Academic literature on the topic 'Ferromagnetism'

We report the discovery of ferromagnetism in the cubic CsCl-type Ti21∼25Nb20∼24Cr5∼10Ru∼49 multicomponent alloy. In metals, the appearance of ferromagnetism due to the Cr magnetic moment is a rare phenomenon. The purest sample shows ferromagnetism with the Curie temperature of 38 K. The effective magnetic moment and the Weiss temperature are 3.67 μ B/Cr and 58 K, respectively, derived from the temperature dependence of dc magnetization. These values mean the ferromagnetic exchange interaction between the localized Cr magnetic moments. The ferromagnetic nature is also confirmed by the isothermal magnetization curve with the highest magnetization of 1.1 μB/Cr at 2 K. The electronic structure calculation also supports a ferromagnetic ground state in the CsCl-type structure. We further investigated the effect of elemental substitution on the ferromagnetic behavior. The partial substitution of Pd for Ru heavily suppresses the Curie temperature, indicating that the Ru atom may play an essential role in sustaining ferromagnetism. Ti21∼25Nb20∼24Cr5∼10Ru∼49 would be the first example of the ferromagnetic Cr-containing multicomponent alloy, and this study shows the usefulness of the large compositional space in exploring novel phenomena.

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Transition metal (TM) doped NiO is a promising candidate of p-type oxide diluted magnetic semiconductors (DMSs), which shows obvious room-temperature ferromagnetism. When researching the magnetic properties of DMSs, it is very important to get rid of ferromagnetic impurity phases by optimizing the preparation process. For this purpose, pure NiO thin films have been deposited by a pulsed laser deposition method and annealed by different annealing process. As-deposited or low-temperature annealed films show room-temperature ferromagnetism and high-temperature annealed films are not ferromagnetic. Nickel metal should be the origin of ferromagnetism in these thin films. On the other hand, high annealing temperature influences the microstructure of the film surface a lot. These results show a useful method to eliminate ferromagnetic impurity in NiO thin films and to optimize the preparation parameters of TM doped NiO thin films.

A robust constitutive model is desirable in order to guide the processing and development of ferromagnetism and for use in the design of ferromagnetic devices. In this paper, a non-linear constitutive model of soft ferromagnets is developed for general magnetomechanical loading histories. The experimental set-up and measured techniques, which were employed to measure the non-linear deformation of both the magnetostrictive materials and the soft ferromagnetic materials subjected to coupled magnetomechanical loading, are introduced. The general structure for the constitutive behaviour of ferromagnets including soft-ferromagnetic and hard-ferromagnetic materials is similar to the classical models of metal plasticity; i.e. a convex yield surface can be identified in a stress-magnetic field space, within which the deformation and magnetization are reversible, and increments of both remanent magnetization and remanent strains are normal to the yield surface. The state of materials is described in terms of the remanent strain and the remanent magnetization that are introduced as internal variables besides stress, strain, magnetic field and magnetization. The one-dimensional model is examined by numerical stimulations and the response of the ferromagnetic materials is discussed in comparison with the measured results. The macroscopic features of ferromagnets, such as hysteresis loop and magnetostrictive hysteresis, are predicted.

In recent years, ferromagnetism induced by natural defects of nonmagnetic semiconductors has been widely investigated and expected to be applied in spintronics. On this basis, we report the ferromagnetic behavior of copper (I) nitride (Cu3N) nanoparticles. A robust room temperature ferromagnetism is found in Cu3N nanoparticles with the saturated magnetization of 4 memu/g (300 K). Based on the element-specific X-ray magnetic circular dichroism (XMCD) and the density functional theory (DFT) analysis, it is concluded that the ferromagnetism of Cu3N nanoparticles originate from the surface Cu vacancies. Moreover, by increasing the surface area of Cu3N, the variation of magnetism is realized, and the surface states related to ferromagnetism is further revealed.

Successful synthesis of room-temperature ferromagnetic semiconductors, (Cu, Co) co-doped ZnO film is obtained by sol-gel method. It is found that the essential ingredient in achieving room-temperature ferromagnetism is Cu co-doping. By Hall-effect measurement ap-type conductivity was observed for the Cu co-doped films, which induced the room-temperature ferromagnetism.

Multiferroic materials have attracted significant research attention due to their technological potential for applications as multifunctional devices. The scarcity of single-phase multiferroics and their low inherent coupling between multiferroic order parameters above room temperature pose a challenge to their further applications. We propose a 3BiFeO3/7BaTiO3 perovskite–perovskite composite that combines ferroelectricity and ferromagnetism. We demonstrate that the sintering temperature can tailor the ferroelectricity and ferromagnetism of the composites. The multiferroicity can be achieved at a low sintering temperature in the composite-like structure ceramics, and its multiferroic properties, especially the ferromagnetism, are superior to those of solid solutions. We also investigate the dynamic evolution of multiferroicity with sintering temperature. We adopt a nano–micro strategy to construct a composite-like microstructure, which results in optimized ferroelectric (1.62 μC cm−2) and ferromagnetic (0.16 emu/g) characteristics at a sintering temperature of 750 °C. We also found experimental evidence of the competition between antiferromagnetic and ferromagnetic interactions in the transition metal cation sublattice. Multiferroic BiFeO3/BaTiO3 composites with combined ferroelectric and ferromagnetic properties have significant potential for various applications.

