Investigation of antiferromagnetic order in FeMnP0.75Si0.25 alloy for magnetocaloric application by first principles calculations

Hexagonal Fe₂P-type FeMnPX (X=Si, Ge, As) alloys have been considered as candidate magneto-caloric materials recently due to their remarkable magnetocaloric properties. Among them, the FeMnPSi alloys are being in intense focus owing to their cheap and non-toxic raw materials, tunable Curie temperature and magnetic transition temperature hysteresis, and good magnetocaloric properties. For FeMnP_0.75Si_0.25 alloy, detailed experimental and theoretical observations indicates that it can be in ferromagnetic (FM, about 4.5 μ_B/unit cell) and/or antiferromagnetic (AFM) states with different atom disorder level between 3f and 3g sites, which depends on the kinetic diffusion process from high temperature partial disordered paramagnetic (PM) state to low temperature ordered FM state. The possible AFM magnetic structure of FeMnP_0.75Si_0.25 alloy was reported to be in a supercell along a-axis. However, the atomic magnetic moments in a-axis supercell and the experimental values show large difference. This motivates us to investigate other types of AFM magnetic structure in FeMnP_0.75Si_0.25 alloy. Here, the supercell along c-axis was investigated. In this work, the freedom offered by ab initio techniques was fully used to investigate the coupling between magnetic and chemical orders in FeMnP_0.75Si_0.25 alloy. The magnetic and chemical disorder was treated by the recently developed alloy theory formulated within the Coherent Potential Approximation (CPA) as implemented in the Exact Muffin-Tin Orbitals (EMTO) method. All calculations were carried out within the generalized gradient approximation using the Per-dew-Burke-Ernzerhof parameterization (PBE). The crystal parameters for the calculations were taken from experiments.