A ferromagnetic particle with single domain, at some kinds of applied field (at some angle or strangth), the particle's free energy would be two state model. The rate of barrier crossing could be solve by Fokker-Planck equation .And use master equation to find out the Total rate between two potential well. In this thysis, we use the upper method to simulate particle's magnetic moment under time varying magnetic field at fixed angle or fixed magnetic applied rotate the particle. In numerical method, we use the back Euler method to prevent the divergence of the calculation.

In the present study it was tried to create a diluted magnetic semiconductor on the basis of GaN and TiO2 by means of ion beam implantation. In most cases, by characterization of structural and magnetic properties, it was possible to prove that the ferromagnetic state is related to either spinodal decomposition or secondary phase formation. In case of Fe implanted GaN spinodal decomposition, epitaxially oriented alpha-Fe or epsilon-Fe3N nanocrystals were found to be responsible for the ferromagnetic behavior. In addition, the formation of gamma-Fe clusters was observed. Similarly, in TiO2 the ferromagnetism is related to the formation of epitaxially oriented alpha-Fe clusters. Dependent on the process parameters during annealing experiments several various secondary phases were formed. A critical examination of the references in literature points out the significance of usage of sensitive and complementary probe techniques (like CEMS, SQUID, XRD, EXAFS), in order to be able to discuss the origin of ferromagnetism in the field of diluted magnetic semiconductors in a proper way. / In der vorliegenden Arbeit wurde versucht, mittels Ionenimplantation verdünnte magnetische Halbleiter auf der Basis von GaN und TiO2 herzustellen. In den meisten Fällen konnte anhand von Charakterisierungen der strukturellen und magnetischen Eigenschaf- ten nachgewiesen werden, dass der ferromagnetische Zustand auf das Vorliegen von entweder spinodaler Entmischung oder kristalliner Ausscheidungen zurückgeführt werden kann. Im Fall von Fe-implantiertem GaN konnten spinodale Entmischung, epitaktisch ausgerichtete alpha-Fe- oder epsilon-Fe3N-Nanokristallite für den Ferromagnetismus verantwortlich gemacht werden. Daneben wird die Bildung von gamma-Fe beobachtet. Bei TiO2 ist Ferromagnetismus ebenfalls auf die Ausscheidung von epitaktisch orientierten alpha-Fe-Clustern zurückzuführen. In Abhängigkeit von den Prozessparametern bei Temperungsexperimenten bildete sich eine Reihe unterschiedlicher Sekundärphasen. Eine kritische Auseinandersetzung mit den Literaturangaben zeigt die Wichtigkeit des Einsatzes sensitiver, sich ergänzender Messmethoden (wie CEMS, SQUID, XRD, EXAFS), um die Ursache des Ferromagnetismus auf dem Gebiet der verdünnten magnetischen Halbleitern zu finden.

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.

Thesis (Ph.D.)--University of Western Australia, 2006.

Thesis (doctoral)--Rheinisch-Westfälische Technische Hochschule Aachen, 2004. / Includes bibliographical references (p. 223-235).

