J. Cabral Neto

Néel temperature and sublattice magnetiztion of a three-dimensional classical and quantum Heisenberg antiferromagnet

The effective-field theory (EFT) of a two-spin cluster is applied to study the critical behavior of a three-dimensional classical and quantum Heisenberg antiferromagnet (AF). Neel temperature (TN) and sublattice magnetization are calculated numerically and compared with the results of these models with ferromagnetic (F) interaction. We find that our results, for the quantum model TN is higher than Tc (Curie temperature, F case) but for the classical model TN=Tc. The difference between F and AF results for the quantum model obtained here by EFT is in accordance with Monte Carlo simulation and high-temperature expansion.

Surface magnetism: phase transitions in quantum and classical models

We study the phase diagram and multicritical behavior of the semi-infinite quantum spin 1/2 anisotropic Heisenberg and classical O(n) models. We use correlated effective field theory. The multicritical point Δc and the crossover exponent φ are obtained as a function of the value n and anisotropy parameter. We have compared the results of systems with ferromagnetic and antiferromagnetic interactions.

Study of surface effects in quantum Heisenberg and XY antiferromagnets

Using a finite-cluster mean-field approximation, we study the characteristics of phase transitions for the semi-infinite simple cubic quantum spin-1/2 anisotropic Heisenberg model with ferromagnetic and antiferromagnetic interactions. The dependence of the critical temperature on the parameter is obtained, where Δ = J s /J b - 1, J s and J b are the nearest-neighbour coupling constants at the surface and bulk, respectively. Particular attention has been drawn to the difference between the criticality for the F (ferromagnetic) and AF (antiferromagnetic) models. It is shown that quantum influence modifies the criticality of the anisotropic Heisenberg model with T c (F case) T N in the bulk-paramagnetic and surface paramagnetic critical lines, respectively. On the other hand, the quantum XY model presents the same criticality for the two types of interactions (F and AF).

Phase diagrams of thin classical n-vector films

Abstract The Curie temperature T c of a classical n -vector ferromagnetic thin film was theoretically investigated in the framework of an effective-field theory. The effect of surface interaction and of film thickness l on the critical temperature was carefully investigated in the case of simple cubic films.