Spin-one ferromagnets with single-ion anisotropy in a perpendicular external field

TL;DR

This paper generalizes the Holstein-Primakoff method using a characteristic angle transformation to better analyze spin-one ferromagnets with single-ion anisotropy under external fields, improving ground state predictions.

Contribution

The authors introduce a new approach that overcomes limitations of the conventional method for spin-one systems with anisotropy, providing more accurate ground state properties.

Findings

The new method accurately predicts ground states for easy-plane anisotropy.

It improves representation for easy-axis anisotropy over conventional methods.

A field-induced phase transition and specific heat peak are identified in the easy-plane model.

Abstract

In this paper, the conventional Holstein-Primakoff method is generalized with the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao, J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with single-ion anisotropies. We find that the weakness of the conventional method for such systems can be overcome by the new approach. Two models will be discussed to illuminate the main idea, which are the ``easy-plane" and the ``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the current approach can give reasonable ground state properties for the magnetic system with ``easy-plane" anisotropy though the conventional method never can, and can give a better representation than the conventional one for the magnetic system with ``easy-axis" anisotropy though the latter is usually believed to be a good approximation in such case. Study of the easy-plane model shows that there is a phase transition induced by the external field, and the low-temperature specific heat may have a peak as the field reaches the critical value.