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Theory of near-gap second harmonic generation in centrosymmetric magnetic semiconductors: Europium chalcogenides

Henriques, Andre Bohomoletz; ABRAMOF, E.; RAPPL, P. H. O.
Fonte: AMER PHYSICAL SOC Publicador: AMER PHYSICAL SOC
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
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Second harmonic generation is strictly forbidden in centrosymmetric materials, within the electric dipole approximation. Recently, it was found that the centrosymmetric magnetic semiconductors EuTe and EuSe can generate near-gap second harmonics, if the system is submitted to an external magnetic field. Here, a theoretical model is presented, which well describes the observed phenomena. The model shows that second harmonic generation becomes efficient when the magnetic dipole oscillations between the band-edge excited states of the system, induced by the excitation light, enter the in-phase regime, which can be achieved by applying a magnetic field to the material.; FAPESP; CNPq; (DAAD) Deutscher Akademischer Austausch Dienst, Germany

Spin-polarized transport in II-VI diluted magnetic semiconductors superlattices

Gomes, J. L.; Rodrigues, S. C. P.; Sipahi, Guilherme Matos; Scolfaro, L.; da Silva, E. F., Jr.
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
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We studied the spin-polarized charge densities in II-VI-based diluted magnetic superlattices formed of p-doped ZnTe:Mg/ZnTe:TM/ZnTe:Mg non-magnetic/magnetic/non-magnetic layers, with TM standing for transition metal. The calculations were performed within a self-consistent k.p method, in which are also taken into account the exchange correlation effects in the local density approximation. Our results show a limit for the width of the non-magnetic layer for which the difference between the opposite spin charge densities is maximized, indicating the best conditions to obtain full polarization by varying the TM content. We also discuss these effects in the calculated photoluminescence spectra. Our findings point to the possibility of engineering the spin-polarized charge distribution by varying the widths of the magnetic and non-magnetic layers and/or varying the TM concentration in the magnetic layers, thus providing a guide for future experiments. (c) 2012 Elsevier B.V. All rights reserved.; CNPq; CNPq [564.739/2010-3/NanoSemiCon, 303.880/2008-2/PQ, 470.998/2010-5/Univ, 472.312/2009-0/Univ, 304936/2009-0/PQ, 303578/2007-6/PQ, 577.219/2008-1/JP]; CAPES; CAPES; FACEPE [0553-1.05/10/APQ]; FACEPE; FAPESP; FAPESP

Propriedades eletrônicas de heteroestruturas semicondutoras magnéticas diluídas.; Electronic properties of diluted magnetic semiconductor heterostructures

Marin, Ivan Silvestre Paganini
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 28/02/2007 PT
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Neste trabalho e apresentado um estudo, via teoria de massa efetiva multibanda autoconsistente de heteroestruturas de semicondutores magnéticos diluídos, generalizada para incluir parâmetros de diferentes materiais. A interacao magnética e descrita por um modelo de campo médio baseado no mecanismo de troca indireta, com a possibilidade de inclusão de diferentes íons magnéticos. As equacoes de massa efetiva são resolvidas de forma autoconsistente com o auxílio da equacao de Poisson. As interacoes de spin-órbita e de troca-correlacao, na aproximacao de densidade local, são incluídas no cálculo. O método e aplicado para o estudo das estruturas de bandas e densidades de carga com separacao por spin do portador de heteroestruturas com dopagem tipo-n e tipo-p, variando a geometria dos pocos magnéticos e também o período da super-rede, as densidades de portadores e as concentracoes de íons magnéticos. Solucoes autoconsistentes da equacao de massa efetiva são encontradas para o oxido semicondutor (Zn,Co)O. Será mostrada a separacao de portadores por spin em funcao dos parâmetros variados, simulando diversas concentracoes possíveis, utilizadas em sistemas descritos na literatura, e será analisado o comportamento dos perfis de potencial. Usando os dados obtidos...