ZnO doped with a few per cent of magnetic ions such as Ni, Fe, Co exhibits room temperature ferromagnetism (RTFM), transforming it into a very promising candidate for future spintronic applications. Two samples i.e. ZnO doped with Ni and Cr (5% each) have been investigated in the present work. The samples were characterized by Rietveld refinement of X-ray diffraction (XRD) patterns and the superconducting quantum interference device (SQUID) magnetometry. Rietveld analysis confirms that both the polycrystalline samples possess wurtzite structure with no evidence of any secondary phase. The SQUID measurements exhibit a diamagnetic state for the pristine ZnO and a paramagnetic state for the as-synthesized (Cr and Ni)-doped ZnO samples. However, the post annealing in H2 and vacuum drive them to a remarkable ferromagnetic state at room temperature. No element specific signature for ferromagnetism was seen. Then the X-ray photoelectron spectroscopic (XPS) measurements were performed to investigate their electronic structure and exploring the origin of ferromagnetism in these diluted magnetic semiconductor materials. The XPS results confirm the creation of oxygen vacancies upon Hydrogen/ vacuum annealing, owned to the (Ni/Cr) 3d¬−O 2p hybridization. The findings suggest oxygen vacancies as the intrinsic origin for ferromagnetism in doped ZnO. The important feature of this work is that the ferromagnetism and the consequent electronic property changes are found to be reversible with regard to re-heating the samples in air, showing a switch “on” and “off” ferromagnetic ordering in the ZnO matrix.

ZnO and doped ZnO films with non-ferromagnetic metal have been widely used as biosensor elements. In these studies, the electrochemical measurements are explored, though the electrical impedance of the system. In this sense, the ferromagnetic properties of the material can be used for multifunctionalization of the sensor element using external magnetic fields during the measurements. Within this context, we investigate the room-temperature ferromagnetism in pure ZnO and Ag-doped ZnO films presenting zigzag-like columnar geometry. Specifically, we focus on the films’ structural and quasi-static magnetic properties and disclose that they evolve with the doping of low-Ag concentrations and the columnar geometry employed during the deposition. The magnetic characterization reveals ferromagnetic behavior at room temperature for all studied samples, including the pure ZnO one. By considering computational simulations, we address the origin of ferromagnetism in ZnO and Ag-doped ZnO and interpret our results in terms of the Zn vacancy dynamics, its substitution by an Ag atom in the site, and the influence of the columnar geometry on the magnetic properties of the films. Our findings bring to light an exciting way to induce/explore the room-temperature ferromagnetism of a non-ferromagnetic metal-doped semiconductor as a promising candidate for biosensor applications.

Abstract Nanocrystalline powders of Fe-doped SnO2 (Sn1-xFexO2) (x = 0.00, 0.01, 0.03, 0.05) were prepared by a hydrothermal method. The powders were calcined in argon atmosphere at 600 °C for 2 h, causing phase transition from diamagnetic and weak ferromagnetic behavior to a ferromagnetic state. No trace and other magnetic impurity phases was detected in the samples with Fe content up to 3%. The calcined samples of Fe-doped SnO2 revealed the room temperature ferromagnetism with highest magnetization values of 434.07 memu/g at 15 kOe for x = 0.05. The room temperature ferromagnetism of samples originated from oxygen vacancies that occurred in the argon calcination process. In particular, oxygen vacancy shows a significant role in ferromagnetic coupling corresponding to F-center interaction.

Orientadora: Fanny Béron
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-28T23:44:04Z (GMT). No. of bitstreams: 1 Soares_Gabriel_M.pdf: 11563028 bytes, checksum: 679e40dbf51a0c4198966c4f0eee8e20 (MD5) Previous issue date: 2015
Resumo: Desde a descoberta dos filmes finos magnéticos com anisotropia perpendicular, ou PMAs (Perpendicular Magentic Anisotropy, em inglês), se tornou possível aumentar drasticamente a densidade de dados em discos rígidos. Uma maneira de continuar a incrementar a densidade final de dados seria desalinhar o eixo de anisotropia uniaxial e o campo magnético de escrita. Para isso, necessita-se de materiais com anisotropia magnética intermediária, i.e, na qual o seu eixo fácil está entre a longitudinal ao plano e a sua perpendicular. A situação ótima seria com o eixo fácil a 450, teoricamente dobrando a densidade final. No entanto, fabricar grãos ou filmes com esta propriedade não é viável do ponto de vista industrial. Todavia, bicamadas podem ser utilizadas para imitar este comportamento, onde um filme com anisotropia longitudinal é depositado sobre um PMA. Além do mais, esta abordagem apresenta a possibilidade de maior velocidade de escrita. Bicamadas de CoCrPt/Ni podem ser utilizadas para este propósito, com a camada de CoCrPt como PMA e a de Ni com anisotropia longitudinal. O experimento de ressonância ferromagnética permite, a priori, resolver as contribuições de anisotropia de cada camada no sistema de bicamadas, juntamente com os mecanismos de amortecimento magnético. O objetivo principal desta dissertação de mestrado é introduzir os conceitos de experimentação e teoria desta técnica, e aplicá-los nas bicamadas de CoCrPt/Ni. Neste trabalho foram utilizados filmes já depositados por sputtering com 10 nm de Co66Cr22Pt12, seguidos de 5 a 40 nm de Ni, para investigar o efeito da camada com anisotropia longitudinal sobre o PMA. Um outro conjunto de amostras contém um espaçador de Ti entre as camadas magnéticas, a fim de se estudar a interação na interface entre elas. Os experimentos de ressonância foram realizados em varredura de frequência num analisador de rede vetorial com campos magnéticos aplicados longitudinais ou perpendiculares e numa cavidade de banda X (9,54GHZ) em função da orientação do campo aplicado. Foi verificado qualitativamente o decréscimo da energia de anisotropia longitudinal com menores espessuras de Ni nas bicamadas. No entanto, não foi possível observar a linha de absorção da camada de CoCrPt, e consequentemente, encontrar unequivocamente as constantes de anisotropia do sistema, muito menos confirmar o comportamento multiaxial encontrado. Não obstante, foi confirmado a natureza de curto alcance da interação entre as camadas. Nova experimentação em outras bandas de frequência é necessária, afim de identificar a absorção do CoCrPt. Uma perspectiva futura interessante é o estudo da largura de linha e o amortecimento magnético do sistema devido a interação envolvida
Abstract: Since the advent of perpendicular magnetic anisotropy media, or PMAs, it was possible to greatly enhance the data density of hard drives. A method to push forward the data density increase is to misalign the anisotropy and applied field axes. This can be achieved by tilted media, i.e., in which the anisotropy axis lays somewhere between in-plane (IP) and out-of-plane (OOP). The optimal condition is with the axis tilted by 45\textsuperscript{0}. However, produce this kind of material is not practible at industrial scale. Thus, composite media can be used to mimic this behavior, where an in-plane anisotropy media is deposited on top of a PMA. Nonetheless, this approach allows to reach faster switching rates. CoCrPt/Ni bilayers may be used for this purpose, with the CoCrPt serving as the PMA and the Ni as the IP anisotropy layer. The ferromagnetic resonance experiment (FMR) can be used to obtain information about the magnetic anisotropy, being able to characterize its constant for each layer in the bilayer system, together with the damping mechanisms. Therefore, the main goal of this master thesis is to introduce the FMR theory and experimentation and apply them in this system. The used films were already deposited by sputtering with 10 nm of Co.66Cr.24Pt.12 under a 5 to 40 nm Ni layer. Another set of samples with a Ti layer between the magnetic layers provides information of the interface coupling. The FMR experiments were partly performed in a broadband vector network analyzer (VNA) with applied magnetic fields IP or OOP, and in a X-band (9,54 GHz) cavity as function of the orientation of the applied magnetic field. It was qualitatively verified a decrease of the longitudinal magnetic anisotropy energy as a function of the Ni thickness in the bilayer system. However, the CoCrPt single layer absorption could not be observed, making it impossible to unequivocally identify each anistropy constant. Nonetheless, the short-range nature of the coupling was confirmed. Further experimentation in higher frequencies is needed in order to find the CoCrPt absorption. Also, promising effects in the damping mechanism due to the coupling are expected
Mestrado
Física
Mestre em Física
2013/1186360
CAPES

