Tuning magnetic properties of rare-earth-free ferromagnetic Fe2Ni2N alloys via strain

초록

This study used first-principles calculations based on density functional theory (DFT) to analyze how uniaxial strain along the c -axis affects the electronic structure and, consequently, the magnetic properties of a tetragonally ordered Fe₂Ni₂N alloy. The saturation magnetization ( M S) of Fe₂Ni₂N decreases to 1.28 T at a + 8% c / a ratio from 1.35 T in the unstrained state. In contrast, the magnetocrystalline anisotropy constant ( K u) increases significantly to 2.75 MJ/m3 at a + 8% c / a ratio, compared to 1.88 MJ/m3 without strain. To assess the thermal stability of the magnetic phase under uniaxial strain, the Monte Carlo method was used. Compressive strain can increase the Curie temperature ( T C) by up to 50 K in comparison to the unstrained state, but tensile strain decreases it by as much as 250 K. At higher tensile strain, Fe₂Ni₂N transitions from a ferromagnetic to a ferrimagnetic state and becomes dynamically unstable, as evidenced by the appearance of imaginary phonon frequencies. Strained Fe2Ni2N can achieve a maximum energy product ( BH )max of 46.11 MGOe (366.9 kJ/m3) at 0 K and 29.3 MGOe (233.2 kJ/m3) at 300 K. The mechanisms of changes in the magnetic properties of Fe₂Ni₂N under uniaxial strain were elucidated through an extensive analysis of the total and d -orbital-resolved density of states (DOS), the anisotropy of the orbital magnetic moments, the band structures, and exchange interaction parameters. Magnetocrystalline anisotropy energy (i.e., K u) is inversely related to the energy difference between unoccupied and occupied electron states. Strain alters this energy difference, which then adjusts K u. Applying strain to Fe2Ni2N effectively tunes the M S, K u, dynamic stability, and T C of nitrogen-doped FeNi-based compounds

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