Schwinger boson theory of anisotropic ferromagnetic ultrathin films

TL;DR

This paper applies Schwinger boson theory to anisotropic ferromagnetic ultrathin films, demonstrating that mean-field results align with known theorems and capturing key magnetic properties and transitions.

Contribution

It introduces a Schwinger boson mean-field approach to model anisotropic ferromagnetic thin films, including two bosonization methods and comparison with Green's function results.

Findings

Qualitatively correct magnetization behavior at mean-field level.

Satisfaction of the Mermin-Wagner theorem in the model.

Analysis of magnetic reorientation and anisotropy effects.

Abstract

Ferromagnetic thin films with magnetic single-ion anisotropies are studied within the framework of Schwinger bosonization of a quantum Heisenberg model. Two alternative bosonizations are discussed. We show that qualitatively correct results are obtained even at the mean-field level of the theory, similar to Schwinger boson results for other magnetic systems. In particular, the Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite temperatures is not found if the ground state of the anisotropic system exhibits a continuous degeneracy. We calculate the magnetization and effective anisotropies as functions of exchange interaction, magnetic anisotropies, external magnetic field, and temperature for arbitrary values of the spin quantum number. Magnetic reorientation transitions and effective anisotropies are discussed. The results obtained by Schwinger boson mean-field theory are compared with the many-body Green's function technique.