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

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 (PM) and ferromagnetic phases (T-c approximate to 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 intralayer 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 T-c. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing T-c. 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.