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Doctoral Thesis
Full name
Wagner Figueiredo
Knowledge Area
Date of Defense
São Paulo, 1980
Becerra, Carlos Castilla
Oliveira, Mario Jose de
Qualifik, Paul
Title in Portuguese
Determinação das fronteiras de fase de sistemas antiferromagnéticos anisotrópicos a baixas temperaturas
Keywords in Portuguese
baixa temperatura
funções de Green
Holstein e Primakoff
materiais antiferromagnéticos
Sistemas antiferromagnéticos anisotrópicos
Abstract in Portuguese
A combinação das funções de Green para operadores de criação e destruição de bosons com as transformações de Holstein e Primakoff é utilizada na análise de sistemas antiferromagnéticos, isolantes e anisotrópicos, na região de baixas temperaturas. Os limites de estabilidade das fases antiferromagnética e "spin-flop", assim como a transição "spin-flop"-paramagnética são determinados em função da temperatura. A Hamiltoniana modelo considerada leva em conta as interações de intercâmbio anisotrópicas entre primeiros e segundos dos vizinhos numa rede cúbica simples e anisotropias de íon único, dos tipos uniaxial e ortorrômbico. Em particular, as fronteiras de fase "spin-flop"-paramagnéticas dos antiferromagnetos NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O e MnCl2.4H2O são determinadas e comparadas com os dados experimentais. As interações dipolares são consideradas explicitamente no circulo da fronteira de fase "spin-flop"-paramagnética do EuTe, obtendo-se boa concordância com a experiência.
Title in English
Determination of the boundaries of the anisotropic antiferromagnetic systems phase at low temperatures
Keywords in English
Anisotropic antiferromagnetic systems
antiferromagnetic materials
Green functions
Holstein and Primakoff
low temperature
Abstract in English
The combination of Greens functions for boson creation and destruction operators with the Holstein - Primakoff transformations is employed in the analysis of insulating and anisotropic antiferromagnetic systems at very low temperatures. The stability limits of antiferromagnetic and spin- flop phases, as well as the spin-flop-paramagnetic transition are determined as a function of temperature. The model Hamiltonian considered takes account of the anisotropic exchange interactions between first and second neighbors for a simple cubic lattice and of single ion anisotropies, of the uniaxial and orthorhombic types. Particularly, the spin-flop-paramagnetic phase boundaries of the antiferromagnetics NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O and MnCl2.4H2O are determined and compared with experimental values. The dipolar interactions are explicitly considered in the calculation of the spin-flop-paramagnetic phase boundary of antiferromagnetic EuTe, and we obtain good agreement with the experiments.
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