Thermodynamic properties of ferromagnetic mixed-spin chain systems

TL;DR

This paper analyzes the thermodynamic properties of mixed-spin ferromagnetic chains using various computational methods, revealing a double-peak specific heat structure and fitting experimental susceptibility data for a specific material.

Contribution

It introduces a new expansion method for evaluating thermodynamic properties across different spin quantum numbers and links results to a solvable classical limit.

Findings

Double-peak structure in specific heat due to excitations

Excellent fit of susceptibility data for MnNi(NO2)4 chain

Extraction of microscopic exchange parameters from experimental data

Abstract

Using a combination of high-temperature series expansion, exact diagonalization and quantum Monte Carlo, we perform a complementary analysis of the thermodynamic properties of quasi-one-dimensional mixed-spin systems with alternating magnetic moments. In addition to explicit series expansions for small spin quantum numbers, we present an expansion that allows a direct evaluation of the series coefficients as a function of spin quantum numbers. Due to the presence of excitations of both acoustic and optical nature, the specific heat of a mixed-spin chain displays a double-peak-like structure, which is more pronounced for ferromagnetic than for antiferromagnetic intra-chain exchange. We link these results to an analytically solvable half-classical limit. Finally, we extend our series expansion to incorporate the single-ion anisotropies relevant for the molecular mixed-spin ferromagnetic chain material MnNi(NO$_{2}$)$_{4}$(ethylenediamine)$_{2}$, with alternating spins of magnitude 5/2 and 1. Including a weak inter-chain coupling, we show that the observed susceptibility allows for an excellent fit, and the extraction of microscopic exchange parameters.