Síntese e caracterização de filmes à base de Si e Ge dopados com espécies magnéticas; Synthesis and characterization of Si and Ge based films doped with magnetic species

Ferri, Fabio Aparecido
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 09/08/2010 PT
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Recentemente, a dopagem de semicondutores (envolvendo compostos II-VI, IV-VI, III-V, e do grupo-IV) com espécies magnéticas tem sido extensivamente investigada em função do seu potencial em spintrônica. Neste contexto, semicondutores magnéticos baseados no Si e no Ge são atraentes devido à sua compatibilidade com a indústria de semicondutores existente. Entretanto, a solubilidade das espécies magnéticas nestes materiais em forma cristalina é muito baixa e, consequentemente, sua atividade magnética é limitada. Este não é o caso para o silício amorfo (a-Si) e o germânio amorfo (a-Ge), que podem conter elementos magnéticos além do limite de solubilidade de seus análogos cristalinos, e apresentar propriedades magnéticas notáveis. Motivado por estes fatos, este trabalho apresenta uma investigação abrangente de filmes finos de Si e Ge contendo diferentes quantidades de Mn e Co, trazendo informações úteis no entendimento das propriedades desta classe de materiais. As amostras foram preparadas por co-sputtering, e possuíram concentrações de Mn na faixa de ~ 0.1-24 at.%, e de Co na faixa de ~ 1-10 at.%. Após a deposição, os filmes foram submetidos a tratamentos térmicos cumulativos até 900 oC, e foram investigados por: espectroscopia de energia dispersiva de raios-x (EDS); espalhamento Raman; difração de raios-x; transmissão óptica; microscopias eletrônica de varredura (SEM)...

Orientação óptica de spin em semicondutores magnéticos - calcógenos de európio; Spin optical orientation in magnetic semiconductors-europium chalcogenides.

Galgano, Giovanni Decot
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 19/06/2012 PT
Relevância na Pesquisa
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A investigação das propriedades ópticas e sua relação com as propriedades magnéticas dos semicondutores é de grande interesse para a comunidade científica, em virtude da enorme demanda por novas tecnologias e funcionalidades que podem surgir dessas pesquisas. Os calcógenos de európio são semicondutores intrinsecamente magnéticos, transparentes na região visível do espectro eletromagnético e integráveis em matrizes de silício e nitreto de gálio, sendo assim fortes candidatos a aplicações tecnológicas envolvendo magnetismo e óptica. Neste trabalho são investigados os espectros de absorção e fotoluminescência dos calcógenos de európio, com base no modelo 4f -> 5d('t IND. 2g') da transição óptica de dipolo elétrico, o qual mostrou-se totalmente adequado para a descrição da absorção óptica nos calcógenos de európio em função do campo magnético aplicado, explicando a presença de linhas de absorção estreitas e dicróicas nos espectros em campo alto e a forma larga dos espectros de absorção em campo nulo. Nos espectros de fotoluminescência do EuTe, entretanto, foram detectados estados eletrônicos não contemplados pelo modelo 4f -> 5d('t IND. 2g') , em especial uma banda de emissão denominada 'MX IND. 0'...

Electronic and optical properties of diluted magnetic semiconductors quantum wells and quantum dots = : Propriedades eletrônicas e ópticas de poços quânticos e pontos quânticos de semicondutores magnéticos diluídos; Propriedades eletrônicas e ópticas de poços quânticos e pontos quânticos de semicondutores magnéticos diluídos

