time contrast ability. The antiferromagnetic MnO NPs attend all these requirements and overcome the drawback of using SPION. In our study, MnO NPs were synthesized by the thermal decomposition of Mn(II) acetylacetonate by a variation of the modified polyol process resulting in spherical nanoparticles with average size of 21 ± 3,9 nm. The ligand-exchange step was used to replace the oleic acid adsorbed on the as-synthesized NPs surface by 3-aminopropyltriethoxysilane (APTMS) and the total free amine groups on the NPs surface was determined. After that, a biocompatible and water-dispersible core/shell structure was obtained by coating with carboxymethyl dextran (CMDex) using the free amine-terminal group from APTMS and the carboxylate groups present in the CMDex molecules conjungation. Surface potential and colloidal stability of these functionalized NPs were evaluated by electrophoretic mobility and dynamic light scattering techniques in both water and artificial blood by using the Simulated Body Fluid (SBF) medium. While the water-dispersible NPs have shown toxicity in the human cell line derived from cervical cancer (HeLa), they have not shown significantly cytotoxicity in the healthy fibroblast cells (cell line L929). Both the as-synthesized and coated NPs present controlled size and shape and the final NPs size distribution and magnetic properties are compatible with the expected for biomedical applications.
Ishida, Lina. "Antiferromagnetismo nos titanatos geometricamente frustrados Sm2Ti2O7 e Nd2Ti2O7." Universidade de São Paulo, 2019. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-01022019-104912/.
Full textRecently the scientific community has shown interest for titanium lanthanides, with the titanates Sm2Ti2O7 and Nd2Ti2O7 being poorly approached, specially at low temperatures. Crystallographic analysis were performed on the single crystal samples of Sm2Ti2O7 and Nd2Ti2O7, which allowed us to confirm its expected cubic and monoclinic structures, respectively; Magnetic and calorimetric data displayed the transition temperature, not previously seen in case of Nd2Ti2O7, of T=0.35K and T=0.62K, respectively, and the antiferromagnetic behaviour of both samples; Anisotropy was observed at high temperatures on the Sm2Ti2O7 data and along all the temperature range studied for the Nd2Ti2O7 sample. Finally, we have identified the entropy of Ising spins on both samples.
Rosa, Priscila Ferrari Silveira 1988. "Physical properties of FeAs-based intermetallic compounds = Propriedades físicas de compostos intermetálicos à base de FeAs." [s.n.], 2013. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277008.
Full textTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: O entendimento microscópico da intrincada relação entre magnetismo e supercondutividade não-convencional e atualmente um dos grandes problemas em aberto em física da matéria condensada. Em especial, compostos com estrutura cristalina tetragonal parecem favorecer a emergência de tal fenômeno. Os compostos intermetálicos tetragonais BaFe2As2 e EuFe2As2 cristalizam na estrutura tetragonal ThCr2Si2 (I 4/mmm) com camadas de FeAs separadas por átomos de bario/europio. Ambos os compostos apresentam uma distorção estrutural (tetragonal para ortorrômbica) acompanhada for uma transição de fase magnética do tipo onda de densidade de spin (ou SDW, na sigla em inglês) em TSDW = 140 K e TSDW = 190 K, respectivamente. E notável que esta fase magnética pode ser suprimida em direção ao estado supercondutor através de substituição quica ou pressão aplicada. Neste trabalho, apresentamos o estudo sistemático das propriedades do composto intermetálico tetragonal BaFe2As2 em função de tr^es parâmetros: substituição de Eu no sitio cristalográfico do Ba; substituição de metais de transição TM = Mn, Co, Ni, Cu e Ru no sitio cristalográfico do Fe e/ou pressão hidrostática. Para tal proposito, primeiramente sintetizamos amostras de alta qualidade através do método alternativo de fluxo metálico de In. A caracterização macroscópica dos compostos foi realizada através das técnicas experimentais de medidas de susceptibilidade magnética, calor especifico, resistividade elétrica em pressão ambiente e sob pressão hidrostática, além da caraterização estrutural através da difração de pó de raios-X. No que diz respeito a caracterização microscópica, a investigação experimental foi realizada através da técnica de ressonância de spin eletrônico (RSE) utilizando como provas os ons paramagnéticos de Eu2+ e Mn2+/Cu2+, além do estudo de absorção de raios-X (EXAFS) tanto na borda K do As quanto na borda do Fe. Dessa forma, foi possível investigar a dinâmica de spins no plano e fora do plano de FeAs e sua relação com as distorções locais do material. Nossos resultados evidenciam que a diminuição da distância Fe-As esta intimamente ligada a supressão da fase SDW e a localização das bandas 3d do Fe no plano Fe-As. O aumento do caráter orbital planar xy/x2 - y2 na superfície de Fermi parece ser um ingrediente propício para a emergência da supercondutividade nessa classe de compostos
Abstract: The microscopic understanding of the intricate interplay between magnetism and unconventional superconductivity is currently one of the great open questions in condensed matter physics. In particular, compounds with a tetragonal crystal structure seems to be favorable to the emergence of such phenomena. The intermetallic compounds BaFe2As2 and EuFe2As2 crystallize in the tetragonal ThCr2Si2-type structure (I 4/mmm) with FeAs sheets separated by barium/europium layers. Both compounds exhibit a structural distortion accompanied by a magnetic spin-density wave (SDW) phase transition at TSDW = 140 K and TSDW = 190 K, respectively. Remarkably, this SDW phase can be tuned toward a superconducting state by substitution and applied pressure. In this thesis, we will present a systematic study of the intermetallic tetragonal compound BaFe2As2 as a function of three parameters: Eu substitution in the Ba crystallographic site, transition metal (TM) substitution (TM = Mn, Co, Ni, Cu, and Ru) in the Fe site, and/or applied hydrostatic pressure. For this purpose, we have grown high-quality single crystals by the alternative In-ux method. The macroscopic characterization has been made by measurements of magnetic susceptibility, specic heat and electrical resistivity at ambient pressure and under hydrostatic pressure. Concerning the microscopic investigation, the experimental approach consists in using electron spin resonance (ESR) technique employing paramagnetic ions of Eu2+ and Mn2+/Cu2+ as probes in the Ba and FeAs planes, respectively and X-ray absorption spectroscopy (XANES and EXAFS) in both As and Fe K edges. In this manner, it was possible to study the site specific spin dynamics and its relation with local distortions in the material. Our results evidentiate that the decrease in the Fe-As distance is intimately related to the SDW phase suppression and to a localization of the Fe 3d bands in the FeAs plane. This increase in the planar xy/x2 - y2 orbital character at the Fermi surface appears to be a propitious ingredient to the emergence of superconductivity in this class of materials
Doutorado
Física
Doutora em Ciências
Jesus, Camilo Bruno Ramos de 1987. "Evolução das propriedades magnéticas ao longo da série de compostos intermetálicos RTBi2 (R = Ce, Pr, Nd, Sm, Gd; T = Cu, Au)." [s.n.], 2016. http://repositorio.unicamp.br/jspui/handle/REPOSIP/320980.
