Cálculo de propriedades magnéticas e magnetocalóricas no contexto do modelo de aproximação de pares

Abstract

This thesis is mainly devoted to the study of the magnetic and magnetocaloric properties of ferromagnetic systems in the context of the pair approximation model. This model considers an exact interaction (Oguchi approximation) of the main ion with the nearest-neighbor and additionally (in some cases) with the next-nearest-neighbor. For the analysis, the Ising (= 1) and Heisenberg ( = 0) models were considered, taking into account two cases, namely, the cubic lattice with common magnetic moment (Si = Sj sublattices) and the cubic lattice with mixed spin (Si 6= Sj). Initially we consider spinonly dependent ferromagnetic system, in which the nearest-neighbor hamiltonian has the Zeeman, bilinear (J1) and biquadratic (J01 ) exchange terms, with and without bilinear exchange anisotropy, = 1 and 0, respectively. The magnetization (ghSz i i) and critical temperature (TC) were investigated through their dependence with the biquadratic exchange parameter (J01 ). It was veri ed, especially in the negative regions of this parameter J01 < 0 < J1, changes on the ground state of the spin con guration, being more visible in the Heisenberg model. The consideration of a next-nearest-neighbor hamiltonian, improves the accuracy in the determination of the spin-lattice correlation, especially in the magnetic phase transition region. In addition, we considered a mixed spin system (Si = 9/2 and Sj = 7/2) with a hamiltonian composed of the Zeeman, bilinear exchange (J1) and uniaxial anisotropy (D) terms. With D favoring the z-axis direction of the spin at site i, the major contributions to ghSz i i of this type of anisotropy are found in the Ising model. Also, the pair model was applied to the RAl2 series (R = Gd, Tb and Tm), considering the cubic crystalline eld, in order to calculate its magnetic and magnetocaloric properties. In each of the cases, from the equation of state, we obtained the low temperature isotherm (T0) and determined the area under the curve in the Hi H Hf eld range. We also determined the area described by the magnetic entropy change (in the same eld range) for T T0. The equality of the mentioned areas validates the area sum-rule of the magnetocaloric e ect. In particular, for T0 = 0, quantum transitions are revealed as magnetic plateaus interrupted by discontinuities in magnetization at critical elds. In this case, from the comparison with the entropy-change curves, the area sum-rule is also validated. Finally, the magnetic contribution to the electrical resistivity was determined and the expected similarity with the magnetic entropy variation was con rmed.