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

CUNHA, Anderson Magno Chaves. Correspondência entre ondas de spin de um ferromagneto em uma rede favo de mel e a banda de energia do grafeno. 2014. 88 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-08-29T18:55:19Z No. of bitstreams: 1 2014_tese_amccunha.pdf: 4561415 bytes, checksum: 8c98814cdc7a5600944fbf5d0bfb400f (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-08-29T18:56:57Z (GMT) No. of bitstreams: 1 2014_tese_amccunha.pdf: 4561415 bytes, checksum: 8c98814cdc7a5600944fbf5d0bfb400f (MD5) / Made available in DSpace on 2014-08-29T18:56:57Z (GMT). No. of bitstreams: 1 2014_tese_amccunha.pdf: 4561415 bytes, checksum: 8c98814cdc7a5600944fbf5d0bfb400f (MD5) Previous issue date: 2014 / Spin waves are collective excitations that occur in magnetic materials. These excitations are caused by disturbances in the magnetic system. For example, a small change in temperature causes the precession of a magnetic dipole moment that interacts with neighboring leading to the spread of this disorder. This disturbance has wave character, and can propagate in the direction of any of the nearest neighbors. These waves of spin can be observed by some experimental methods, such as: the inelastic neutron scattering, inelastic scattering of light including Raman and Brillouin scattering, to name a few. The importance of spin waves emerges clearly when magnetoelectronic devices are operated at low frequencies. This situation, the generation of spin waves can sing in a significant loss of energy of these systems, because the excitation of such waves consumes a small part of the energy of the system, becoming important in the innovation process of electronic systems. These waves can be studied using mathematical models like the Heisenberg, Ising, among others. In this model, we can calculate the dispersion relation of the spin waves. The Heisenberg model can be written in terms of operators of creation and destruction through the Holstein-Primakoff transformations. The Hamiltonian that describes the spin waves is now written in terms of bosonic operators. This mathematical description is similar to Tight-Binding Hamiltonian for fermions. This Hamiltonian described, for example, graphene, a material that has recently been discovered and is being treated with much optimism for having a two-dimensional structure that leads to amazing properties. Many possibilities of applications for it have been studied. Our goal here is to make an analogy between the graphene and a magnetic system on a honeycomb lattice. In the magnetic system, we use the Heisenberg model to find the dispersion relations and understand the behavior of the spin waves of the same. While in graphene, we used the Tight-Binding model to find the energy spectrum. Underscoring we use a mathematically identical method for both and found that the curves for power modes have similar behaviors, respecting the particularities of each. Then, we calculate how these modes behave introduction of impurities in substitution sites on one or two lines of the crystal lattice. / Ondas de spin são excitações coletivas que surgem em materiais magnéticos. Essas excitações são causadas por perturbações no sistema magnético. Por exemplo, uma pequena variação na temperatura provoca a precessão de um momento de dipolo magnético que interage com seus vizinhos levando à propagação dessa perturbação. Essa perturbação tem caráter ondulatório, e pode se propagar na direção de qualquer um dos vizinhos próximos. Essas ondas de spin podem ser observadas através de alguns métodos experimentais, tais como: espalhamento inelástico de nêutrons, espalhamento inelástico de luz incluindo espalhamento Raman e Brillouin. A importância das ondas de spin surge claramente quando aparelhos magnetoeletrônicos são operados a baixas frequências. Nessa situação a geração de ondas de spin pode ser um processo significante na perda de energia desses sistemas, pois a excitação de tais ondas consome uma pequena parte da energia do sistema, as tornando importante no processo de inovação dos sistemas eletrônicos. Essas ondas podem ser estudadas através de modelos matemáticos como o de Heisenberg, Ising, dentre outros. Nesse modelo, podemos calcular a relação de dispersão das ondas de spin. O modelo de Heisenberg pode ser escrito em termos de operadores de criação e destruição através das transformações de Holstein-Primakoff. O Hamiltoniano que descreve as ondas de spin é agora escrito em termos de operadores bosônicos. Essa descrição matemática é semelhante ao Hamiltoniano Tight-Binding para férmions. Tal Hamiltoniano descreve, por exemplo, o grafeno, um material que foi descoberto recentemente e vem sendo tratado com muito otimismo, por ter uma estrutura bidimensional que leva a propriedades surpreendentes. Muitas possibilidades de aplicações para ele vêm sendo estudadas. Nosso objetivo aqui é fazer uma analogia entre o grafeno e um sistema magnético em uma rede favo de mel. No sistema magnético, utilizamos o Modelo de Heisenberg para encontrar as relações de dispersão e conhecer o comportamento das ondas de spin do mesmo. Enquanto no grafeno, utilizamos o modelo Tight-Binding para encontrar o espectro de energia. Ressaltando que utilizamos um método matematicamente idêntico para ambos e que as curvas encontradas para os modos de energia são idênticas. Então, calculamos como esses modos se comportam com a introdução de impurezas em substituição em sítios de uma ou duas linhas da rede cristalina.