Full textTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho investigamos as propriedades estruturais e magnéticas da família de compostos intermetálicos RTBi2 (R = Ce, Pr, Nd, Sm, Gd; T = Cu, Au) além do composto CeCd0,7Sb2. As amostras foram crescidas pelo método de fluxo metálico, a maioria delas de forma inédita. Medidas macroscópicas de resistividade elétrica, susceptibilidade magnética e calor específico foram realizadas, além de medidas de espectroscopia de raios X de energia dispersiva ¿ EDS e difração de raios X. Medidas microscópicas de difração magnética de raios X e ressonância magnética nuclear (RMN) foram realizadas, com ajuda de colaboradores, no composto CeCuBi2. Um modelo de campo médio que inclui uma interação de troca isotrópica entre primeiros vizinhos e efeitos de campo cristalino tetragonal foi utilizado para ajustar as medidas de magnetização e calor específico. Esses ajustes forneceram os parâmetros e esquemas de campo cristalino e os valores das constantes da interação de troca. A partir dessas análises encontramos como que efeitos de campo cristalino e a competição entre interações magnéticas, ferromagnética e antiferromagnética, governam as propriedades magnéticas desses compostos
Abstract: In this work we have investigated the structural and magnetic properties of the family of intermetallic compounds RTBi2 (R = Ce, Pr, Nd, Sm, Gd, T = Cu, Au) and, additionally, the CeCd0.7Sb2 compound. The samples were grown by Bi metallic flux method, most of them for the first time. Macroscopic measurements of electrical resistivity, magnetic susceptibility, specific heat were carried out with supplementary measurements of Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction. Microscopic measurements of magnetic X-ray diffraction and nuclear magnetic resonance (NMR) were carried out with the help of collaborators in the CeCuBi2 compound. A mean-field model that includes an anisotropic exchange interaction between nearest neighbors and tetragonal crystal field effects was used to fit the magnetization and specific heat data. The best fits yield the crystal field parameters and schemes of level and the values of the constants of exchange interaction for each compound in the RTBi2 series. From these analyzes, we have described how the crystal field effects and the competition between magnetic interactions, ferromagnetic and antiferromagnetic, govern the magnetic properties of these compounds
Doutorado
Física
Doutor em Ciências
1141235/2012
CAPES
Molina, Ruiz Manel. "Nanocalorimetric studies of size effects in magnetic oxides and formation kinetics in silicides." Doctoral thesis, Universitat Autònoma de Barcelona, 2014. http://hdl.handle.net/10803/298057.
Full textThe advances in Nanoscience and Nanotechnology have been paved by the continuous development of new techniques adapted to small samples. In that framework, calorimetry is a technique suitable to measure thermodynamic properties and energetic processes, such as phase transitions, through the heat absorbed or released by the system. Taking profit of advances in microfabrication techniques a new family of nanocalorimeters, based on ultra-light calorimetric cells and built up onto thin film dielectric membranes, has emerged demonstrating enhanced sensitivities compared with traditional calorimeters, reaching levels better than 1 nJ K-1 mm-2. This technique has permitted to several research groups to explore new physical phenomena inaccessible before. The present research work deals with the development and optimization of this technique: the nanocalorimetry. We enlarge the dynamic ranges of applicability from ultrafast heating rates to quasi-static ones, and we demonstrate its utility in the study of different phase transitions at nanoscale. In the first Chapter, a general introduction sets out the necessity of developing new characterization techniques to give service to the scientific community, and also provides an historical overview about the calorimeters development. Subsequently, Chapter 2 provides an overview about nanocalorimeters microfabrication process, and about design and fabrication of specific experimental setups to carry on nanocalorimetric experiments from 80 to 1200 K in adiabatic conditions. Chapter 3 presents the nanocalorimetric tools used during the Thesis and the improvements that we have implemented. The two major advances achieved are: (i) The expansion of the operating heating rates, where the combination of pulsed and power compensated methods have permitted to cover the ranges from 0.1 to 106 K/s. Moreover, a thorough study of the heat capacity calculation, and the analysis of signal noise led us to obtain a methodology that improves qualitatively the results obtained from data processing. (ii) The development of a new quasi-static technique that combines the better specificities of DC techniques, like the huge signal enhancement of adiabatic nanocalorimetry, and the averaging capabilities of AC calorimetry. This new technique has been named microsecond-pulsed heating nanocalorimetry, and it allows measuring second order phase transitions with a very high sensitivity (less than 75 pJ K-1 mm-2 Hz-1/2), with better thermal maps in the sensing area (less than 1 K thermal gradient) than using pulsed methods. We deeply study the effect of dimensionality in two different physical systems: the magnetic order-disorder transition in thin films of CoO, and the formation kinetics of thin films of Pd2Si from Pd/Si bilayers. In Chapter 4 we present how microstructure influences the antiferromagnetic interaction in CoO thin films. We study size effects in thin film samples ranging from 1 to 20 nm, and the influence of grains and boundaries sizes from a thermodynamic perspective in samples of 20 nm. We relate thermodynamic properties to magnetic ones by means of nanocalorimetric, structural, and magnetic characterization. In Chapter 5 the formation kinetics of Pd2Si is analyzed. We pursuit understand the reaction kinetics over a wide range of heating rates spanning six orders of magnitude. To achieve this purpose, we combine conventional calorimetry and nanocalorimetry. This study is complemented by the structural characterization of the samples. We also use a kinetic model to obtain the most relevant kinetic parameters by fitting the calorimetric curves.
Beach, Kevin S. D. (Kevin Stuart David) 1975. "Magnetic-field-induced antiferromagnetism in the Kondo lattice." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/28645.
Full textIncludes bibliographical references (p. 109-111).
The half-filled Kondo lattice model, augmented by a Zeeman term, serves as a useful model of a Kondo insulator in an applied magnetic field. A variational mean field analysis of this system on a square lattice, backed up by quantum Monte Carlo calculations, reveals an interesting separation of magnetic field scales. For Zeeman energy comparable to the Kondo energy, the spin gap closes and the system develops transverse staggered magnetic order. The charge gap, however, remains robust up to a higher hybridization energy scale, at which point the canted antiferromagnetism is exponentially suppressed and the system crosses over to a nearly-metallic regime. The quantum Monte Carlo simulations are performed using a determinant Monte Carlo method that has been extended to handle mixed spin and fermionic degrees of freedom. The formulation is sign-problem-free for all values of the Kondo coupling and magnetic field strength. The matrix operations are specially organized to maintain numerical stability down to arbitrarily low temperatures. Spectral data is extracted from the imaginary-time correlation functions using an improved analytic continuation technique. The weak, secondary peaks of the single-electron spectral function are resolvable, and their response to the magnetic field is carefully tracked. An unusual rearrangement of spectral weight is found at the onset of the antiferromagnetism.
by Kevin Stuart David Beach.
Ph.D.
Koz, Cevriye. "Investigations on the parent compounds of Fe-chalcogenide superconductors." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-205808.
Full textHooley, Chris. "The Kagome antiferromagnet and related models : studies in low-dimensional geometrically frustrated quantum magnetism." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301860.
Full textJames, Andrew J. A. "Dynamical Correlations of Low Dimensional Quantum Magnets." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.490085.
Full textFong, Manson Cheuk-Man. "Heisenberg model with spin anisotropy on the Kagomé lattice /." View abstract or full-text, 2008. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202008%20ZFONG.
Full textPathak, Shashi. "A theoritical [sic] study of the magnetic polaron /." free to MU campus, to others for purchase, 2002. http://wwwlib.umi.com/cr/mo/fullcit?p3052206.
Full textRodriÌguez, GonzaÌlez AgustiÌn. "The helical antiferromagnetism and ferromagnetism in GD-Y alloys." Thesis, University of Bristol, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399394.
Full textFreye, Dominik Matthias. "The border of antiferromagnetism in CeIn₃ : possible unconventional superconductivity." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627256.
Full textLengyel, Edit. "Antiferromagnetism and superconductivity in ce-based heavy-fermion systems." Göttingen Cuvillier, 2007. http://d-nb.info/990402991/04.
Full textJonen, Shintaro. "Estudo das transições metamagnéticas em compostos intermetálicos R IND. 6 Fe IND. 14 Al IND. X (R=Nd, La)." Universidade de São Paulo, 2002. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-13122013-113657/.