The goal of this work is to investigate the magnetic deflagration phenomena in two very different magnetic systems, using two very different experimental techniques. In the first chapter of this dissertation I introduce the concept of magnetic deflagra-tion, together with a description of the state of the art of the field. In the second chapter of this thesis I present the study of magnetic deflagration in single crystals of the prototypical single molecule magnet Mn12—ac, using the magneto-optical imaging method; never done before. In the chapter the reader will find out that, effectively, the deflagration process can be explored using this technique. The second part of the thesis include the other five chapters. Such a difference in length between both parts is due two factors. The first factor is that the Mn12—ac is a very well known system, with few new experiments to study; while on the contrary, the Nd5Ge3 intermetallic compound has been barely studied in its single crystalline form, and therefore there are many experiments to conduct to understand its dynamics. The second factor is the rich phenomena that this intermetallic compound presents directly related with the abrupt magnetic changes that possesses. In this system sudden changes occur in every studied physical property when the magnetization changes in a stepped manner. Two fundamental properties of the magnetic dynamics of the Nd5Ge3 system make it so remarkable. First, it is one of few systems with a large irreversible AFM-+FM transition induced by the magnetic field, and second, the magnetic changes during its magnetization processes occur in a stepped manner. Therefore, the motivation to investigate the possibility of finding for the first time magnetic deflagration phenomena in a ferromagnet, and also for the first time in two different magnetic phases of a given material, was very high. In the first two chapters dedicated to the Nd5Ge3 I explore the magnetic properties, the heat capacity and the electric resistivity. Some of the measurements were not previously reported in a single crystal. In the fifth chapter I explore explicitly the spon-taneous field-induced avalanches in magnetism, heat capacity and resistivity. The reason of this three chapters is to have as much ingredients as possible to be able to answer the questions that the possible magnetic deflagrations would raise. I say possible because it is not until the fifth chapter of this thesis when we get the experimental confirmation that the field-induced spontaneous avalanches that the compound present correspond to magnetic deflagration phenomena. In this chapter the magnetic deflagrations were studied in detail in the AFM-+FM transition and in the FM reversal, as a function of the magnetic field and the temperature. The spontaneous and induced deflagrations are explored, obtaining good estimations of the propagation speed and the temperature of the deflagration flame. Using the theoretical framework of the magnetic deflagrations we fit the speed propagation of the front to the experimental data using only one pa-rameter, the heat diffusivity of the material; obtaining a value within the range of heat diffusivities found in other intermetallic compounds.
El objetivo que persigue esta tesis es impulsar el estudio de las deflagraciones magnéti-cas gracias, por una parte al descubrimiento del fenómeno en un sistema nuevo y pro-metedor como es el compuesto intermetálico Nd5Ge3, y por otra a la presentación de un método nuevo de medición de las dependencias espacio-temporales de las mismas utilizando técnicas magneto-ópticas. Manteniendo el hilo conductor del fenómeno de las deflagraciones magnéticas, esta tesis doctoral se divide en dos partes. En la primera parte presento mis investigaciones en el estudio del sistema Mn12—ac. A partir de los tratamientos de los videos obtenidos se confirma la presencia de deflagraciones magnéticas. La segunda parte de la tesis está dedicada al compuesto intermetálico Nd5Ge3. Éste compuesto se trata de uno de los pocos sistemas en los que mediante un campo magnéti-co externo se induce espontáneamente un estado ferromagnético (FM) con gran irre-versibilidad proviniendo de un estado antiferromagnético (AFM). Además, los cambios magnéticos que experimenta el sistema, tanto dicha transición AFM—>FM como la in-versión de la magnetización en el estado FM, ocurren de forma muy abrupta, siendo también uno de los escasos sistemas que presenta esta propiedad. Dedico tres capítulos al estudio de sus propiedades magnéticas, térmicas y eléctri-cas, tanto estáticas como dinámicas. En esas medidas encuentro fenómenos interesantes, desde generación espontánea de voltaje durante las deflagraciones magnéticas, hasta la aparición de saltos espontáneos de la magnetización con el tiempo (manteniendo la tem-peratura y el campo magnético constantes), pasando por la obtención de términos de origen antiferromagnético en la dependencia térmica de la capacidad calorífica del estado ferromagnético saturado, o una magnetorresistencia gigante entre ambos estados, entre otros. En el sexto capítulo, las medidas experimentales confirman la existencia del fenómeno de la deflagración magnética en ambas fases, AFM y FM. La velocidad de propagación del frente obtenida en la teoría de deflagraciones se ajusta bien a los datos experimen-tales. Utilizando la bondad del ajuste, extrapolamos la velocidad teórica hacia campos magnéticos elevados y encontramos la posibilidad de que ésta iguale o supere la velocidad del sonido en el material. Lo más remarcable es que esta posible transición se observa en la extrapolación para campos menores de 50 kOe. Por lo que, en principio, reduciendo la temperatura podríamos ser capaces de obtener medidas de dicha transición. Sin em-bargo, el estudio de las deflagraciones espontáneas en función de la temperatura llevado a cabo en un criostato de dilución resultó un claro ejemplo de serendipia. En vez de alcanzar velocidades supersónicas, lo que encontré fueron unas discontinuidades de salto en los campos de deflagración espontánea no predichas. Por lo tanto, el capítulo pasa a enfocarse en su estudio, concluyendo que su origen está relacionado con propiedades intrínsecas del Nd5Ge3.

