Temperature, magnetic field, and pressure dependence of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2)

TL;DR

This study investigates how temperature, magnetic field, and pressure influence the crystal and magnetic structures of Mn1.1Fe0.9(P0.8Ge0.2), revealing a single combined phase transition and the effects of external variables on the magnetocaloric properties.

Contribution

It provides detailed neutron diffraction analysis of the phase transition and demonstrates how sample homogeneity affects phase coexistence in this magnetocaloric compound.

Findings

Single magnetic and structural transition at ~255 K.

Pressure decreases transition temperature, magnetic field increases it.

Phase coexistence is linked to sample inhomogeneity and can be reduced by heat treatment.

Abstract

Neutron powder diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. The data reveal that there is only one transition observed over the entire range of variables explored, which is a combined magnetic and structural transformation between the paramagnetic to ferromagnetic phases (Tc~255 K for this composition). The structural part of the transition is associated with an expansion of the hexagonal unit cell in the direction of the a- and b-axes and a contraction of the c-axis as the FM phase is formed, which originates from an increase in the intra-layer metal-metal bond distance. The application of pressure is found to have an adverse effect on the formation of the FM phase since pressure opposes the expansion of the lattice and hence decreases Tc. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing Tc. We find that the substantial range of temperature/field/pressure coexistence of the PM and FM phases observed is due to compositional variations in the sample. In-situ high temperature diffraction measurements were carried out to explore this issue, and reveal a coexisting liquid phase at high temperatures that is the origin of this variation. We show that this range of coexisting phases can be substantially reduced by appropriate heat treatment to improve the sample homogeneity.