Full textRare earth-based intermetallic compounds R IND.6Fe IND.14-xAl IND.x with R=(Nd, La) are antiferromagnetic and undergo a metamagnetic transition at 2~3 T applied fields. In order to further characterize their magnetic behavior, polycrystalline samples were investigated with magnetometry, magneto resistance, magnetostriction and neutron diffraction. Samples have been prepared by arc melting under argon and annealed at 750°C for four weeks. The crystal structure is tetragonal of La IND.6Co IND.11Ga IND.3-type (space group 14/mcm). In contrast to La-based compounds, Nd IND.6Fe IND.14-xAl IND.x exhibits a large transition hysteresis and magnetic history effects at T<30 K, and antiferromagnetic ordering of Nd moments at 70 K. The T IND.N was determined by magnetic susceptibility measurements in the 150-310 K and 100-230 K range for the Nd and La compounds, respectively. The metamagnetic transition was observed for all samples and a ~3-5% resistivity drop was found to occur concomitantly, suggesting a magnetic multilayer-like structure. Unfortunately the magnetic structure of this compound could not be determined in this work by neutron diffraction. However, neutron diffraction measurements in the Nd IND.6Fe IND.10Al IND.4 compound revealed a change of space group from 14/mcm to P4 IND.2/mcm at 70 K, where the magnetic susceptibility exhibits a sharp peak. Mossbauer spectroscopy measured in the 4.2-100 K range has shown no evidence of Fe spin rotation, suggesting an isolated Nd phenomenon. Another result from neutron diffraction measurements was the presence of a magnetic (001) reflection that grows below 30 K, indicating the development of a spin component parallel to the basal plane. No such effect was observed with La-based samples. This increase is likely to be due to a canting of Nd moments towards the basal plane. The coincidence of this effect with the onset temperature for irreversible behavior in magnetic measurements is consistent with this interpretation.
Coutrim, Leandro Tolentino. "Propriedades estruturais, eletrônicas e magnéticas dos óxidos La2−xCaxCoIrO6." Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/4574.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this work we describe the synthesis of the novel series La2−xCaxCoIrO6 and its structural, electronic and magnetic characterization. The main objective of this project is to investigate the magnetic evolution of the system when modified due to the chemical substitution of Ca at La site, which can induce changes in the magnetic interactions. The policristaline samples were synthesized by Solid State Reaction process, and studied combining global characterization techniques as powder x-ray diffraction, temperature dependence of magnetic susceptibility and magnetization as a function of applied magnetic field. All compounds crystallize in monoclinic structure, with space group P21/n. Magnetic susceptibility results indicate that the superexchange antiferromagnetic interaction between Co2+ and Ir4+ in La2CoIrO6 lead to ferrimagnetic behavior due to the different magnetic moment of the transition metal ions. For the Ca2+ doped at La3+ site compounds, a magnetic evolution is observed with doping, induced by Co and Ir valence changes. As expected, this system presents a strong correlation between its structural, electronic and magnetic properties, in a way that combining the different results of each compound we can explain its behavior and understand the big picture of this system.
Neste trabalho descrevemos a síntese e a caracterização estrutural, eletrônica e magnética dos compostos policristalinos da série inédita La2−xCaxCoIrO6. O principal objetivo deste projeto foi investigar a evolução da magnetização do sistema quando os materiais são modificados a partir da substituição química do Ca no sítio do La, o que pode induzir mudanças na natureza das interações magnéticas. As amostras policristalinas foram sintetizadas pela técnica de Reação de Estado Sólido convencional, e estudadas combinando-se técnicas de caracterização globais como difração de raios X de pó, magnetização em função da temperatura e em função do campo magnético aplicado, e também calor específico em função da temperatura. Todos os compostos da série cristalizam-se em estrutura monoclínica, com grupo espacial P21/n. As medidas de susceptibilidade magnética indicaram que a interação de supertroca antiferromagnética entre os metais de transição Co2+ e Ir4+ leva o composto La2CoIrO6 a apresentar comportamento ferrimagnético devido aos diferentes momentos magnéticos apresentados pelos dois íons, com TC em torno de 100K. Para os compostos dopados com Ca2+ no sítio do La3+, observou-se mudanças nas propriedades magnéticas, induzidas pelas alterações nas valências dos íons de Co e Ir. Como esperado, este sistema apresenta uma forte correlação entre as suas propriedades estruturais, eletrônicas e magnéticas, de modo que a partir da interpretação conjunta dos dados acerca da estrutura e da magnetização, podemos entender o comportamento de cada composto da série.
Chan, Chi Hung. "3-dimensional Heisenberg antiferromagnet in cubic lattice under time periodic magnetic field /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202009%20CHANC.
Full textSaunders, Timothy E. "Phase transitions in geometrically frustrated antiferromagnets." Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670121.
Full textSchierle, Enrico. "Antiferromagnetism in thin films studied by resonant magnetic soft x-ray scattering." [S.l.] : [s.n.], 2007. http://www.diss.fu-berlin.de/2007/176/index.html.
Full textThomas, Kennedy Jessica 1975. "Antiferromagnetism, ferromagnetism and magnetic phase separation in Bi₂Sr₂O₆â[delta]." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/29312.
Full textIn title on t.p. "[delta]" appears subscript as the lower-case Greek letter.
Includes bibliographical references (p. 154-157).
Neutron scattering, magnetization and transport measurements were performed on single crystals of Bi2Sr2CoO6+[delta] to study the evolution of the magnetic properties as a function of the oxygen content . The oxygen content was varied by annealing single crystals in either a reducing or oxidizing environment to obtain an experimental range of 0.25 =/< [delta] =/< 0.5 and a corresponding average Co valence between +2.5 and +3. We show that the as-grown samples, which are oxygen rich ([delta approximately] 0.5) and therefore contain mostly Co3+ ions, enter an antiferromagnetic (AF) phase with a Neel temperature - 250 K. On the other hand, annealing as-grown crystals in vacuum to reach [delta approx.] 0.25 destroys the AF phase; these samples exhibit predominantly ferromagnetic (FM) behavior with Tc [approx.] 100 K. At intermediate doping, 0.25 < [delta] < 0.5, we find evidence for co-existence of FM and AF domains, which are characteristic of the [delta] = 0.25 and [delta] = 0.5 phases, respectively. The signature of the co-existence is the presence of simultaneous FM and AF magnetic Bragg peaks in the neutron diffraction pattern. Polarized neutron scattering measurements confirm that the FM and AF peaks do not arise from different components of a canted antiferromagnet.
(cont.) The FM regions give rise to a ferromagnetic-like peak in the susceptibility at the same temperature as the spins in the AF phase order. In addition, the FM regions exert a random field in the AF phase, above a critical field Hc. We explain the field dependence of the two-phase samples with a microscopic model. We propose that the FM clusters within the AF phase are the result of regions which are rich in Co2+. Furthermore, we suggest that oxygen facilitates the formation of electronically inhomogeneous regions.
by Kennedy Jessica Thomas.
Ph.D.
Adriano, Cris 1980. "Efeitos da substituição química nos diagramas de fases dos férmions pesados Ce2MIn8 (M = Rh e Ir)." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278513.
