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Previous issue date: 2015-05-08 / Nesta tese tratamos teoricamente algumas propriedades magnéticas e excita c~oes de
spins em materiais ferromagn eticos e antiferromagn eticos com o objetivo de interpretar
as medidas e observa c~oes experimentais. Usamos a formula c~ao de ondas de spins de Holstein-Primako para investigar propriedades da din^amica de propaga c~ao de ondas de spins no material ferrimagn etico YIG e tamb em no sistema antiferromagn etico FeF2. Para cada um deles, desenvolvemos modelos te oricos que permitiram ajustar as rela c~oes de dispers~ao de ondas de spins medidas experimentalmente nesses. Al em disso, investigamos a renormaliza c~ao da energia de m agnons usando o formalismo das fun c~oes de Green de dois tempos dependentes da temperatura de Bogoliubov-Tyablikov. A aplica c~ao para o YIG, permitiu calcular as varia c~oes da rela c~ao de dispers~ao para m agnons com a temperatura. Tamb em se investigou a renormaliza c~ao da energia de m agnons no sistema FeF2, mas esta vez considerando uma aproxima c~ao do tipo RPA1. Na renormaliza c~ao da energia foram consideradas duas contribui c~oes: exchange isotr opica tipo Heisenberg e anisotropia uniaxial. Em nossas considera c~oes, foi inclu da a depend^encia com a temperatura da anisotropia uniaxial que permitiu estimar o campo critico de transi c~ao de spin-
op para o limite de estabilidade da fase antiferromagn etica.
Tamb em, foi feito um estudo te orico e experimental da anisotropia magneto cristalina
do material antiferromagn etico RbMnF3, baseado nas intera c~oes de campo cristalino
e intera c~ao de spin- orbita, com o objetivo de calcular a varia c~ao com a temperatura
do campo de spin- op. As ondas de spins em YIG foram estudadas em mais detalhes
calculando as taxas de relaxa c~ao por processos de espalhamento de 3-m agnons
e 4-m agnons. Igualmente foram estimadas empregando campos magn eticos intensos
da ordem de 102 kOe. Finalmente, estudamos propriedades t ermicas de transporte
de m agnons em YIG submetido a campos magn eticos intensos. Calculamos o calor
1Random Phase Approximation especi co de m agnons e a condutividade t ermica em baixas temperaturas e campos magn eticos externos intensos para serem comparados com as medidas experimentais encontrando um bom acordo entre teoria e experimento [145] / In this thesis we theoretically treat some magnetic properties and excitations of spins
in ferromagnetic and antiferromagnetic materials in order to interpret measurements
and experimental observations. We use the formulation of quantization of spin waves
of Holstein-Primako to investigate properties of the spin wave propagation dynamics
in the ferrimagnetic material YIG and also in the antiferromagnetic system FeF2. For
each of them, we have developed theoretical models to adjust the dispersion relations of
spin waves measured experimentally in these systems. In addition, we investigated the
magnon energy renormalization using the formalism of two-time temperature dependent
Green's functions of Bogoliubov-Tyablikov. The application for the YIG allowed
calculate the variation of the dispersion relation for magnons with temperature. We
also investigate the magnon energy renormalization in the system FeF2, but this time
considering an approximation of the RPA type. In the energy renormalization were
considered two contributions: exchange isotropic Heisenberg and uniaxial anisotropy
type. In our calculations the temperature dependence of the uniaxial anisotropy eld
allows estimating the critical spin- op transition for the antiferromagnetic phase stability
limit. Also, we have undertaken a theoretical and experimental study of the
magneto crystalline anisotropy of the antiferromagnetic material RbMnF3, based on
the crystal eld interactions and spin-orbit interaction, in order to calculate the variation
with temperature of the eld spin- op. The spin waves in YIG were studied in
more detail calculating the relaxation rate due to 3-magnons and 4-magnons scattering
processes considering strong magnetic elds of hundreds of kOe. Finally, we have
studied thermal transport properties of magnons in YIG subject to intense magnetic
elds. We have, calculate of the speci c heat and thermal conductivity of magnons
at low temperatures and high elds to compare with the experimental measurements
nding a good agreement between theory and experiment [145].
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufpe.br:123456789/16560 |
| Date | 08 May 2015 |
| Creators | LÓPEZ ORTIZ, Javier del Cristo |
| Contributors | REZENDE, Sérgio Machado |
| Publisher | Universidade Federal de Pernambuco, Programa de Pos Graduacao em Fisica, UFPE, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Breton |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFPE, instname:Universidade Federal de Pernambuco, instacron:UFPE |
| Rights | Attribution-NonCommercial-NoDerivs 3.0 Brazil, http://creativecommons.org/licenses/by-nc-nd/3.0/br/, info:eu-repo/semantics/openAccess |
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