Slobodan M. Radošević

Phase diagram of spin-12 quantum Heisenberg J1–J2 antiferromagnet on the body-centered-cubic lattice in random phase approximation

Abstract Magnetic properties of spin 1 2 J 1 – J 2 Heisenberg antiferromagnet on the body centered cubic lattice are investigated. By using two-time temperature Greens functions, sublattice magnetization and critical temperature depending on the frustration ratio p = J 2 / J 1 are obtained in both stripe and Neel phases. The analysis of ground state sublattice magnetization and phase diagram indicates the critical end point at J 2 / J 1 = 0.714 , in agreement with previous studies.

Magnon energy renormalization and low-temperature thermodynamics of O(3) Heisenberg ferromagnets

Abstract We present the perturbation theory for lattice magnon fields of the D -dimensional O(3) Heisenberg ferromagnet. The effective Hamiltonian for the lattice magnon fields is obtained starting from the effective Lagrangian, with two dominant contributions that describe magnon–magnon interactions identified as a usual gradient term for the unit vector field and a part originating in the Wess–Zumino–Witten term of the effective Lagrangian. Feynman diagrams for lattice scalar fields with derivative couplings are introduced, on the basis of which we investigate the influence of magnon–magnon interactions on magnon self-energy and ferromagnet free energy. We also comment appearance of spurious terms in low-temperature series for the free energy by examining magnon–magnon interactions and internal symmetry of the effective Hamiltonian (Lagrangian).

Multipath Metropolis simulation: An application to the classical Heisenberg model

This study explores the Multipath Metropolis simulation of the classical Heisenberg model. Unlike the standard single-path algorithm, the Metropolis algorithm applied to multiple random-walk paths becomes an embarrassingly parallel algorithm in which many processor cores can be easily utilized. This is important since processor cores are progressively becoming less expensive and thus more accessible. The most obvious advantage of the multipath approach is in employing independent random-walk paths to produce an uncorrelated simulation output with a normal distribution allowing for straightforward and rigorous statistical analysis.

Magnon–magnon interactions in O(3) ferromagnets and equations of motion for spin operators

The method of equations of motion for spin operators in the case of O(3) Heisenberg ferromagnet is systematically analyzed starting from the effective Lagrangian. It is shown that the random phase approximation and the Callen approximation can be understood in terms of perturbation theory for type B magnons. Also, the second order approximation of Kondo and Yamaji for one dimensional ferromagnet is reduced to the perturbation theory for type A magnons. An emphasis is put on the physical picture, i.e. on magnon-magnon interactions and symmetries of the Heisenberg model. Calculations demonstrate that all three approximations differ in manner in which the magnonmagnon interactions arising from the Wess-Zumino term are treated, from where specific features and limitations of each of them can be deduced.

Magnetic Susceptibility of Quasi-two-dimensional Cuprate Antiferromagnets

Abstract Parallel magnetic susceptibility temperature dependence of the high-TC superconducting parent compound La2CuO4 is calculated in both antiferromagnetic (AFM) and paramagnetic phase. By making use of the quantum Heisenberg three-dimensional AFM model including the in-plane spin anisotropy, the calculation is performed within the framework of three different theories: Green’s function theory in random-phase approximation (RPA), linear spinwave (LSW) theory and mean-field (MF) theory. The results suggest that at low temperatures quantum spin fluctuations play an important role, while at the temperatures above the critical one short-range correlations have a great impact on the behavior of the system. This leads to the discrepancy between RPA and MF results, since the later neglects the above phenomena. Further, LSW theory expectedly agrees with RPA results only at low temperatures where the magnon interactions are negligible. Comparison to the theoretical and experimental results quoted in literature confirms that RPA method presents the most appropriate method among the applied ones, suggesting that this approach is satisfactory in the case of the parallel magnetic susceptibility, while in order to reproduce the transversal one, spin-orbit coupling must be included.

Elementary Excitations in Ferromagnetic Semiconductor Superlattice in the Narrow‐band Limit

The transfer matrix method, developed in our previous paper [1], is used to study bulk and surface magnetic excitations of a semi‐infinite ferromagnetic semiconductor (FMS’s) superlattice. Results are discussed in the narrow‐band limit. The spin‐wave frequencies for a semi‐infinite narrow‐band semiconductor are analyzed in the low‐frequency, as well as in the high‐frequency region. In the frame of the same methodology, bulk and surface magnetic excitations of thin FMS’s films are analyzed in dependence of the parameters of the system. Results are analyzed numerically, discussed and plotted.