Full textTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Este trabalho apresenta estudos sistemáticos das propriedades magnéticas dos compostos Ce2MIn8 (M = Rh, Ir) dopados com Cd, Sn e Ga no sítio cristalográfico do In. Foram utilizadas as técnicas de medidas macroscópicas de susceptibilidade magnética, calor específico e resistividade elétrica, além de medidas de caracterização estrutural pelo método de difração de pó de raios-x e medidas microscópicas da determinação da estrutura magnética usando a técnica de espalhamento magnético de raios-x e nêutrons para os compostos dopados com Cd. Os compostos Ce2MIn8 (chamados de 218) são tetragonais (grupo espacial P4/mmm) e são a versão bi-camada da família de férmions pesados supercondutores Cem MnIn3m+2n(M = Co, Rh, Ir; m = 1,2; n = 0,1), na qual os compostos do tipo CeMIn5 (chamados de 115) foram muito estudados por apresentarem uma variedade de propriedades físicas interessantes, tais como efeito Kondo, supercondutividade não convencional, ordenamento magnético, comportamento non-Fermi-Liquid, e ocorrência de pontos críticos quânticos. Mais interessante ainda é o fato de que estas propriedades podem ser controladas por dopagem, pressão externa e/ou campo magnético gerando diagramas de fases bastante interessantes. O objetivo de se estudar os parentes Ce2MIn8 é entender o papel da dimensionalidade nas propriedades físicas destes compostos e comparar os resultados encontrados com aqueles observados para os 115. Foram construídos os diagramas de fase inéditos para as séries de Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, C e2Rh0.5Ir0.5In8-xC dx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax a partir de dados obtidos utilizando as técnicas mencionadas acima. Foi visto que a dopagem de Cd nos compostos Ce2MIn8 favorece o AFM através do aumento da TN em função da concentração de Cd. Os resultados sugerem que o Cd está agindo de modo a diminuir a densidade de carga local e isso está favorecendo o ordenamento magnético. Porém, resultados da variação da temperatura do máximo da resistividade elétrica em função da concentração de Cd, sugerem que efeitos de campo cristalino também estão presentes nestes compostos. A estrutura magnética dos compostos de Ce2RhIn8-xCdx e C e2IrIn8-xCdx dopados com x = 2.6 % de Cd foram estudados em baixas-T e encontrou-se um vetor de propagação do tipo (½ ½ 0) consistente com ordenamento AFM abaixo de TN. Verificou-se que a orientação do momento magnético tende a ir na direção do plano-ab com dopagem de Cd nestes compostos. A substituição de In por Sn nos compostos de Ce2MIn8 mostrou que esta dopagem está desfavorecendo o ordenamento magnético através da diminuição do valor da TN em função da concentração de Sn. Os resultados sugerem que o Sn está favorecendo o efeito Kondo e que efeitos de campo cristalino são menos observáveis nos compostos dopados com Sn. Foi visto que a dopagem com Ga no sítio do In igualmente diminui a temperatura de ordenamento magnético e favorece o efeito Kondo. Porém, os resultados sugerem que efeitos de campo cristalino estão presentes pela variação não usual da temperatura do máximo das curvas de resistividade elétrica em função da dopagem com Ga. Neste trabalho também são apresentados estudos da evolução do magnetismo dos elétrons 4f, em compostos relacionados que não são férmions pesados para R2CoGa8, para R = Gd, Tb, Dy, Ho, Er e Tm. Nestes compostos podemos observar a evolução do magnetismo e dos efeitos de campo cristalino sem a presença do efeito Kondo
Abstract: This thesis describes the investigation of the physical properties of the (Cd, Sn, Ga)-doped Ce2MIn8 (M = Rh, Ir) intermetallic compounds. Temperature dependent magnetic susceptibility, heat capacity and electrical resistivity measurements were performed to characterize the macroscopic properties of the synthesized compounds. X-ray powder diffraction were used to determine their phase purity and crystal structure and X-ray and neutron magnetic scattering techniques were used to study the microscopic magnetic structure in low temperature. The Ce2MIn8 (M = Rh, Ir) compounds belong to the family of the heavy-fermions Ce-based compounds CenMIn3n+2 (M = Co, Rh, Ir; n = 1, 2), where the occurrence of unconventional superconductivity (USC) in many of its members has been motivating intense work in past few years. In particular these compounds present a wide range of interesting properties as Kondo effect, USC, magnetism, non Fermi liquid and Fermi liquid behavior and the occurrence of quantum critical points. Particularly interesting is the tunability of their ground state by pressure and chemical doping that revealed very interesting and rich phase diagrams with properties that are unprecedented for this class of compounds. Indeed the study of the 218 parent compounds is a great opportunity to further study the relationship between magnetism, USC and crystal structure in this family. We report in this work five unpublished phase diagrams of this family: Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, Ce2Rh0.5Ir0.5 In8-xCdx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax. Our results revealed that Cd-doping Ce2MIn8 enhances the antiferromagnetic ordering temperature as a function of the Cd concentration. The results suggest that the Cd-doping in these compounds is inducing an electronic tuning by locally decreasing the Ce 3+ density of states and this is favoring the AFM ordering according com a Doniach-like scenario. However the evolution of the maximum in the resistivity as a function of the Cd concentration and the small variation of the magnetic moment orientation of the doped compounds suggest that Cd-doping is also changing the crystal field (CEF) parameters in these compounds. The magnetic structure studied by X-ray and neutron magnetic scattering showed the Cd-doped samples present just below TN a commensurate antiferromagnetic structure with a propagation vector (½ ½ 0). The magnetic structure determination indicates that the magnetic moment orientation of the Cd doped samples tend going to the ab-plane. The Sn chemical substitution in Ce2MI n8 (M = Rh, Ir) showed Sn-doping is decreasing the magnetic order by favoring the Kondo effect. This is the opposite of the Cd-doping effect because Sn is now increasing the local density of states of the Ce3+ions. As Sn-doping drives 4f to a more itinerant character the CEF effect are less important for this compounds. On the other hand, the Ga-doped Ce2RhI n8 similarly show a decreasing of the magnetic order due to the enhancement of the Kondo effect by chemical pressure. However the unexpected evolution of the maximum in the resistivity as a function of the Ga suggests in this dopant is changing the crystal field is still an important effect in this compound. Finally we present the 4f magnetism evolution studies on the series of the Ga-based R2CoGa8 for R = Gd ¿ Tm compounds, where one can follow the evolution of the magnetism and the CEF parameters without the presence of the Kondo effect
Doutorado
Física da Matéria Condensada
Doutor em Ciências
An, Ran. "Study of the field-induced phase transition for the antiferromagnetic chain /." View abstract or full-text, 2006. http://library.ust.hk/cgi/db/thesis.pl?MATH%202006%20AN.
Full textPfeffer, James. "Magnetic resonance measurements in some zircon-structure compounds." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294327.
Full textWong, Ming-Wai. "Studying the mutual interaction between ferromagnet and antiferromagnet planes by using the Schwinger-Boson mean field theory /." View Abstract or Full-Text, 2002. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202002%20WONG.
Full textPalmer, Stephanie E. "Order and disorder in two geometrically frustrated antiferromagnets." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342663.
Full textCAMARENA, MARIELLA ALZAMORA. "ANTIFERROMAGNETISM AND QUANTUM CRITICAL POINT IN CECOGE(2,1)SI(0,9) COMPOUND UNDER PRESSURE." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2007. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=11502@1.
Full textCENTRO LATINO-AMERICANO DE FÍSICA
Estudos no sistema pseudoternário CeCoGe(3-x)Six (com 0 = < x = < 3) mostraram que o sistema evolui continuamente de um estado antiferromagnético da rede de Kondo (CeCoGe(3) com ~21k) para um composto de valência intermediária (CeCoSi(3) com Tw ~230k). O sistema apresenta comportamento tipo não- líquido de Fermi (NLF) em torno do ponto crítico quântico (PCQ) na concentração crítica xC = 1,25. A substituição isoeletrônica dos átomos de por não aumenta o grau de desordem magnética, sendo ideal para o estudo de efeitos intrínsecos das variações das constantes de interação da rede Kondo. Estudamos este sistema em concentrações próximas à concentração crítica através de medidas de resistividade elétrica AC sob pressão (x=0,9) e campo magnético (x =1), em amostras policristalinas. Nossos resultados mostram que a ordem magnética de longo alcance presente na amostra CeCoGe(2,1)Si(0,9) é suprimida com o aumento da pressão e, para a pressão crítica PC ( aprox. 6,2 kbar) TN é aproximadamente zero. Para temperaturas inferiores a TN, as medidas de resistividade são bem descritas considerando um espalhamento de elétrons de condução por mágnons antiferromagnéticos anisotrópicos. Acima de PC observa-se o comportamento líquido de Fermi. Na região crítica o estado NLF foi observado. A análise do comportamento da linha crítica na proximidade do PCQ indica que as flutuações magnéticas relevantes são tipicamente bidimensionais. Por outro lado, no composto CeCoGe2Si, que apresenta ordem magnética de curto alcance com TN aproximadamente 2K, observa-se que a temperatura de ordenamento é reduzida com o aumento do campo magnético e, para campos acima de 3 T, surge o comportamento tipo líquido de Fermi.
