Numerical interchain mean-field theory for the specific heat of the bimetallic ferromagnetically coupled chain compound MnNi(NO$_2$)$_4$(en)$_2$ (en = ethylenediamine)

TL;DR

This study combines experimental measurements and numerical simulations to analyze the specific heat of a bimetallic ferromagnetic chain compound, revealing the effects of magnetic fields and interchain coupling on its thermal properties.

Contribution

The paper introduces a numerical interchain mean-field approach to accurately model the specific heat of MnNi(NO$_2$)$_4$(en)$_2$, integrating experimental data with advanced simulations.

Findings

Double-peak structure in specific heat due to acoustic and optical modes.

Excellent agreement between experimental data and numerical models.

Strong magnetic field effects suggest potential magnetocaloric applications.

Abstract

We present a detailed study of the field-dependent specific heat of the bimetallic ferromagnetically coupled chain compound MnNi(NO$_2$)$_4$(en)$_2$, en = ethylenediamine. For this material, which in zero field orders antiferromagnetically below $T_N=2.45$ K, small fields suppress magnetic order. Instead, in such fields a double-peak like structure in the temperature dependence of the specific heat is observed. We attribute this behavior to the existence of an acoustic and an optical mode in the spin wave dispersion as result of the existence of two different spins per unit cell. We compare our experimental data to numerical results for the specific heat obtained by exact diagonalization and Quantum-Monte-Carlo simulations for the alternating spin chain model, using parameters that have been derived from the high-temperature behavior of the magnetic susceptibility. The interchain coupling is included in the numerical treatment at the mean-field level. We observe remarkable agreement between experiment and theory, including the ordering transition, using previously determined parameters. Furthermore, the observed strong effect of an applied magnetic field on the ordered state of MnNi(NO$_2$)$_4$(en)$_2$ promises interesting magnetocaloric properties.