Udson Cabral Mendes
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 21/03/2014 PT
Relevância na Pesquisa
67.84%
Nesta tese, investigamos teoricamente as propriedades eletrônicas e ópticas de poços quânticos e pontos quânticos de semicondutores magnéticos diluídos. Este estudo é fortemente motivado por muitos resultados experimentais sobre as propriedades ópticas desse materiais. Usando a teoria do funcional da densidade dependente de spin descrevemos os estados eletrônicos como função do campo magnético externo para poços quânticos que possuem barreiras dopadas com impurezas magnéticas. Nosso modelo leva em conta os efeitos de muitos-corpos do gás de buracos e as interações entre portadores e os íons magnéticos. Comparamos nossos resultados com os dados experimentais disponíveis, que apresentam forte oscilações da luz polarizada circularmente como função do campo magnético. Nossos resultados apresentam excelente concordância qualitativa e quantitativa com os resultados experimentais. Mostramos que os efeitos de troca do gás de buraco são responsáveis pela forte oscilação observada na fotoluminescência. Também realizamos uma investigação sistemática dos parâmetros da heteroestrutura afim de aumentar a interação de troca entre portadores e íons de Mn. Com o nosso modelo entedemos os diferentes regimes de relaxação de spin do elétron em poços quânticos com barreiras dopadas com impurezas magnéticas. Nós também investigamos as propriedades eletrônicas e ópticas de pontos quânticos carregados dopados com uma única impureza magnética em seu centro. Usando métodos de diagonalização exata mostramos que os elétrons que não estão diretamente acoplados com o íon de Mn acoplam-se via uma interação indireta que é mediada pela interação elétron-elétron. Este acoplamento indireto entre elétrons e Mn pode ser tanto ferromagnético quanto antiferromagnético dependendo de ambos confinamento e número de camadas eletrônicas confinadas no ponto quântico. Demonstramos que este acoplamento indireto é um efeito importante mesmo quanto o íon de Mn não esta no centro do ponto quântico. O acoplamento indireto existe independentemente do tipo de interação direta entre portadores e a impureza magnética. Também extendemos a teoria de fotoluminescência para essa heteroestrutura. Observamos que a interação indireta entre portadores e íon magnético gera uma estrutura fina em ambos os estados iniciais e finais da emissão...

Effects of disorder on the exchange coupling in (Ga,Mn)As diluted magnetic semiconductors

Silva,Antônio J. R. da; Fazzio,A.; Santos,Raimundo R. dos; Oliveira,Luiz E.
Fonte: Sociedade Brasileira de Física Publicador: Sociedade Brasileira de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2006 EN
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A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga1-xMn xAs diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J$^{Mn-Mn}_n$, from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J$^{Mn-Mn}_n$ with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.

Femtosecond optical characterization and applications in Cd(Mn)Te diluted magnetic semiconductors

Wang, Daozhi (1978 - ); Sobolewski, Roman
Fonte: University of Rochester Publicador: University of Rochester
Tipo: Tese de Doutorado Formato: Number of Pages:xiv, 115 leaves
ENG
Relevância na Pesquisa
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Thesis (Ph. D.)--University of Rochester. Dept. of Electrical and Computer Engineering, 2008.; This thesis is devoted to the optical characterization of Cd(Mn)Te single crystals. I present the studies of free-carrier dynamics and generation and detection of coherent acoustic phonons (CAPs) using time-resolved femtosecond pump-probe spectroscopy. The giant Faraday effect and ultrafast responsivity of Cd(Mn)Te to sub-picosecond electromagnetic transients are also demonstrated and discussed in detail.
The first, few-picosecond-long electronic process after the initial optical excitation exhibits very distinct characteristic dependence on the excitation condition, and in case of Cd(Mn)Te, it has been attributed to the collective effects of band filling, band renormalization, and two-photon absorption. A closed-form, analytic expression for the differential reflectivity induced by the CAPs is derived based on the propagating-strain-pulse model and it accounts very well for our experimental observations. The accurate values of the Mn concentration and longitudinal sound velocity vs in Cd(Mn)Te were obtained by fitting the data of the refractive index dependence on the probe wavelength to the Schubert model. In Cd0.91Mn0.09Te...

Optical Properties of Sb2Te3, and Dilute Magnetic Semiconductors Sb1.97 VO.03 Te3 and Sb1.94 CrO.06 Te3

Manson, Jason
Fonte: Brock University Publicador: Brock University
Relevância na Pesquisa
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This thesis reports on the optical properties of the dilute magnetic semiconductors, Sb1.97 V 0.03 Te3 and Sb1.94Cr0.06Te3, along with the parent compound Sb2Te3' These materials develop a ferromagnetic state at low temperature with Curie temperatures of 22 K and 16 K respectively. All three samples were oriented such that the electric field vector of the light was perpendicular to the c-axis. The reflectance profile of these samples in the mid-infrared (500 to 3000 cm-1) shows a pronounced plasma edge which retracts with decreasing temperature. The far-infrared region of these samples exhibits a phonon at ~ 60 cm-1 which softens as temperature decreases. Kramers-Kronig analysis and a Drude-Lorentz model were employed to determine the optical constants of the bulk samples. The real part of the optical conductivity is shown to consist of intraband contributions at frequencies below the energy gap (~0.26 eV) and interband contributions at frequencies above the energy gap. The temperature dependence of the scattering rate show that a mix of phonon and impurity scattering are present, while the signature of traditional spin disorder (magnetic) scattering was difficult to confirm.