Studies on the pseudo ternary system CeCoGeSix (where 0 = < x = < ) have shown that the system evolves continuously from a Kondo lattice antiferromagnetic state ( CeCoGe(3) with ) towards a mixed valent compound ( CeCoSi(3)with Tw ~230k ). The system displays a non-Fermi-liquid-type behavior (NFL) in the vicinity of the quantum critical point (QCP) at the critical concentration . Isoelectronic substitution of atoms for does not enhance the degree of magnetic disorder, rendering it ideal for the study of the Kondo lattice¡ s interaction constants intrinsic effects. We have studied this system in polycrystalline samples at concentrations close to the critical one through AC electrical resistivity under pressure (x= 0,9) and magnetic field (x=1) measurements. Our results show that the long range magnetic order present in the CeCoGe(2,1)Si(0,9) sample is suppressed as pressure is increased, and that for the critical pressure PC (aprox. 6,2 kbar), TN (aprox. zero) . For temperatures below TN, the resisitivity data are well described considering conduction electron scattering by anisotropic antiferromagnetic magnons. Above PC we observe the Fermi liquid behavior. At the critical region, a NFL state with exponents close to 1 was found. The analysis of the behavior of the critical line in the neighborhood of the QCP indicates that the relevant magnetic fluctuations are typically two-dimensional. On the other hand, the CeCoGe(2)Si compound displays short range order (TN ~ 2k). The ordering temperature is reduced under an increase of an applied magnetic field, and for magnetic fields above 3 T a Fermi liquid behavior arises.
Feldmaier, Teresa [Verfasser], and Maria [Akademischer Betreuer] Daghofer. "Excitonic antiferromagnetism in two-dimensional t4 2g systems / Teresa Feldmaier ; Betreuer: Maria Daghofer." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2020. http://d-nb.info/120406122X/34.
Full textShi, 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.
Full textTitle 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).
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.
Full textTitle 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).
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/.
Full textIn 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.
Lugas, Massimo. "d − wave Superconductivity and antiferromagnetism in strongly correlated systems by a new variational approach." Doctoral thesis, SISSA, 2007. http://hdl.handle.net/20.500.11767/3915.
Full textHatakeyama, Yuhki. "Theoretical study of antiferromagnetism induced by paramagnetic pair-breaking in a strong-coupling superconducting phase." 京都大学 (Kyoto University), 2014. http://hdl.handle.net/2433/188469.
Full textLalis, Diovana de Mello. "Descrição teórica do diagrama de fases dos compostos de férmions pesados a partir do modelo de Anderson Periódico." Universidade do Estado de Santa Catarina, 2015. http://tede.udesc.br/handle/handle/1990.
Full textCoordenação de Aperfeiçoamento de Pessoal de Nível Superior
Os compostos de férmions pesados são geralmente descritos no âmbito do modelo da rede de Kondo, que pode ser derivado a partir do modelo de Anderson. As suas propriedades peculiares são regidas pela competição entre o efeito Kondo e a interação Ruderman-Kittel-Kasuya-Yosida (RKKY), que permite esboçar o diagrama de Doniach. A investigação de compostos ferromagnéticos de férmions pesados, em particular, tem se intensificado recentemente, com a possibilidade de aproximação do ponto crítico quântico (QCP) por meio do ajuste da pressão, da concentração de elétrons, ou pela aplicação de campo magnético. Neste trabalho, o modelo de rede de Anderson (ou modelo de Anderson periódico) é utilizado diretamente para descrever estes compostos, incorporando o efeito das flutuações de valência, que podem desempenhar um papel importante na vizinhança do QCP. Empregamos o método da equação do movimento para as funções de Green, que permite desacoplamentos sucessivos da cadeia de equações. Exploramos a estabilidade das soluções de AF, FM e não-magnéticas em função dos parâmetros do modelo, da temperatura e da concentração de életrons. O diagrama de fases é obtido a partir das curvas de magnetização e pela comparação das energias das soluções envolvidas.
Rudisch, Christian. "Nuclear Magnetic Resonance on Selected Lithium Based Compounds." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-130485.
Full textFabbris, Gilberto Fernandes Lopes. "Estudo da estrutura local da família RMn2O5 (R=Bi, Tb, Gd, Pr)." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277890.
Full textDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Materiais multiferróicos apresentam pelo menos duas propriedades ferróicas na mesma temperatura: (anti) ferroeletricidade, (anti) ferromagnetismo, ou ferroelasticidade. Esses materiais têm atraído grande atenção da comunidade científica nos últimos anos devido às suas potenciais aplicações e ao complexo acoplamento entre suas propriedades, o que ainda não é bem compreendido. Neste trabalho, nos concentramos nos materiais multiferróicos da família RMn2O5, que possuem estrutura ortorrômbica com grupo espacial Pbam. Nestes compostos, a transição antiferromagnética acontece a ~40K e a ferroelétrica em torno de 39K para todos os membros da família. A ferroeletricidade vista nesse material é incompatível com o centro de simetria existente no grupo espacial Pbam. Resultados de EXAFS para o composto TbMn2O5 existentes na literatura indicam uma distribuição bimodal de Tb-O na primeira camada de coordenação. Este desdobramento da primeira camada de coordenação independe da temperatura e indica uma possível variação do grupo espacial Pbam. Tais resultados nos motivaram a empreender o estudo da estrutura local da família RMn2O5 a fim de entendermos a correlação entre a estrutura local e a natureza do íon R. Medidas do espectro de XAFS como função da temperatura foram realizadas na linha XAFS2 do Laboratório Nacional de Luz Síncrotron (LNLS). Foram realizadas varreduras na borda K do Mn e em torno da borda L3 dos íons R (Bi, Gd, Pr, Tb). Os resultados para a borda do manganês em todos os compostos indicam que os poliedros de MnO são rígidos. Já na borda do íon R há comportamentos distintos. Enquanto no composto com bismuto observamos uma possível vibração rígida dos poliedros de MnO, para os outros compostos estudados identificamos uma distorção rígida desses poliedros. O comportamento distinto do composto com Bi, em relação aos demais membros da família, parece estar relacionado ao lone pair do bismuto
Abstract: Multiferroic materials present, at the same temperature, at least two of the so-called ferroic properties: (anti) ferroelectricity, (anti) ferromagnetism and ferroelasticity. They have attracted great attention in the last few years due to their potential applications as well as from the basic science point of view, given the intricate coupling between their physical properties, which remains poorly understood. In this work, we have focused on the study of the RMn2O5 family multiferroic materials. Their crystalline structure belongs to the Pbam orthorhombic space group, with an antiferromagnetic and ferroelectric phase transition temperatures below 40 K and 39K, respectively. Such ferroelectric phase is incompatible with the inversion center of symmetry in the Pbam space group. Recently published EXAFS results for TbM n2O5 revealed a first coordination shell with a bimodal Tb-O bond length distribution. Such bond length splitting is temperature-independent and may be related to a change of the Pbam space group. Such results were our main motivation to undertake a systematic study of the local structure of the RMn2O5 multiferroic family, aiming at a better understanding the correlation between the ion R and the local structure. Mn K-Edge and R-ion L3 Edge (R=Bi, Gd, Pr, Tb) temperature-dependent XAFS measurements were performed at the Brazilian Synchrotron Light Laboratory XAFS2 beam line. Results for the Mn K-edge in all studied compounds reveal the Mn-O coordination polyhedral are rigid. The R-ion results reveal distinct behaviours. For the R=Bi compound, we identified that low energy Mn-O rigid unit vibrational modes are likely to be operative. For the other compounds, we have identified a rigid distortion of the Mn-O polyhedra. The distinct behavior of the BiMn2O5 seems to be related to the Bi 6s2 lone pair
Mestrado
Física da Matéria Condensada
Mestre em Física
Souza, dos Santos Edgar Gonzaga. "Etude théorique et expérimentale des propriétés magnétiques des oxydes de métaux de transition quasi-bidimensionnels du type AB2O6." Thesis, Grenoble, 2012. http://www.theses.fr/2012GRENY047/document.
