Thermo-magnetic properties of the magnetocaloric layered materials based upon FeMnAsP: a Green function-method approach

TL;DR

This paper employs a Green function approach to analyze the thermo-magnetic properties of FeMnAsP-based magnetocaloric materials, incorporating detailed crystal structure and spin correlations, and achieving good agreement with experimental data.

Contribution

It introduces a Green function theoretical method for magnetocaloric materials, improving upon simple models by including crystal structure details and spin correlations.

Findings

Accurately reproduces first-order phase transitions.

Matches experimental magnetocaloric data.

Predicts magnetic field dependence consistent with Landau theory.

Abstract

The compounds FeMnAsxP1-x are very promising as far as commercial applications of the magnetocaloric effect are concerned. However, the theoretical literature on magnetocaloric materials still adopts simple molecular-field models in the description of important properties like the entropy variation that accompanies applied isothermal magnetic field cycling, for instance. We apply a Green function theoretical treatment for such analysis. The advantages of such approach are well-known since the details of the crystal structure are incorporated in the model, as well as a precise description of correlations between spins of the transition metal ions can be obtained. For the sake of simplcity we adopt a simple one-exchange parameter Heisenberg model, and the observed first-order phase transitions are reproduced by the introduction of a biquadratic term in the hamiltonian. Good agreement with experimental magnetocaloric data for FeMnAsxP1-x compounds is obtained, as well as an agreement with the magnetic field dependence for these properties predicted from the Landau theory of continuous phase transitions.