Structure, phase transitions and hard magnetic properties of ternary Fe1.93(P1-xSix) compounds with x ≤ 0.5

Fe₂(P,Si) ternary derivatives from the Fe₂P parent hexagonal compound have so far received less attention than their quaternary counterparts, such as the (Mn,Fe)₂(P,Si) giant magnetocaloric materials; Yet, Fe₂(P,Si) compounds present an intriguing phase diagram with the development of a body-centered orthorhombic (BCO) structure when Si substitutes P. Here, we revisit their crystal structure and magnetic properties with the objective of establishing the properties of orthorhombic Fe_1.93P_1-xSi_x compounds. The BCO to Fe₂P-type transition observed in Si substituted samples is found to be of first-order type and associated with a significant latent heat, a large volume discontinuity and an electrical transport anomaly. Furthermore, the potential for inducing this transition through the application of external fields was investigated. It revealed that the relatively modest difference in magnetization between the hexagonal and BCO structures renders the transition more sensitive to physical pressure. The recent surge of interest in the application of Fe₂P-type materials as permanent magnets also prompted us to investigate their magneto-crystalline anisotropy. A uniaxial anisotropy is found even in the BCO phase, with a large anisotropy constant of approximately 0.86 MJm⁻³ at room temperature. In combination with Curie temperatures which are much higher than room temperature, this makes BCO compounds potential rare-earth free permanent magnets, as demonstrated by the observation of a finite coercivity in BCO Fe_1.93P_0.6Si_0.4 ball milled powders (H_C ≈ 1.5 kOe). This work reveals that the exploration of Fe₂P materials for permanent magnet applications should not be limited to the hexagonal structure.