Orientador: Ezequiel Costa Siqueira
Banca: Victor Ciro Solano Reynoso
Banca: Rodrigo Yoshikawa Oeiras
Resumo: Neste trabalho é proposto a investigação teórica de uma junção formada por metal supercondutor e um metal ferromagnético acoplados através de dois canais. O primeiro canal consiste de um acoplamento direto entre os metais, enquanto que o segundo canal é formado através um ponto quântico composto por um nível discreto. Para isso utilizamos como ferramenta as funções de Green de não-equilíbrio, por meio das quais obtemos o cálculo e as curvas para a corrente elétrica, números de ocupação e transmitância. É demonstrado que podemos alterar o tipo de spin no ponto quântico, e este sobrevive a presença de um campo magnético por meio de um efeito de interferência. Por meio deste resultado, é possível manipular um spin em um ponto quântico por meio de variáveis externas, o que pode ser de interesse em aplicações na computação quântica
Abstract: In this work is proposed the theorical investigation of a double-path junction formed by a ferromagnetic and a supercondutor lead. The first path connects superconductor and ferromagnet by an insulator barrier while in the second path these metals are connected by a single level quantum dot. We have used the nonequilibrium Green's functions to perform the calculations as well as to obtain the curves for electrical current, occupation numbers and transmittance. It is shown that the spin within the quantum dot can be manipulated by means of external parameters which can be of interesed in quantum computation applications
Mestre