Made available in DSpace on 2014-10-09T12:26:12Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:32Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Neste trabalho, utilizamos o método de primeiros princípios RS-LMTOASA (Real Space - Linear Muffin-Tin Orbital - Atomic Sphere Approximation), baseado na Teoria do Funcional da Densidade (DFT Density Functional Theory) e implementado para o cálculo de estruturas magnéticas não colineares, para investigar as propriedades magnéticas de nanoestruturas adsorvidas em superfícies metálicas. Primeiramente, devido ao aparecimento de estruturas magnéticas complexas, foram estudadas nanoestruturas magnéticas depositadas em substratos ferromagnéticos. Foram consideradas nanoestruturas de Mn tais como nanofios adsorvidos, pirâmides e aglomerados adsorvidos e embebidos, com o tamanho destas nanoestruturas variando de dois até nove átomos depositados em Fe(001) e Fe(110). Nossos cálculos indicam uma interação de troca magnética de longo alcance entre os átomos de Mn-Mn e Mn-Fe. Além disto, a presença de uma forte dependência destas interações de troca magnética com o meio local, frustrações magnéticas e o acoplamento spin-órbita forneceram a possibilidade da presença de estruturas magnéticas complexas tais como, por exemplo, spin espiral e half-skyrmion. Por fim, com o objetivo de estudar nanoestruturas que podem apresentar altos valores de momento magnético, foram investigados nanofios de FexCo1-x adsorvidos em uma superfície de Pt(111) . Nossos resultados indicam que os momentos magnéticos de spin dos átomos de Fe e Co são independentes da concentração de Fe e apresentam valores superiores quando comparados aos das ligas FeCo bcc, enquanto que o momento magnético médio de spin do nanofio FexCo1-x varia linearmente com a concentração de Fe, comportamento este que é diferente da curva de Slater-Pauling observada na liga FeCo bcc. O momento magnético orbital médio do nanofio de FexCo1-x é monotonicamente decrescente com a concentração de Fe que se apresenta bastante diferente a monocamada FexCo1-x sobre Pt(111). / We use the first principles RS-LMTO-ASA (Real Space - Linear Muffin- Tin Orbital - Atomic Sphere Approximation) method, in the framework of the Density Functional Theory and implemented to calculate noncollinear magnetic structures, to investigate the magnetic properties of nanostructures adsorbed on metallic surfaces. First, due to presence of the complex magnetic properties, we investigated magnetic nanostructures deposited on a ferromagnetic substrate. We have considered a variety of nanostructures such as adsorbed wires, pyramids, at and intermixed clusters with sizes varying from two to nine atoms deposited on Fe(001) and Fe(110). Our calculations reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. Moreover, the presence of the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion. Finally, we also investigated FexCo1-x nanowires deposited on Pt(111) surface aiming to investigate materials with large local magnetic moment. Our results reveal that the Fe and Co spin magnetic moment are independent of the Fe concentration with the enhancement of the spin magnetic moment when compared with the FeCo bcc alloys, while the average spin magnetic moment is a linear function of the Fe concentration. This is in contrast to the Slater-Pauling model observed in the FeCo bcc alloys. The average orbital magnetic moment shows a linearly decreasing behavior with the Fe concentration which is in contrast to the behavior of FexCo1-x monolayer on Pt(111) surface.

Determinamos a tensão superficial de um defeito do tipo parede de domínio, utilizando a aproximação semiclássica no contexto da teoria de campos à temperatura finita para o modelo ? (?2)2 com interação invariante por simetria O(N), obtendo em seguida a temperatura crítica Tc de transição de fase e cálculo dos expoentes críticos ? e ?, aplicando então os resultados obtidos para a descrição de um modelo fenomenológico descrevendo o ferromagnetismo. / We determine the surface tension of a domain wall defect by using the semi classical approach in the context of field theory at finite temperature for the ? (?2)2 model with O(N) invariant interaction obtaining the critical temperature Tc of phase transition and the calculation of the critical exponents ? and ? applying then the obtained results to a description of a phenomenological model describing ferromagnetism.