Ferromagnetic phase of a uniaxial magnet with anisotropic biquadratic exchange

TL;DR

This paper investigates the ferromagnetic phase of a uniaxial magnet with anisotropic biquadratic exchange, analyzing spin excitation spectra, stability conditions, and reentrance phenomena at finite temperatures.

Contribution

It provides new analytical expressions for spin excitation spectra and stability diagrams considering anisotropic biquadratic exchange in uniaxial magnets with S=1.

Findings

Spectral mode stability can be violated and restored with temperature changes.

Reentrance phenomenon observed in spectral mode stability diagram.

Transition temperature depends significantly on biquadratic exchange anisotropy.

Abstract

The ferromagnetic phase (FMP) of a uniaxial magnet with the easy-plane single-ion anisotropy (SIA) and the anisotropic biquadra\-tic exchange interaction (BQEI) has been studied. The case $S=1$ for the site spin $S$ has been considered. Expressions for two branches of the spin excitation spectrum at finite temperatures $T$ have been obtained, and the conditions for spectral mode stability have been determined. The spectral mode stability diagram in the $T-h$ coordinates has been constructed. The diagram testifies that, under certain conditions, the temperature decrease is accompanied by a violation of the spectral mode stability followed, as the temperature decreases further, by its restoration; i.e. the reentrance phenomenon is observed. The temperature of the second-order phase transition (PT) from the FMP into the phase with spontaneously broken symmetry has been demonstrated to depend considerably on the BQEI anisotropy constant.