Multiferroic materials, in which two or more ferroic ordering take place in the same phase, have driven major interest in the last few years, not only due to the possibility of exploring novel physical properties in those materials, but also the implications that such properties show in novel technological applications. From those materials, the especially interesting are those in which the ferromagnetic (FM) and ferroelectric (FE) ordering take place, due to their direct application in magnetodielectric devices. In the field of multiferroic materials such materials could play an important role in a new generation of none volatile magnetic random access memories (M RAM), in which a sufficiently strong magnetodielectric coupling could allow for the modification of the magnetic state, not only with a magnetic field, but with an electric field. This fact would allow for a dramatic reduction in energy consumption and would promote the further technological integration (the major commercial drawback of MRAMs), due to the fact that an electric field, contrary to the magnetic field, can be applied locally. Additionally, such multiferroic materials could prove useful in magnetic tunnel junctions, in which the ferroelectric and ferromagnetic nature would allow them to codify four resistive states, instead of the traditional two states of ferroelectric or ferromagnetic junctions, allowing for the implementation of a generation of four state memories. The materials with perovskite structure, ABB"03 (A=Rare Earth, Bismuth, Lead and Yttrium), bring a broad spectrum of possibilities when it comes to design of multifunctional materials. This is due to the wide variety of A, B, B" cations that are compatible with such structure. However, in the case of R(NiMn)03, such oxides have been poorly studied and many detailed studies, both in bulk and thin films are needed. The cation selection of B and B' seems to transform the paramagnetic ordering (PM) into FM below room temperature. The multiferroicity of these materials is typically provided by the A cation of the perovskite formula, which can be Bi or Pd, in order to create a Type 1 multiferroic. In this type of materials, i.e: Bi2NiM n06, the ferroelectricity and ferromagnetism arose by separate mechanisms, the FE is provided by the A cation, with so called long pair electrons, which are free electrons in the valence band that do not participate in any chemical reaction in the compound, while the Ni2+(d8) and (M n4+) (d3) provides the FM. However, even though the materials are multiferroic, their magnetodielectric coupling, crucial for future industrial applications, is weak, due to the different mechanisms that provide their FM and FE ordering. On the other hand, the FE induction by geometrical distortion of the perovskite lattice, for example in YM n03, is an interesting case since rotations of the M nO6 octahedrons promote an important structural change, in which the oxygen atoms move closer to the Y and, due to a large dipole interaction, generate a stable FE state. Moreover, the deformation of the unit cell generates a weak spin canting on the Mn cations, that can be promoted by Li doping or lattice distortions. This behavior could prove useful in the R(NiM n)06 family, which shows strong FM . This thesis is devoted to the study of R(Ni0.5M n0.5)03 (Y,Sm, Nd and Pr) and Bi(Fe0.5M n0.5)06 grown in thin films by pulsed laser deposition technique. Firstly, this thesis focuses on the growth and characterization of thin films of Y(Ni0.5M n0.5)03 (YNM 0) on strontium titanate substrates SrTiO3(001) (STO). The influence of the deposition parameters, such as temperature, fluence and ablation frequency, on the morphology and crystalline quality of the films is investigated. The study reveals that the YNMO films grown on STO(001,011 and 111) substrates are epitaxial and that their crystalline quality and epitaxial relationship are similar to those of the YMO compound. In particular, it is observed that a single out of plane domain is the norm for all the substrate orientations, while there are various in-plane domains. Moreover, chemical composition studies reveal Ti diffusion from the substrate to the YNMO film when STO(111) substrates are used. Once the growth conditions of YNMO are optimized, the magnetic and dielectric properties are studied. All the films show a paramagnetic to ferromagnetic transition at a temperature around 95K, with a magnetic moment of YNMO(001) = 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirming the ferromagnetic nature of the samples. The dielectric characterization reveal a FE ordering on the YNMO films, and what is more, the existence of a dielectric anisotropy on the films, that is characterized by the absence of ferroelectric response on YNMO samples deposited on STO(001), while YNMO samples on STO(111) show a strong FE response. This anisotropy could be explained, according to recent theoretical studies, in the improper origin of the observed ferroelectriciy. The coexistence of FM and FE response shows in a conclusive manner the multiferroic nature of the YNMO compound. Secondly, studies similar to those previously presented are performed for thin films of R(Ni0.5Mn0.5)O3 (Sm, Nd and Pr) compounds grown on STO(001). In this case the deposition temperature turns out to play a crucial role on the epitaxial growth of all the studied compounds. It is shown that the ratio between the b/a lattice parameters influences the epitaxial growth of the films, being the decisive factor between single or multi domain films. All the samples show a PM to FM transition at temperatures around 190K Finally, films of Bi(Fe0.5Mn0.5)O6 have been grown on STO(001) substrates. The films are epitaxial and grow under epitaxial strain. Samples show a FM behavior at room temperature with a weak signal of 7,42 emu/cm3 and 0,4 µB/f.u(Fe-Mn). The dielectrical characterization shows the influence of external magnetic fields on the dielectric properties of the film above room temperature.
Los materiales multiferroicos, en los que dos o más ordenes ferroicos tienen lugar en la misma fase, ha despertado gran interés en los últimos años debido, no solo al hecho de explorar nuevas propiedades físicas en los materiales, sino también a las implicaciones de las nuevas propiedades funcionales en las aplicaciones tecnológicas. De dichos materiales resultan especialmente interesantes aquellos que presentan un orden ferroeléctrico (FE) y ferromagnético (FM) debido a su aplicación directa en dispositivos magnetoelectrónicos. En este ámbito los materiales multiferroicos podrían tener una gran relevancia en una nueva generación de memorias magnéticas RAM (MRAM) de control eléctrico, no volátiles, en las que, si el acoplamiento magnetoeléctrico es suficientemente grande, se podría modificar el estado magnético no con un campo magnético sino con un campo eléctrico. Este hecho permitiría una reducción radical en el consumo de potencia y favorecería a su vez una mayor integración (la principal desventaja de las MRAMs para competir en el mercado), ya que el campo eléctrico, a diferencia del campo magnético, puede aplicarse de forma muy localizada. Por otro lado, dichos materiales multiferroicos podrían emplearse en una nueva generación de uniones túnel, en las que el carácter ferroeléctrico y ferromagnético permitiría codificar información en cuatro estados resistivos en lugar de en dos, como viene siendo hasta ahora en las convencionales uniones túnel magnéticas o ferroeléctricas, dando lugar a una nueva generación de memorias de cuatro estados. Los materiales con estructura perovskita, ABB '03, (A=Tierra Rara, Bismuto, Plomo e Ytrio) ofrecen una gran versatilidad a la hora de diseñar materiales funcionales debido a la gran variedad de cationes A, B y B' compatibles con tal estructura. Sin embargo en el caso de R(NiMn)03, estos óxidos han sido poco estudiados y muchos carecen de estudios detallados tanto en forma másica como en capa fina. Esta selección de cationes en la posición B y B' parece transformar la estructura perovskita la cual típicamente presenta un ordenamiento paramagnético (PM) en FM a temperaturas inferiores a la ambiente. El carácter multiferroico de estos materiales es típicamente aportado por el catión A en la formula perovskita, el cual puede ser un átomo de Bi, o Pb, para crear un multiferroico tipo 1. En los materiales de este tipo, por ejemplo el Bi2NiMnO6, la ferroelectricidad y el ferromagnetismo provienen de fuentes diferentes, el carácter FE es aportado por el catión A con -lone pairs electrons-, los cuales son electrones libres en la banda de valencia que no participan en las reacciones químicas del compuesto, mientras la combinación Ni2+ (d8) and Mn4+ (d3) aporta el FM. Pese al carácter multiferroico de estos materiales su acoplamiento magnetoelectrico, indispensable para sus aplicaciones industriales futuras, es débil, puesto que su FE y FM provienen de efectos independientes. Por otra parte la inducción de FE por distorsiones geométricas de la celda perovskitas, como es el caso de YMnO3 (YMO), es un caso interesante de considerar ya que la rotación de los octaedros Mn05 genera un cambio estructural importante, en el cual los oxígenos se desplazan a una posición más cercana al Y, esto sumado a una larga interacción de los dipolos conduce al material a un estado FE estable. Además la deformación de la celda genera un débil FM en este material, el cual proviene un pequeño giro en los espines del Mn ya sea debido a un dopaje con Li o por la deformación de la celda. Este comportamiento podría resultar interesante en la familia de perovskitas R(NiMn)03 las cuales presentan un fuerte FM. Esta tesis está dedicada al estudio de la perovskitas R(Ni0.5Mn0.5)O3 (Y, Sm, Nd y Pr) y Bi(Fe0.5Mn0.5)O6 crecidas en capa fina usando la técnica de depósito mediante ablación por láser pulsado. En primer lugar, esta tesis se centra en el crecimiento y caracterización de capas finas del compuesto Y(Ni0.5Mn0.5)O3 (YNMO) sobre substratos de titanato de estroncio, SrTiO3(001) (STO). Se estudia la influencia de los parámetros de depósito tales como temperatura, fluencia y frecuencia de ablación sobre la morfología y la calidad cristalina de las capas obtenidas. El estudio pone de manifiesto que las capas de YNMO crecidas sobre substratos de STO(001,011 y 111) son epitaxiales de YNMO y que la calidad cristalina y las relaciones epitaxiales entre la capa y el substrato son semejantes a las obtenidas en el compuesto YMO. En particular se observa un único dominio cristalino fuera del plano independientemente de la orientación del sustrato, mientras que dentro del plano se presentan varios dominios cristalinos. Por otra parte, los estudios de composición química revelan una difusión de Ti desde el sustrato hacía la capa de YMNO cuando se utilizan substratos STO(111).. Una vez optimizadas las condiciones de crecimiento del compuesto YNMO, se estudian sus propiedades magnéticas y dieléctricas. Todas las capas presentan una transición de fase paramagnetica a ferromagnética a una temperatura alrededor de 95K con un momento magnético de YNMO(001)= 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirmando el carácter ferromagnético de las muestras. La caracterización dieléctrica revela el carácter FE de las capas de YNMO y lo que es más interesante, la existencia de anisotropía dieléctrica en las capas, ésta se pone de manifiesto en la ausencia de respuesta FE en capas YNMO sobre STO(001) que contrasta con la fuerte respuesta de las capas de YNMO sobre STO(111). Esta anisotropía puede tener su origen, a la luz de los recientes estudios teóricos, en el carácter impropio de la ferroelectricidad observada, a la luz de recientes estudios teóricos. La coexistencia de FM y FE muestra de manera conclusiva el carácter multiferroico del compuesto YNMO. En segundo lugar se han realizado estudios similares a los anteriores para el caso de capas finas de los compuestos del tipo R(Ni0.5Mn0.5)O3 (Sm, Nd y Pr) crecidas en STO(001). En este caso la influencia de la temperatura de depósito resulta ser un factor importante para la obtención, en todos los compuestos estudiados, de crecimiento epitaxial. Se observa que el cociente b/a entre las constantes red juega un factor importante en la epitaxia de las capas, siendo este cociente un factor determinante en el crecimiento mono-dominio o multi-dominio de las capas. Todas las muestras presentan transiciones PM a FM a temperaturas alrededor de 190K. Por último, se han crecido y estudiado capas finas del compuesto Bi(Fe0.5Mn0.5)O6 depositadas sobre STO(001). Las capas obtenidas son epitaxiales y crecen sometidas a estrés inducido por el substrato. Presentan comportamiento FM a temperatura ambiente pero con una débil señal de 7,42 emu/cm3 y 0,4 µB/f.u(Fe-Mn). La caracterización dieléctrica pone de manifiesto la influencia, a temperaturas superiores a la ambiente, de la presencia de campo magnético sobre las propiedades dieléctricas.

