Effect of pressure-induced lattice distortion on physical properties of L10-FeNi ordered alloy

The ground-state properties of tetragonal L1₀ type FeNi alloy have been thoroughly studied, while its physical properties under high pressure are poorly understood. Here, the effect of pressure on the structural, magnetic, mechanical and dynamical properties of L1₀-FeNi are systematically investigated from the first principles calculations. The critical pressure of ferromagnetic collapse is detected to be 250 GPa, and the system is mechanically and dynamically stable below the critical pressure. The pressure-induced lattice distortion is identified in the pressure range of 80–150 GPa. Within this pressure range, the magnetic moments of the system decrease dramatically, the elastic constants C₁₂, C₁₃, C₃₃ and bulk modulus B show softening behavior, and consequently the ductility, longitudinal sound velocity, and acoustic Grüneisen constant exhibit softening behavior, while the elastic anisotropy increase sharply. Furthermore, there are significant variations in magnetocrystalline anisotropy, coercivity, maximum magnetic energy product and magnetic hardness parameters within the pressure range of lattice distortion. More interesting is the discovery that the pressure-induced lattice distortion triggers a transition from semi-hard to hard magnet near 130 GPa.