Full textThis work aims to contribute to the study of magnetism in insulating oxides of the type AB_2O _6 (A = Fe, Co, Ni, Ta = B, Sb). These compounds crystallize in a trirutile structure and present a rich variety of magnetic phases. All the observed orderings can be classified as antiferromagnetic, but they differ from a simple Néel type order. Moreover, substitutions such as Fe_xCo_{1-x}Ta_2O_6, Fe_xNi_{1-x}Ta_2O_6 and Co_xNi_{1-x}Ta_2O_6 produce several regions of phase coexistence and bicritical points in the phase diagram T vs. x. A particularly interesting characteristic of these compounds is the quasi-two-dimensional nature of their magnetism, since the transition-metal ions A appear in layers separated by two planes of the non-magnetic ions B. They also show strong crystal-field anisotropy on the magnetic sites due to axial distortion of the oxygen octahedra surrounding the A ions. Our approach involves two complementary aspects: theoretical and experimental. From the theoretical point of view, a reformulation of the two-dimensional model used until now to describe the paramagnetic susceptibility allows us to obtain exchange parameters compatible with the observed planar magnetic orderings. Considering that the three-dimensional character the magnetic structures revealed by neutron diffraction indicates that the coupling between planes, although weak, plays an important role to low temperatures, we propose a three-dimensional model consistent with the observed orderings both in the ab plane and along the c axis. From the experimental point of view, we make substitutions on the non-magnetic sites, in order to systematically modify the spacing between planes and, consequently, the coupling between them. In particular, we have synthesized compounds of type ANb_xTa_{2-x}O_6 with A = Fe, Ni and Co, remaining in the stability region of tetragonal phase, since the presence of Nb favors an orthorhombic phase. The structural and magnetic characterization of the systems is done via the X-ray and neutron diffraction as well as measurements of magnetic susceptibility, specific heat, and magnetization as a function of the applied field. We also study series with Sb in place of Nb where we additionally include substitutions in the magnetic site
Haykal, Angela. "Exploring the antiferromagnetic order with a single-spin magnetometer." Thesis, Montpellier, 2020. http://www.theses.fr/2020MONTS057.
Full textAntiferromagnetic (AF) materials are currently attracting considerable excitement for low dissipative and ultrafast spintronic devices. However, most of conventional real-space magnetic microscopy techniques cannot probe the AF order at the nanoscale because magnetic moments are mostly compensated, resulting in very low magnetic signals. This is a major obstacle to the fundamental understanding of nanoscale AF order and its response to external stimuli, such as spin polarized currents or electric fields. To release the full potential of antiferromagnets for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be extended to AF materials. In this thesis we prove that scanning magnetometry based on a single nitrogen–vacancy (NV) defect in diamond is ideally suited for imaging complex AF orders at the nanoscale, even under ambient conditions.A promising platform for AF spintronics is BiFeO3 (BFO), a prototypical room-temperature multiferroic material in which the AF order is intimately linked to the ferroelectric one via magnetoelectric coupling. Scanning NV magnetometry here demonstrates its ability to image the AF cycloidal order in BFO by mapping the magnetic stray field it produces. It also permits inferring interesting quantities such as the uncompensated magnetic moment of the spin density wave, and the real-space visualization of the intimate link between the ferroelectric and the AF orders. In order for BFO to make its way into device applications, the thin film form must be employed. The effect of epitaxial constraint on the behaviour of the AF order in strained BFO thin films is investigated. NV magnetometry proved that strain-tuning and electric-field switching, can stabilize a wide variety of complex antiferromagnetic spin textures in BFO thin films.Beyond imaging the static stray field, we demonstrate a new approach for imaging AF textures. It consists of mapping the magnetic noise they locally produce rather than their static magnetic fields. This technique exploits the strong dependence of the NV defect photoluminescence on magnetic fluctuations at the NV spin resonance frequency. As a proof of principle of the efficiency of the technique, the high tunability of synthetic antiferromagnets that host spin waves, is exploited to stabilize different spin textures. These AF textures ranging from domain walls, to exotic spin-spirals and AF skyrmions are imaged through this novel relaxometry technique
Panigrahy, Sujit. "Magnetic skyrmions in cobalt-based synthetic antiferromagnets." Electronic Thesis or Diss., université Paris-Saclay, 2023. http://www.theses.fr/2023UPASP187.
Full textMagnetic skyrmions are topologically protected textures behaving as particles, and that could be used in future spintronic devices. In ferromagnets, their velocity induced by electrical current is limited and they suffer from a deflection related to their non-trivial topology. In this work, we explore their properties in synthetic antiferromagnets (SAF), that have been proposed to overcome these limits. Using simulations and models, we show that such materials are indeed promising and give to skyrmions new properties, like a mass. Then, in two experimental chapters, we show how to design suitable samples. We first develope a methodology to overcome the application of an external magnetic field that is usually required in ferromagnetic films, but whose application would not be efficient in antiferromagnets. Then, we finally demonstrate the current induced skyrmion motion in synthetic antiferromagnets, and show velocities five time larger than in ferromagnets and deflectionless motion. The results obtained hence are both fundamentally and technologically relevant for SAF based skyrmionics devices beyond the conventional ferromagnetic systems
Sarthour, Roberto S. "Magnetism in rare-earth superlattices and alloys." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.312552.
Full textBodensiek, Oliver [Verfasser], Thomas [Akademischer Betreuer] Pruschke, and Matthias [Akademischer Betreuer] Vojta. "Superconductivity and Antiferromagnetism in the Kondo-Lattice Model / Oliver Bodensiek. Gutachter: Thomas Pruschke ; Matthias Vojta. Betreuer: Thomas Pruschke." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2013. http://d-nb.info/1044770279/34.
Full textArdavan, Arzhang. "Millimetre-wave magneto-optics of correlated systems." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297940.
Full textFulton, Sharon. "Scattering studies of excitations and phase transitions." Thesis, University of Oxford, 1993. http://ora.ox.ac.uk/objects/uuid:45f661ac-61cc-4a0c-9e96-636866a99cf2.
Full textAraujo, Alexandre Abdalla. "Propriedades estruturais, eletrônicas e magnéticas de filmes finos de materiais magnéticos." [s.n.], 2008. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277462.