Orientador: Amir Ordacgi Caldeira
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Por muitas décadas, fenômenos quânticos foram observados com partículas microscópicas, tais como átomos, elétrons e fótons. No entanto, avanços na fabricação e controle de sistemas físicos com dimensões extremamente reduzidas vêm permitindo a manifestação de eventos quânticos em proporções gigantescas. Por exemplo, existem evidências de superposições quânticas com uma supercorrente composta por bilhões de elétrons num SQUID (Superconducting Quantum Interference Device). Motivados por tais evidências, nosso objetivo reside na busca de novos dispositivos capazes de exibir efeitos quânticos macroscópicos. Em particular, estamos interessados em sistemas ferromagnéticos que manifestem CQM (Coerência Quântica Macroscópica), isto é, ferromagnetos nos quais o campo de magnetização tunela periodicamente no tempo entre dois estados topologicamente distintos e degenerados. Nesta tese, sugerimos dois dispositivos: um ?o ferromagnético no qual uma parede de domínio tunela entre dois centros de aprisionamento arti?ciais; e um MQUID (Magnetic Quantum Interference Device), isto é, um análogo magnético do SQUID que permite efeitos de tunelamento com uma ¿supercorrente¿ formada por vórtices de spin. Esses dispositivos são úteis não só na exploração dos limites de validade da mecânica quântica, mas também abrem novas possibilidades de implementação de um bit quântico
Abstract: For many decades, quantum phenomena were observed with microscopic particles, such as atoms, electrons and photons. However, advancements in manufacture and control of physics systems with very small dimensions have allowed verifying quantum events in large proportions. For instance, there are evidences of quantum superposition with a supercurrent formed by billions of electrons on a SQUID (Superconducting Quantum Interference Device). Such evidences have driven our work in a way to investigate new devices that are capable to exhibit macroscopic quantum effects. In particular, we are interested in ferromagnetic systems that present MQC (Macroscopic Quantum Coherence), in other words, ferromagnets in which the magnetization ?eld tunnels periodically in time between two distinct and degenerate topological states. In this thesis, we have suggested two devices: a ferromagnetic wire in which a domain wall tunnels between two arti?cial pinning centers; and a MQUID (Magnetic Quantum Interference Device) that is a magnetic device analogous to SQUID that permit quantum tunneling effects with a supercurrent formed by spin vortices. These devices are useful to explore the limits of validity of quantum mechanics, as well they open new possibilities to put into operation a quantum bit
Doutorado
Física
Doutor em Ciências

