The influence of interplanar coupling on the entropy and specific heat of the bilayer ferromagnet

TL;DR

This paper investigates how interplanar coupling affects entropy and specific heat in a bilayer ferromagnet using the Pair Approximation, revealing a double peak structure and effects of magnetic anisotropy.

Contribution

It applies the Pair Approximation to the Ising-Heisenberg bilayer, incorporating quantum effects and analyzing entropy and specific heat with a focus on weak interplanar coupling.

Findings

Double peak structure in specific heat and entropy change.

Quantum effects significantly influence thermodynamic properties.

Comparison with exact finite cluster calculations confirms the approximation's validity.

Abstract

The Pair Approximation method is applied to studies of the magnetic Ising-Heisenberg bilayer with simple cubic structure and spin $S=1/2$. The method allows to take into account quantum effects related to Heisenberg couplings. In the paper the entropy and magnetic contribution to the specific heat are calculated. A special attention is paid to the case when the planes are magnetically non-equivalent and the interplanar coupling is relatively weak. A double peak structure in the temperature dependence of the specific heat and the entropy change in the external field is found. When the system is strongly anisotropic (the case of Ising couplings) the Exact Calculations for Finite Cluster are performed for comparison with the Pair Approximation method.