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Journal of Physics C Solid State Physics

The three-dimensional quantum Heisenberg ferromagnet with random anisotropy

Rita Maria Zorzenon dos Santos, Ananias M. Mariz, Raimundo R. dos Santos, Constantino Tsallis

Published Oct 10, 1985• 6 citations
Publisher

Abstract

The authors study the critical properties of the 3D quantum Heisenberg ferromagnet with random anisotropies; that is, the coupling between any pair of nearest-neighbouring spins can be either isotropic (Heisenberg) or anisotropic (Ising-or XY-like) at random. Within a Migdal-Kadanoff approximation they obtain the full critical frontier and correlation length critical exponents. They found that the isotropic Heisenberg model is unstable (in the context of universality classes) in the presence of a small concentration of couplings with lower symmetry.

DOI
10.1088/0022-3719/18/28/016

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The authors study the critical properties of the 3D quantum Heisenberg ferromagnet with random anisotropies; that is, the coupling between any pair of nearest-neighbouring spins can be either isotropic (Heisenberg) or anisotropic (Ising-or XY-like) at random. Within a Migdal-Kadanoff approximation they obtain the full critical frontier and correlation length critical exponents. They found that the isotropic Heisenberg model is unstable (in the context of universality classes) in the presence of a small concentration of couplings with lower symmetry.
We consider the square-lattice spin-(1/2) anisotropic Heisenberg ferromagnet with interactions whose symmetry can independently (quenched model) and randomly be of two competing types: namely, the isotropic Heisenberg type and the Ising type. Within a real-space renormalization-group framework, we perform a quite precise numerical calculation of the critical frontier, and establish its main asymptotic behaviors. We also characterize the relevant universality classes through the analysis of the correlation-length critical exponent.
The anisotropic nearest-neighbour Heisenberg model for the simple cubic lattice has been investigated by interpolating the anisotropy between the Ising and isotropic Heisenberg limits via general spin high-temperature series expansions of the zero-field suspectibility. This is done by estimating the critical temperature (Tc(3)) and the susceptibility exponent γ from the analysis of the series by the Ratio and Pade approximants methods. It is noted that Tc(3) varies with anisotropy while γ is almost the same for the anisotropic system, and a jump in it occurs for the isotropic case in agreement with the universality hypothesis. The effect of anisotropy on the susceptibility is also shown. Further, it is seen that estimates of γ for the two extreme limits agree well with those of previous theoretical as well as experimental investigations. In addition, critical temperatures have been summarised in a relation, and expressions for the magnetisation have been derived.
We have studied the critical properties of the three-dimensional random anisotropy Heisenberg model by means of numerical simulations using the Parallel Tempering method. We have simulated the model with two different disorder distributions, cubic and isotropic ones, with two different anisotropy strengths for each disorder class. For the case of the anisotropic disorder, we have found evidence of universality by finding critical exponents and universal dimensionless ratios independent of the strength of the disorder. In the case of isotropic disorder distribution the situation is very involved: we have found two phase transitions in the magnetization channel which are merging for larger lattices remaining a zero magnetization low-temperature phase. Studying this region using a spin-glass order parameter we have found evidence for a spin-glass phase transition. We have estimated effective critical exponents for the spin-glass phase transition for the different values of the strength of the isotropic disorder, discussing the crossover regime.
We use a Migdal-Kadanoff-like renormalization group approach to study the critical behaviour of a semi-infinite simple cubic Ising ferromagnet whose (1, 0, 0) free surface contains anisotropic (in spin space) Heisenberg ferromagnetic interactions. The phase diagram presents three phases (namely the paramagnetic, the bulk ferromagnetic and the surface ferromagnetic ones) which join on a multicritical point The location of this point is calculated as a function of the anisotropy. The various universality classes of the problem are exhibited.
We study the ±J three-dimensional (3D) Ising model with a spatially uniaxial anisotropic bond randomness on the simple cubic lattice. The ±J random exchange is applied on the xy planes, whereas, in the z direction, only a ferromagnetic exchange is used. After sketching the phase diagram and comparing it with the corresponding isotropic case, the system is studied at the ferromagnetic-paramagnetic transition line using parallel tempering and a convenient concentration of antiferromagnetic bonds (p(z)=0;p(xy)=0.176). The numerical data clearly point out a second-order ferromagnetic-paramagnetic phase transition belonging in the same universality class with the 3D random Ising model. The smooth finite-size behavior of the effective exponents, describing the peaks of the logarithmic derivatives of the order parameter, provides an accurate estimate of the critical exponent 1/ν=1.463(3), and a collapse analysis of magnetization data gives an estimate of β/ν=0.516(7). These results are in agreement with previous papers and, in particular, with those of the isotropic ±J three-dimensional Ising model at the ferromagnetic-paramagnetic transition line, indicating the irrelevance of the introduced anisotropy.
We study the critical dynamics of the three-dimensional Heisenberg model with random cubic anisotropy in the out-of-equilibrium and equilibrium regimes. Analytical approaches based on field theory predict that the universality class of this model is that of the three-dimensional site-diluted Ising model. We have been able to estimate the dynamic critical exponent by working in the equilibrium regime and by computing the integrated autocorrelation times obtaining z=2.50(5) (without taking into account scaling corrections) and z=2.29(11) (by fixing the scaling corrections to that predicted by field theory). In the out-of-equilibrium regime we have focused on the study of the dynamic correlation length, allowing us to compute the dynamic critical exponent obtaining z=2.38(2), which is compatible with the equilibrium ones. Finally, both estimates are also compatible with the most accurate prediction z=2.35(2), from numerical simulations of the 3D site-diluted Ising model, in agreement with the predictions based on field theory.