Defect-induced ferromagnetism is attracting intensive research interest. It not only challenges the traditional opinions about ferromagnetism, but also has some potential applications in spin-electronics. SiC is a new candidate for the investigation of defect-induced ferromagnetism after graphitic materials and oxides due to its high material purity and crystalline quality. In this thesis, we made a comprehensive investigation on the structural and magnetic properties of ion implanted and neutron irradiated SiC sample. In combination with X-ray absorption spectroscopy and first-principles calculations, we try to understand the mechanism in a microscopic picture. For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. We qualitatively explain the magnetic properties as a result of the intrinsic clustering tendency of defects. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling. Thus, we provide a direct correlation between the collective magnetic phenomena and the specific electrons/orbitals. With the aim to verify if the defect-induced magnetization can be increased by orders of magnitude, i.e., if a sample containing defects through its bulk volume can persist ferromagnetic coupling, we applied neutron irradiation to introduce defects into SiC. Besides a weak ferromagnetic contribution, we observe a strong paramagnetism, scaling up with the neutron fluence. The ferromagnetic contribution induced by neutron irradiation only occurs in a narrow fluence window or after annealing. It seems non-realistic to make the bulk specimens ferromagnetic by introducing defects. Instead, we speculate that defect-induced ferromagnetism rather locally appears in particular regions, like surface/interface/grain boundaries. A comparable investigation on neutron irradiated graphite supports the same conclusion.

