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Doctoral Thesis
DOI
10.11606/T.43.1993.tde-26022014-145535
Document
Author
Full name
Jair Libero Cadorin
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1993
Supervisor
Committee
Yokoi, Carlos Seihiti Orii (President)
Coutinho Filho, Mauricio Domingos
Salinas, Silvio Roberto de Azevedo
Tsallis, Constantino
Wreszinski, Walter Felipe
Title in Portuguese
Estudo de diagramas de fases de alguns variantes do modelo ANNNI e de um modelo planar clássico helicoidal
Keywords in Portuguese
Mecânica estatística
Abstract in Portuguese
Nesse trabalho estudamos dois tipos de modelos para a descrição dos sistemas magnéticos modulados. O primeiro é o modelo ANNNI ("Axial Next-Nearest-Neighbor Ising"). Dentro dos limites da aproximação de campo médio estudamos vários efeitos que surgem quando as interações intraplanares são menores que as interações interplanares. Investigamos em particular a ocorrência de fases desordenadas, assim denominadas por apresentar planos com magnetização nula. Estudamos ainda o comportamento dos pontos de acumulação de pontos de ramificação que determinam o limite entre as transições do tipo comensurável-comensurável e comensurável-incomensurável. Também investigamos dois variantes do modelo ANNNI, um com spin qualquer e outro com spin 1 e anisotropia de campo cristalino. O segundo tipo de modelo estudado, apropriado para sistemas apresentando modulações helicoidais das magnetizações, é um modelo planar clássico com interações competitivas entre primeiros e segundos vizinhos numa direção axial. Restringimos o nosso estudo ao zero absoluto de temperatura. Fizemos expansões das magnetizações em série de Fourier para campos baixos e altos. Também fizemos análise de estabilidade das fases comensuráveis, dentro de uma aproximação contínua, utilizando a teoria dos sólitons. Finalmente, estudamos numericamente o diagrama de fases do modelo aplicando o método dos potenciais efetivos.
Title in English
Study Phase Diagrams Variants Mode Model ANNNI Classic Planar Coil
Keywords in English
Statistical mechanics
Abstract in English
In this work we have studied two types of models for the description of modulated magnetic systems. The first is the ANNNI (Axial NextNearest-Neighbor Ising) model. Within the mean-field approximation we have studied various effects which comes about when the intralayer interactions are weaker than the interlayer interactions. We have investigated in particular the possibility of partially disordered phases, characterized by the presence of layers with zero magnetization. We also have studied the behavior of the accumulation points of the branching points, which separates the commensurate-commensurate transitions from the commensurate-incommensurate transitions. We have also investigated two variants of the ANNNI model, one with an arbitrary spin and the other with spin 1 and a term of crystal-field anisotropy. The second type of the model we have studied, appropriate for systems presenting helicoidal modulation of the magnetization, is a classical planar model with nearest and next-nearest interactions along an axial direction. \'Ve have restricted our study to the absolute zero of the temperature. The magnetization is expanded in Fourier series for high and low fields. We also analyse the stability of the commensurate phases, within a continuum approximation, using the soliton theory. Finally, we construct numerically the phase diagram of the model using the method of effective potentials.
 
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Publishing Date
2014-02-26
 
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