Full textTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: A Física de superfícies, interfaces e filmes finos vem se desenvolvendo muito rapidamente nas últimas décadas com o aparecimento de inúmeras técnicas experimentais para estudo das propriedades de superfície. Por outro lado, tem ocorrido um grande avanço dos equipamentos de informática e dos métodos computacionais, com o desenvolvimento de novos algoritmos, os quais já permitem o estudo de sistemas mais complexos como interfaces, defeitos, filmes-finos e nanofios, contendo um número cada vez maior de átomos. Um considerável interesse em superfícies e na deposição de filmes finos sobre superfícies, envolvendo metais, tem sido motivado pela possibilidade de se conseguir novas propriedades magnéticas e eletrônicas, incluindo temperaturas acima da temperatura ambiente, visando avanços tecnológicos em dispositivos eletrônicos. Nosso trabalho representa uma estratégia bastante promissora nessa área, pois nele identificamos claramente a possibilidade de produção de filmes finos com caráter ferromagnético half-metallic (isto é, com um canal de condução eletrônico semicondutor e outro metálico). Conforme pudemos mostrar, este caráter foi atingido a partir de pequenas variações de parâmetro de rede, de espessura de filme e de composição atômica. As propriedades observadas em nossos resultados teóricos sinalizam a importância de aplicação de diferentes materiais tais como CrAs, CrTe, CrAs(1-x)Sex, CrAs(1-x)T e x, CrSe(1-x) Tex, objetivando suas utilizações em Spintrônica. Desta forma, realizamos um estudo sistemático desses materiais, verificando suas propriedades eletrônicas e magnéticas e suas viabilidades de aplicações em novos dispositivos. Dois métodos de cálculo de estrutura eletrônica: o RS-LMTO-ASA (Real-Space ¿ Linear Muffin-Tin ¿ Atomic Sphere Approximation) e o FLAPW (Full Potential - Linearized Augmented Plane wave), assim como o método da Matriz Transferência foram utilizados em nossas investigações. Em primeiro lugar, apresentamos estudos teóricos sobre as fases estruturais e magnéticas observadas nas primeiras camadas de filmes finos de CrAs, crescidos sobre substratos de GaAs(001). Esses estudos englobaram processos de otimização de geometria, realizados através do método FLAPW, baseados em cálculos autoconsistentes de primeiros princípios, levando em consideração a polarização de spin. Em segundo lugar, estudamos as propriedades eletrônicas e magnéticas das superfícies CrAs(001) através do RS-LMO-ASA e determinamos as dispersões dos estados eletrônicos de superfície segundo direções de alta simetria na zona de Brillouin bidimensional. A seguir, como os resultados apontaram a possibilidade de obtermos mais materiais com comportamento ferromagnético half-metallic, passamos a investigar toda uma classe de materiais com estruturas volumétricas ou de filmes finos envolvendo os elementos Cr, As, Te, e Se, arranjados em ligas binárias (CrAs, CrSe, CrTe) e ternárias (CrAs(1-x)Sex, CrAs(1-x)T ex, CrSe(1-x)Tex), em diferentes concentrações e diferentes regiões superficiais. Como conseqüência, um amplo conjunto de resultados interessantes foi conseguido, confirmando nossas expectativas de que pequenas variações de parâmetro de rede, de espessura e de composição atômica são ingredientes fundamentais a serem considerados para se atingir uma transição do regime ferromagnético metálico para half-metallic e que isto representa uma área bastante promissora, que deverá estimular novos experimentos, com a produção de novos tipos de filmes finos, com espessura e composição controladas. Por último, apresentamos um estudo teórico do composto Fe2CoAl, no qual a precisão de nossos cálculos é comparada a medidas experimentais
Abstract: In the last decades, the Physics of Surfaces, Thin Films and Interfaces has motivated a great advance of the experimental techniques applied to study surface properties. In addition, a fast progress in the computational area has also occurred, with the development of powered computers, new methods of calculations, and new algorithms, which already allow the description of more complex systems, such as interfaces, defects, thin films and nanowires. A considerable interest in the deposition of thin films on surfaces, involving metals, has been motivated by the possibility of producing new devices using the fascinating electronic and magnetic properties, in order to produce technological advances in electronic devices. This work represents a promising strategy in this area, because we identify, clearly, the possibility of producing thin films with half-metallic character (that is, with a semiconductor electronic spin channel and a metallic spin channel, simultaneously). As we showed, this character was attained from small variations of lattice parameter, film thickness or atomic composition. The results of our theoretical calculations have pointed the importance of some materials such as CrAs, CrTe, CrAs(1-x)S ex, CrAs(1-x)Tex, CrS e(1-x)Tex to be used in the Spintronic branch. So, we carry out a systematic analysis of these new materials, emphasizing its structural, electronic and magnetic properties and the viability of using these materials in new electronic devices. Two different methods of electronic structure calculations: the RS-LMTO-ASA (Real-space - Linear Muffin-Tin - Atomic Sphere Approximation) and the LAPW (Linearized Augmented Plane-Wave), as well as the Matrix Transfer method have been used in our studies. Initially, we present the theoretical results of the structural and magnetic phases, observed in the first layers of thin films of orthorhombic CrAs, grown on a GaAs(001) substrate. Two geometry optimization processes have done with the Full-Potential Linearized Augmented Plane-Wave (FLAPW) method, based on first principles, self-consistent calculations, taking in account the spin polarization, at the scalar relativistic level. Secondly, we study the electronic and magnetic properties of the CrAs(001) surfaces, via the RS-LMTO-ASA, and determined the energy dispersion of the electronic surface states along two highly symmetric directions in the two-dimensional Brillouin zone. Then, as the results suggested the possibility of obtaining new thin films, with ferromagnetic half-metallic behavior, we started to investigate a large class of materials, with volumetric and thin films structures, of binary (CrAs, CrSe, CrTe) and ternary (CrAs(1-x)Sex, CrAs(1-x) Te, CrSe(1-x)Tex) systems, in different atomic concentrations and with different superficial regions. Consequently, a large quantity of interesting results was obtained for these ferromagnetic materials, confirming that small variations of lattice parameters, film thickness and atomic composition are the fundamental ingredients to be considered, in order to reach the transition from metallic regime to ferromagnetic half-metallic regime and that our results can stimulate new experiments with the aim of producing new thin films, with controlled thicknesses and atomic compositions. Finally, we present a theoretical study of the inter-metallic compound Fe2CoAl, by comparing the precision of our calculations with experimental measurements
Doutorado
Física da Matéria Condensada
Doutor em Ciências
Sena, Filho Roberto Ferreira. "Teoria microscópica de ondas de spin em nanofios magnéticos." reponame:Repositório Institucional da UFC, 2007. http://www.repositorio.ufc.br/handle/riufc/12372.
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The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra.
O comportamento dinâmico de spins em materiais magnéticos é influenciado pela geometria que eles apresentam. Além disso, outro aspecto relevante é a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnéticas em sistemas de baixa dimensionalidade, que é devido em grande parte as aplicações tecnológicas, tais como: nanosensores, gravadores magnéticos de alta densidade, dispositivos magneto-eletrônicos, etc. Neste trabalho estudamos a propagação de ondas de spin em nanofios magnéticos cilíndricos, onde a abordagem é feita utilizando teoria microscópica, através do Hamiltoniano de Heisenberg, em que os spins são considerados fixos nos sítios da rede e cuja geometria da seção transversal dos cilindros é hexagonal. Entre as interações magnéticas estudadas consideramos: a interação de troca que pode ser ferromagnética se os primeiros vizinhos dos spins estão numa configuração paralela, ou antiferromagnética se estiverem antiparelelos; a interação Zeeman que é devido ao campo magnético externo aplicado ao sistema; a interação de Anisotropia, esta sendo responsável pela direção de magnetização preferida que diversos sistemas magnéticos reais apresentam e a interação dipolar de natureza magnetostática, presente em todos os materiais. O formalismo leva em consideração a dependência espacial dos spins no sistema, onde os operadores de spin do hamiltoniano são escritos em termos de operadores bosônicos de criação e aniquilação através da Representação de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direção devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equações matriciais no espaço dos vetores de onda. A partir desse sistema de equações obtemos vários espectros de excitação como: a relação de dispersão para as ondas de spin, que é o gráfico onde mostra como a frequência de ondas de spin varia em função do vetor de onda e a variação da energia do sistema com o campo aplicado.
Sugimoto, Koudai. "Theoretical Study of Electron Dynamics in Multi-Orbital Antiferromagnetic Metals." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199089.
Full textAlmeida, Dalson Eloy 1989. "Coexistência microscópica de antiferromagnetismo e supercondutividade não-convencional." [s.n.], 2017. http://repositorio.unicamp.br/jspui/handle/REPOSIP/331437.