Anomalous lattice parameter of magnetic semiconductor alloys

CAETANO, Clovis; MARQUES, Marcelo; FERREIRA, Luiz G.; TELES, Lara K.
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
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The addition of transition metals to III-V semiconductors radically changes their electronic, magnetic, and structural properties. We show by ab initio calculations that in contrast to the conventional semiconductor alloys, the lattice parameter in magnetic semiconductor alloys, including those with diluted concentration, strongly deviates from Vegard's law. We find a direct correlation between the magnetic moment and the anion-transition metal bond lengths and derive a simple and general formula that determines the lattice parameter of a particular magnetic semiconductor by considering both the composition and magnetic moment. This dependence can explain some experimentally observed anomalies and stimulate other kind of investigations.; Brazilian funding agencies CNPq; FAPESP[05/52231-0]; FAPESP[06/05858-0]

Ferromagnetism from localized deep impurities in magnetic semiconductors

Barzykin, Victor
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/11/2003
Relevância na Pesquisa
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We propose that localized defects in magnetic semiconductors act as deep impurities and can be described by the Anderson model. Within this model, hybridization of d-orbitals and p-orbitals gives rise to a non-RKKY indirect exchange mechanism, when the localized d-electrons are exchanged through both conduction and valence bands. For semiconductors with indirect band gap the non-RKKY part of exchange integral is antiferromagnetic, which suppresses ferromagnetism. In case of direct band gap, this exchange mechanism can, under certain conditions, lead to enhancement of ferromagnetism. The indirect exchange intergral is much stronger than RKKY, and can be sufficiently long range. Thus, a potentially new class of high-temperature magnetic semiconductors emerges, where doped carriers are not necessary to mediate ferromagnetism. Curie temperatures in such magnetic semiconductors are determined mostly by the interaction between localized impurities, not Zener mechanism. This effect could also be responsible for unusually high Curie temperatures in some magnetic semiconductors with direct band gap, such as GaMnAs.; Comment: 16 pages, 8 figures

Kinetic exchange vs. room temperature ferromagnetism in diluted magnetic semiconductors

Blinowski, J.; Kacman, P.; Dietl, T.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/01/2002
Relevância na Pesquisa
47.71727%
Guided by the internal-reference rule and the known band offsets in III-V and II-VI diluted magnetic semiconductors, we discuss the feasibility of obtaining p-type conductivity, required for the carrier-induced ferromagnetism, as well as the cases for which the doping by shallow impurities may lead to the ferromagnetism driven by the double exchange. We consider the dependence of kinetic exchange on the p-d hybridization, on the electronic configurations of the magnetic ions, and on the energies of the charge transfer between the valence band of host materials and the magnetic ions. In the case of Mn-based II-VI compounds, the doping by acceptors is necessary for the hole-induced ferromagnetism. The latter is, however, possible without any doping for some of Mn-, Fe- or Co-based III-V magnetic semiconductors. In nitrides with Fe or Co carrier-induced ferromagnetism with T_C>300K is expected in the presence of acceptor doping.; Comment: 6 pages, 1 figure

Role of Correlation and Exchange for Quasi-particle Spectra of Magnetic and Diluted Magnetic Semiconductors