The goal of this thesis is to explore and control the magnetization dynamics on magnetic multilayered thin films through two different techniques: the application of strain and spin- polarized currents, which represent lower-power consumption approaches to the control of magnetization dynamics compared with conventional techniques. The ferromagnetic materials with nanometric thickness used in this thesis are magnetic materials widely used in research. Aside the purely scientific interest, these materials are potentially applicable in telecommunications or technologies for storing and transmitting information at high speeds. 1. Magnetization Dynamics Induced by the Application of Oscillating Strain The first part of the thesis studies the magnetization dynamics induced by the application of dynamic strain on the magnetic material. The strain deforms the magnetic material and induces a change in the direction and intensity of the magnetic anisotropy. Therefore, the magnetic states are affected by this variation and align with the new direction of magnetic anisotropy inducing dynamics in the magnetization. The main result of the first part of the thesis is the simultaneous time- and space-resolved observation of both the piezoelectric voltage wave associated to the SAW and the induced magnetization excitations on the ferromagnetic thin film of Nickel (Ni). We have found that manipulation of magnetization states in ferromagnetic thin films with SAWs is possible at the picosecond scale with efficiencies as high as for the static case. In Chapter 3 we have studied Ni nanostructures whose magnetization dynamics are governed by the intrinsic configuration of the magnetic domains and by their orientation with respect to the SAW- induced strain resulting in considerable delays between strain and magnetization. In Chapter 4 we have studied extended Ni thin film, on which SAWs induce spin waves that propagate millimeter distances and have a rotation amplitude of about 25 deg. 2. Magnetization Dynamics Induced by the Spin-Polarized Current The second part of the thesis studies the magnetization dynamics induced by the application of spin-polarized current through the magnetic material that exchanges magnetic moment with the magnetic spins of the electrons in the current. The current density has to be high to induce dynamics on the magnetization (~106-107 A/cm2) and this results in a reduction of the diameter of the electrical contact 50-200 nm. The main results of the second part of the thesis are related with the stability and the nucleation process of magnetic solitons. On the one hand, we have showed that magnetic solitons can exhibit multiple stable states, which are tunable with current or magnetic field. We also have correlated the existence of unstable states with an increment of low- frequency noise. Using simulations, we have identified the low-frequency spectra with the existence of drift resonances and we have observed that any asymmetry on the effective magnetic field suffered by the magnetic soliton can leads to drift resonances. On the other hand, we have experimentally observed that the processes of nucleation and annihilation of magnetic solitons have different intrinsic times, and using simulations we have identified a waiting time associated with the creation process, which make it a longer than annihilation. We also have studied, using micromagnetic simulations, the initial magnetization states that lead to the nucleation of topological and non-topological magnetic solitons.
La tesis gira en torno al estudio de la dinámica de la magnetización en capas y multicapas delgadas ferromagnéticas. Sin embargo, los sistemas estudiados son diversos y pueden clasificarse por la técnica utilizada para la excitación de la dinámica de la magnetización. Este hecho queda plasmado en la estructura de la tesis que consta de una introducción general, Capítulo 1, y luego de dos partes independientes y separadas, a su vez, en varios capítulos. El orden en la exposición de los resultados pretende seguir una linea lógica para su compresión. Como contrapartida, los resultados son presentados sin seguir un orden cronológico. La primera parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de tensión dinámicamente sobre el material magnético, que al deformarlo induce en él un cambio en la dirección e intensidad de la anisotropía magnética. Por lo tanto, los estados magnéticos se ven afectados por esta variación y cambian para alinearse con la nueva dirección de anisotropía magnética induciendo dinámica en la magnetización. La segunda parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de corriente polarizada a través del material magnético que intercambia momento magnético con los espines magnéticos de los electrones de la corriente. Para que esta transferencia de momento magnético sea efectiva la densidad de corriente ha de ser elevada (~106-107 A/cm2) y para conseguirla se reduce hasta los 50-200 nm el diámetro del contacto eléctrico. Los materiales ferromagnéticos con grosor nanométrico usados en esta tesis son materiales magnéticos usados ampliamente en la investigación. Aparte del interés puramente científico, estos materiales son potencialmente aplicables en telecomunicaciones o tecnologías del almacenaje y transmisión de información a altas velocidades.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Conselho Nacional de Desenvolvimento Científico e Tecnológico
Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro
Dois fenômenos de natureza antagônica juntos em um sistema híbrido podem apresentar propriedades muito diferentes, e um exemplo é o sistema híbrido de supercondutividade e ferromagnetismo, onde diversos novos fenômenos podem ser observados, como vórtices espontâneos. Aqui, dois sistemas híbridos supercondutor-ferromagneto foram estudados: Um consistindo de uma camada de Nb(200 nm) entre duas camadas de nanopartículas ferromagnéticas de Ni( ∼5 nm), preparadas por ablação a laser, com os gases Ar e O2 para produção das nanopartículas; O outro consiste de uma primeira camada feita de nanodiscos ferromagnéticos de Py( ∼1 m) desenvolvidos com litografia por feixe de elétrons, com uma disposição de rede quadrada com determinadas distâncias entre nanodiscos adjacentes, cobertos por uma segunda camada de Al2O3, ambos depositados por pulverização catódica, e por fim uma terceira camada supercondutora de Nb(200 nm) preparada por ablação a laser. As nanopartículas de Ni no primeiro sistema estão em contato direto com a camada de Nb e como resultado, o efeito de proximidade está presente no sistema. Diferentemente, os nanodiscos de Py no segundo sistema estão eletricamente isolados da camada de Nb, que pode eliminar o efeito de proximidade, assim a interação entre nanodiscos magnéticos e o Nb supercondutor ocorre somente através dos campos magnéticos remanescentes dos nanodiscos de Ni. A microestrutura estudada mostra que as nanopartículas feitas em gás Ar e O2 possuem formatos muito diferentes: uma (preparada em Ar) é cubica e a outra (preparada em O2) é esférica. Os diferentes formatos das nanopartículas de Ni apresentam influência muito diferente sobre as propriedades supercondutoras da camada de Nb: a amostra com nanopartículas de Ni(Ar) não apresenta uma transição de vortex vidro e a amostra com nanopartículas de Ni(O2) mostra um estado de vortex vidro bem claro sem qualquer campo magnético externo aplicado, indicado pelas medidas V(I). No segundo sistema, as medidas de transporte indicam a formação de clusters de vórtices na camada supercondutora sobre os nanodiscos magnéticos devido aos momentos magnéticos deles, e os vórtices induzidos por um único nanodisco podem formar uma fase de vortex vidro. A dimensão do espaçamento entre discos desempenha também um papel muito importante. A amostra com uma distância muito grande entre nanodiscos não mostrou uma curva V(I) com formato ’S’ mas possui uma fase vortex vidro; quando diminui a distância entre discos, as curvas V(I) próximas à temperatura de transição vortex vidro deformaram para um formato ’S’, indicando que os vórtices induzidos pelos diferentes nanodiscos estão interagindo uns com os outros quando as distâncias entre discos são menores do que um valor crítico.
Two phenomena with antagonistic nature together in a hybrid system can have very different properties and one of the samples is the hybrid of superconductivity and ferromagnetism in which many new phenomena can be observed, such as spontaneous vortices. Here two superconducting-ferromagnetic hybrid systems have been studied: one consists of a Nb layer(200 nm) between two layers of ferromagnetic Ni nanoparticles( ∼5 nm), which is prepared by pulsed laser deposition(PLD) with Ar and O2 for the production of Ni nanoparticles; the other consists of a first layer made of ferromagnetic permalloy (Py) nanodisks( ∼1 m)developed by e-beam lithography, with the arrangement of square lattice with certain distances between two adjacent nanodisks, covered by a second layer of Al2O3, both deposited by magnetron sputtering, and finally a third layer of superconducting Nb(200 nm) prepared by PLD. The Ni nanoparticles in the first system are in direct contact with the Nb layer and as a result, the proximity effect in the system is presented. In contrast, the Py nanodisks in the second system are electrically insulated from the Nb layer which can eliminate the proximity effect, thus the interaction between the magnetic nanodisks and superconducting Nb is through the magnetic stray fields of Ni nanodisks only. The microstructure study shows that the nanoparticles made in Ar and O2 gases have very different shapes: one (prepare in Ar) is cubic and the other (prepared in O2) is spherical. The different shapes of the Ni nanoparticles have very different influence on the superconducting properties of the Nb layer: the sample with Ni (Ar) nanoparticles does not show a vortex glass transition and the sample with Ni (O2) nanoparticles shows a very clear votex glass state without any external magnetic field applied, indicated by the V(I) measurements. In the second studied system, the transport measurements indicate the formation of vortex clusters in the superconducting layer on the top of the magnetic nanodisks due to the magnetic moments of them and the vortices induced by a single nanodisk may form a vortex glass phase. The spacing dimension between the disks plays a very important role as well. The sample with very large distance between the nanodisks does not show an ’S’ shape V(I) curve but has a vortex glass phase; when decrease the distance between the disks, the V(I) curves near the vortex glass transition temperature deformed to a ’S’ shape, indicating that the vortices induced by different nanodisks are interacting with each other when the distance between the disks are smaller then an critical value.

Ferroelectric and ferromagnetic alloy clusters are produced and studied in molecular beams. Nb clusters doped with 1-3 impurity atoms are ferroelectric with low transition temperatures. The alloy clusters with an even number of valence electrons have larger dipole moments than those with odd number of valence electrons. The ferroelectricity is suppressed by magnetic impurities or thermal excitations, and is enhanced by Au and Al doping. The observations strongly suggest that electron-pairing interactions exist in Nb clusters, which indicates Cooper pairing in clusters. The magnetic moments of Co clusters doped with small fraction of Mn,V and Al are studied and compared with those of the bulk alloys. CoMn alloy clusters have enhanced average magnetic moments with Mn doping, which is opposite to the behavior of bulk CoMn. CoV and CoAl alloy clusters behave similarly to their bulk counterparts. We explain the experimental results using the virtual-bound-state model. Finally, the magnetic properties of BiMn clusters are studied in molecular beams. The Mn local moments are found to couple ferromagnetically or ferrimagnetically depending on the composition of the clusters.