Iron-based Magnetocaloric Materials Design

Search for magnetocaloric materials that do not contain rare earth or heavy metal elements, but compatible in both performance and price to state-of-the-art gadolinium (Gd) with excellent magnetocaloric effect (MCE) characteristics, is underway. In this presentation, we report results of our recent first-principles density functional theory (DFT) calculations on the ground state magnetic and electronic properties, and quasi harmonic and micromagnetic simulations of temperature and grain size dependent M-H loops and entropy change effects of Fe₂P_1-xSi_x based alloys. We find a sharp drop in the c/a ratio that occurs for 0.12<x<0.16, which indicates the onset of a hexagonal-to-orthorhombic transition, in agreement with recent room-temperature experimental finding. Further calculations show that Si substitution increases the saturation magnetization in both polymorphs, while Monte Carlo simulations reveal a corresponding rise in Curie temperature up to room temperature with increasing Si content. This enhancement is attributed to stronger interlayer exchange coupling between Fe atoms as the c/a ratio decreases, which stabilizes ferromagnetic ordering at higher temperatures. The thermodynamics of magnetocaloric and lattice entropy change, obtained via the quasi-harmonic approximation within DFT approach, will also be discussed to provide further insight into a possible second order phase transition and magnetostructural coupling in Fe₂P-based compounds.