Kuzemsky, A. L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/03/2004
Relevância na Pesquisa
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Theoretical foundation and application of the generalized spin-fermion (sp-d) exchange lattice model to magnetic and diluted magnetic semiconductors are discussed. The capabilities of the model to describe spin quasi-particle spectra are investigated. The main emphasis is made on the dynamic behavior of two interacting subsystems, the localized spins and spin density of itinerant carriers. A nonperturbative many-body approach, the Irreducible Green Functions (IGF) method, is used to describe the quasi-particle dynamics. Scattering states are investigated and three branches of magnetic excitations are calculated in the regime, characteristic of a magnetic semiconductor. For a simplified version of the model (Kondo lattice model) we study the spectra of quasi-particle excitations with special attention given to diluted magnetic semiconductors. For this, to include the effects of disorder, modified mean fields are determined self-consistently. The role of the Coulomb correlation and exchange is clarified by comparing of both the cases.; Comment: 34 pages

Dilute magnetic topological semiconductors: What's new beyond the physics of dilute magnetic semiconductors?

Kim, Kyoung-Min; Jho, Yong-Soo; Kim, Ki-Seok
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.996846%
Role of localized magnetic moments in metal-insulator transitions lies at the heart of modern condensed matter physics, for example, the mechanism of high T$_{c}$ superconductivity, the nature of non-Fermi liquid physics near heavy fermion quantum criticality, the problem of metal-insulator transitions in doped semiconductors, and etc. Dilute magnetic semiconductors have been studied for more than twenty years, achieving spin polarized electric currents in spite of low Curie temperatures. Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Increasing disorder strength which corresponds to the size distribution of ferromagnetic clusters, we suggest a novel disordered metallic state, where Weyl metallic islands appear to form inhomogeneous mixtures with topological insulating phases. Performing the renormalization group analysis combined with experimental results, we propose a phase diagram in $(\lambda_{so},\Gamma,T)$, where the spin-orbit coupling $\lambda_{so}$ controls a topological phase transition from a topological semiconductor to a semiconductor with temperature $T$ and the distribution for ferromagnetic clusters $\Gamma$ gives rise to a novel insulator-metal transition from either a topological insulating or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands. Since electromagnetic properties in Weyl metal are described by axion electrodynamics...

Effective Interaction Hamiltonian of Polaron Pairs in Diluted Magnetic Semiconductors

Angelescu, D. E.; Bhatt, R. N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/12/2000
Relevância na Pesquisa
47.778135%
The magnetic interaction of a pair of bound magnetic polarons (BMP) in diluted magnetic semiconductors (DMS) is analyzed via a generalized Hubbard-type Hamiltonian for two carriers in the presence of effective magnetic fields arising from the magnetic polarization of their respective polarons. For the case where the magnetic fields at the two sites have equal magnitude but are allowed to have arbitrary directions, it is shown that the energy of the two polarons is minimized for a ferromagnetic configuration of the carrier spins (in contrast to the case of hydrogenic centers in nonmagnetic semiconductors) if polaron fields are strong enough. A modified Heisenberg-type Hamiltonian is constructed to describe the low energy states of the resulting system.; Comment: 20 pages,3 figures

Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors

Durst, Adam C.; Bhatt, R. N.; Wolff, P. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 26/11/2001
Relevância na Pesquisa
47.75193%
The magnetic behavior of insulating doped diluted magnetic semiconductors (DMS) is characterized by the interaction of large collective spins known as bound magnetic polarons. Experimental measurements of the susceptibility of these materials have suggested that the polaron-polaron interaction is ferromagnetic, in contrast to the antiferromagnetic carrier-carrier interactions that are characteristic of nonmagnetic semiconductors. To explain this behavior, a model has been developed in which polarons interact via both the standard direct carrier-carrier exchange interaction (due to virtual carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due to the interactions of polarons with magnetic ions in an interstitial region). Using a variational procedure, the optimal values of the model parameters were determined as a function of temperature. At temperatures of interest, the parameters describing polaron-polaron interactions were found to be nearly temperature-independent. For reasonable values of these constant parameters, we find that indirect ferromagnetic interactions can dominate the direct antiferromagnetic interactions and cause the polarons to align. This result supports the experimental evidence for ferromagnetism in insulating doped DMS.; Comment: 11 pages...

