Fe-Ni based alloys as rare-earth free gap permanent magnets

L 1 0 -ordered FeNi phase has potential as a rare-earth free permanent magnet due to its large magnetization and high Curie temperature. However, low long-range crystal ordering and weak magnetocrystalline anisotropy ( K u ) impede its practical use in a technologically relevant permanent magnet. Employing density functional and Monte Carlo simulations, we demonstrate the substantial improvements on structural and thermal stability, disorder-order phase transition, and intrinsic permanent magnetic properties of Fe − Ni structures. We propose that this is achievable through the complementary elemental substitutions by metal elements (M) and interstitial doping with 2 p elements. Fe 1 -x M x Ni with simple metal (M = Ga and Al) and metalloid (M = Ge and Si) elemental substitutions at x = 0.5 are broadly known as Heusler ( L 2 1 ) structures with no permanent magnet characteristics. On the contrary, we find that for x = 0 − 0.5, L1 0 -type structure is energetically favored over the Heusler-type structures. The predicted structures exhibit K u values up to approximately 2.1 MJ/m 3 , which is roughly three times that of FeNi phase (0.68 MJ/m 3 ), where the absolute value of K u depends on the degree of L 1 0 crystal ordering. Interstitial doping with B elevates K u further up to a maximum value of 3.9 MJ/m 3 (at 0 K), leading to room-temperature intrinsic permanent magnetic properties, including maximum energy density product ( B H ) max , anisotropy field μ 0 H a , and hardness parameter κ , comparable to the widely investigated MnBi and MnAl permanent magnets. These results may serve as a guideline in designing Fe − Ni based rare-earth free gap permanent magnetic materials that were previously overlooked.