Full textTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Nesta tese estudamos a relação entre antiferromagnetismo e supercondutividade em pnictídeos à base de ferro. Este estudo será feito através da análise de uma energia livre de Ginzburg-Landau de parâmetros de ordem acoplados que será derivada de um modelo microscópico. Em particular, estamos interessados em saber se a transição entre os estados ordenados é de primeira ordem ou se as duas ordens podem coexistir. Para o caso de supercondutividade convencional as duas fases puras nunca coexistem. Entretanto, quando a supercondutividade é não-convencional e a condição de nesting perfeito não é satisfeita, pode haver um regime intermediário de coexistência microscópica das duas ordens. Nesta nova fase termodinâmica, as simetrias de rotação no espaço de spins, de reversão temporal e U(1) são quebradas simultânea e localmente. Logo, os canais de supercondutividade singleto e tripleto se misturam quanticamente. Em outras palavras, uma componente tripleto secundária do estado supercondutor é gerada. Os diagramas de fases do sistema são apresentados e analisamos também como flutuações magnéticas, acima da temperatura de Néel pura, afetam a temperatura de transição tripleto. Investigamos também o efeito da magnetização alternada no efeito Josephson, i.e., na supercorrente que flui através de uma junção entre dois supercondutores na fase de coexistência. Por fim, mas não menos importante, estudamos o efeito de proximidade em uma interface entre um supercondutor e um antiferromagneto. Veremos que os pares de Cooper podem penetrar a região magnética e em consequência, uma componente tripleto é induzida próximo da interface
Abstract: In this thesis, we study the interplay between antiferromagnetism and superconductivity in iron pnictides. This study will be done analyzing a free energy of coupled order parameters which will be derived from a microscopic model. In particular, we are interested if the phase transition between the ordered states is first order or if the two orders can coexist. For the case of conventional superconductivity, the two phases cannot coexist. However, when superconductivity is unconventional and the perfect nesting condition is not satisfied, there can exist an intermediary state of microscopic coexistence of the two orders. In this new thermodynamic phase, spin rotation, time reversal and U(1) symmetries are simultaneously and locally broken. Therefore, the singlet and triplet superconductivity channels are quantum mechanically mixed. In other words, a secondary triplet component is generated. The phase diagrams of the system are presented and we also analyze the effect of magnetic fluctuations above the pure Néel temperature on the triplet temperature transition. We also investigate the effects of the staggered magnetization on the Josephson effect, i.e., on the supercurrent that flows through a junction of two superconductors in the coexistence phase. Last, but not least, we study the proximity effect at an interface between a superconductor and an antiferromagnet. We will see that the Cooper pairs can penetrate the magnetic region and consequently a triplet component is induced near the interface
Doutorado
Física
Doutor em Ciências
140834/2013-3
2342/15-4
CNPQ
CAPES
BEX
Amow, Gisele. "Structural and physical properties of the vacancy doped systems R(1-x)TiO3 (R = Nd for 0.00< x < 0.33 and Sm for 0.00< x < 0.17) : an investigation of metal-insulator transitions /." *McMaster only, 1999.
Find full textFilho, Roberto Ferreira Sena. "Teoria microscÃpica de ondas de spin em nanofios magnÃticos." Universidade Federal do CearÃ, 2007. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=428.
Full textO comportamento dinÃmico de spins em materiais magnÃticos à influenciado pela geometria que eles apresentam. AlÃm disso, outro aspecto relevante à a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnÃticas em sistemas de baixa dimensionalidade, que à devido em grande parte as aplicaÃÃes tecnolÃgicas, tais como: nanosensores, gravadores magnÃticos de alta densidade, dispositivos magneto-eletrÃnicos, etc. Neste trabalho estudamos a propagaÃÃo de ondas de spin em nanofios magnÃticos cilÃndricos, onde a abordagem à feita utilizando teoria microscÃpica, atravÃs do Hamiltoniano de Heisenberg, em que os spins sÃo considerados fixos nos sÃtios da rede e cuja geometria da seÃÃo transversal dos cilindros à hexagonal. Entre as interaÃÃes magnÃticas estudadas consideramos: a interaÃÃo de troca que pode ser ferromagnÃtica se os primeiros vizinhos dos spins estÃo numa configuraÃÃo paralela, ou antiferromagnÃtica se estiverem antiparelelos; a interaÃÃo Zeeman que à devido ao campo magnÃtico externo aplicado ao sistema; a interaÃÃo de Anisotropia, esta sendo responsÃvel pela direÃÃo de magnetizaÃÃo preferida que diversos sistemas magnÃticos reais apresentam e a interaÃÃo dipolar de natureza magnetostÃtica, presente em todos os materiais. O formalismo leva em consideraÃÃo a dependÃncia espacial dos spins no sistema, onde os operadores de spin do hamiltoniano sÃo escritos em termos de operadores bosÃnicos de criaÃÃo e aniquilaÃÃo atravÃs da RepresentaÃÃo de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direÃÃo devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equaÃÃes matriciais no espaÃo dos vetores de onda. A partir desse sistema de equaÃÃes obtemos vÃrios espectros de excitaÃÃo como: a relaÃÃo de dispersÃo para as ondas de spin, que à o grÃfico onde mostra como a frequÃncia de ondas de spin varia em funÃÃo do vetor de onda e a variaÃÃo da energia do sistema com o campo aplicado.
The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra.
Mbulunge, Masevhe Hamisi. "Giant Magnetocaloric effect and Magnetic Properties of selected Rare-Earth compounds." University of the Western Cape, 2021. http://hdl.handle.net/11394/7926.
Full textRare-earth (RE) compounds have been an attractive subject, based on the unique electronic structures of the rare-earth elements. In particular, the RETX (RE = rare-earth, T = 3d/4d/5d, transition metals, and X = p – block elements) series is a large family of intermetallic compounds which crystallizes in different crystal structure depending on the constituents. Most of these compounds crystalize in the hexagonal, orthorhombic, and tetragonal crystal structure. On the other hand, the family of compounds RET2X2 adopted the tetragonal crystal structure of the ThCr2Si2 or the CaBe2Be2 with different space groups. Owing to the different crystal structure, these compounds show versatile magnetic and electrical properties such as Kondo effect, complex magnetic behaviour, valence fluctuation, unconventional and conventional superconductivity, heavy fermion behaviour, Fermi and non – Fermi liquid behaviour, metamagnetism, spin – glass, memory effect, crystal electric field (CEF), magnetoresistance and magnetocaloric effect. The history of magnetism reveals that it is closely related to practical applications and magnetic materials from the most vital components in many applications. These are memory devices, permanent magnets, transformer cores, magneto-mechanical devices and magneto-electronic devices. Recent additions to this list include magnetic refrigeration through the studies of magnetocaloric effect as well as spintronics. Magnetic refrigeration (MR) is an emerging technology and shows real potential to enter conventional markets and the principles of MR obeys the magnetocaloric effect (MCE), which is based on the effect caused by a magnetic field on the materials that accept the property of varying the magnetic entropy, as well as its temperature when varying the magnetic field. In this thesis, we report giant magnetocaloric effect and magnetic properties of NdPd2Al2 and RECuGa (RE = Nd, Dy, and Ho) compounds. These investigations were done through measurements of X – ray diffraction (XRD), magnetic susceptibility, ((T)), magnetization, (M(H)), isothermal magnetization, (M(H, T)), heat capacity, (Cp(T)) and electrical resistivity, ((T)). MCE has been studied from the isothermal magnetization and heat capacity measurements.The first chapter of the thesis describes the theoretical background from which the experimental results have been analyzed and interpreted. This is followed by the chapter which presents experimental details and methodology carried out in this thesis. Chapter three presents the results and discussion of the transport, magnetic and magnetocaloric properties of NdPd2Al2 compounds. XRD studies confirm the tetragonal CaBe2Ge2 – type structure with space group P4/nmm (No. 129). The results of (T), (T) and Cp(T) indicate a putative antiferromagnetic (AFM) phase transition at low temperature at, TN = 3 K. On the other hand, (T) data at high temperatures follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent Nd3+ ion. The magnetization results indicate metamagnetic – like transition at a low field that bears a first-order character which corroborates with the Below – Arrott plots. Giant MCE was obtained for the NdPd2Al2 compound similar to those reported for potential magnetic refrigerant materials. Chapter four discusses the magnetic and thermodynamic properties of the series of compounds RECuGa where RE = Nd, Dy, and Ho. XRD studies indicate the orthorhombic CeCu2 – type crystal structure with space group Imma (No. 74) for all three compounds. Magnetic measurements indicate a putative AFM phase transition below 𝑇𝑁 = 7.1, 8.5, and 3.7 K for Nd, Dy, and Ho compounds, respectively. The high-temperature (T) data for all three compounds follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent rare-earth ion. Again, large MCE were obtained for all three compounds similar to those reported for materials that can be used as magnetic refrigerant materials.