Spin dynamics in paramagnetic diluted magnetic semiconductors

Phan, Van-Nham; Tran, Minh-Tien
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/10/2015
Relevância na Pesquisa
47.75193%
Microscopic properties of low-energy spin dynamics in diluted magnetic semiconductor are addressed in a framework of the Kondo lattice model including random distribution of magnetic dopants. Based on the fluctuation-dissipation theorem, we derive an explicit dependence of the spin diffusion coefficient on the single-particle Green function which is directly evaluated by dynamical mean-field theory. In the paramagnetic state, the magnetic scattering has been manifested to suppress spin diffusion. In agreement with other ferromagnet systems, we also point out that the spin diffusion in diluted magnetic semiconductors at small carrier concentration displays a monotonic $1/T$-like temperature dependence. By investigating the spin diffusion coefficient on a wide range of the model parameters, the obtained results have provided a significant scenario to understand the spin dynamics in the paramagnetic diluted magnetic semiconductors.; Comment: 6 pages, 4 figures

Scattering by Atomic Spins and Magnetoresistance in Dilute Magnetic Semiconductors

Petukhov, M. Foygel A. G.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/02/2007
Relevância na Pesquisa
47.81513%
We studied electrical transport in magnetic semiconductors, which is determined by scattering of free carriers off localized magnetic moments. We calculated the scattering time and the mobility of the majority and minority-spin carriers with both the effects of thermal spin fluctuations and of spatial disorder of magnetic atoms taken into account. These are responsible for the magnetic-field dependence of electrical resistivity. Namely, the application of the external magnetic field suppresses the thermodynamic spin fluctuations thus promoting negative magnetoresistance. Simultaneously, scattering off the built-in spatial fluctuations of the atomic spin concentrations may increase with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive magnetoresistance. We discuss the role of the above effects on magnetoresistance of non-degenerate semiconductors where magnetic impurities are electrically active or neutral.

Surface characterization of Mn(x)Ge(1−x) and Cr(y)Mn(x)Ge(1−x−y) dilute magnetic semiconductors

Gambardella, Pietro; Claude, L.; Rusponi, S.; Franke, K. J.; Brune, H.; Raabe, J.; Nolting, F.; Bencok, Peter; Hanbicki, A. T.; Jonker, B. T.; Grazioli, C.; Veronese, M.; Carbone, C.
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Artículo Formato: 849828 bytes; application/pdf
ENG
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57.71727%
7 pages, 7 figures.-- PACS nrs.: 75.50.Pp; 75.70.-i; 78.70.Dm; 75.20.Hr.; We have used x-ray photoemission electron microscopy (XPEEM) and x-ray absorption spectroscopy (XAS) to characterize Mn(x)Ge(1−x) and Cr(y)Mn(x)Ge(1−x−y) films grown by molecular beam epitaxy. The surface layers of the as-grown films probed by XPEEM present segregation of Mn-rich phases. XAS using both total electron yield and fluorescence yield detection shows that the films are heavily oxidized after exposure to air. Etching in HF and HCl can be used to reduce oxidation, but inhomogeneities in the surface composition might not be completely eliminated depending on the Mn concentration. X-ray magnetic circular dichroism (XMCD) measurements reveal that neither the etched nor the as-grown films present remanent ferromagnetic behavior down to a temperature of 5 K within the probing depth of the fluorescence yield (~20 nm). Mn is paramagnetic in both the oxidized and etched samples, with an increased tendency to order magnetically toward the interior of the films. Cr in Cr(y)Mn(x)Ge(1−x−y) possesses a paramagnetic moment only in the oxidized form. A comparison of the XAS line shapes obtained in the present study with those of Mn impurities deposited on Ge and GaAs surfaces demonstrates that the interpretation of XAS spectra of Mn-doped dilute magnetic semiconductors in the literature is often affected by residual oxidation.; Part of this work was performed at the Swiss Light Source...

Electrical gating effects on the magnetic properties of (Ga,Mn)As diluted magnetic semiconductors

Owen, Man Hon Samuel
Fonte: University of Cambridge; Department of Physics Publicador: University of Cambridge; Department of Physics
Tipo: Thesis; doctoral; PhD
EN
Relevância na Pesquisa
57.75193%
The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1?xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ?2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ?30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga...