Thermodynamics of S>=1 ferromagnetic Heisenberg chains with uniaxial single-ion anisotropy

TL;DR

This paper investigates the thermodynamic behavior of S>=1 ferromagnetic chains with uniaxial anisotropy using Green-function and exact diagonalization methods, revealing temperature-dependent features and matching experimental data.

Contribution

It provides a combined theoretical approach and experimental comparison for understanding thermodynamics in anisotropic ferromagnetic chains, highlighting the impact of anisotropy on specific heat.

Findings

Two maxima in specific heat for D/J > 7.4 in S=1 chains.

Agreement between Green-function and exact diagonalization results.

Predictions for spin-wave spectrum, correlation length, and susceptibility.

Abstract

The thermodynamic properties of S>=1 ferromagnetic chains with an easy-axis single-ion anisotropy are investigated at arbitrary temperatures by both a Green-function approach, based on a decoupling of three-spin operator products, and by exact diagonalizations of chains with up to N=12 sites using periodic boundary conditions. A good agreement between the results of both approaches is found. For the S=1 chain, the temperature dependence of the specific heat reveals two maxima, if the ratio of the anisotropy energy D and the exchange energy J exceeds a characteristic value, D/J >7.4, and only one maximum for D/J <7.4. This is in contrast to previous exact diagonalization data for comparably small chains (N <= 7) using open boundary conditions. Comparing the theory with experiments on di-bromo Ni complexes the fit to the specific heat yields concrete values for D and J which are used to make predictions for the temperature dependences of the spin-wave spectrum, the correlation length, and the transverse magnetic susceptibility.