Uniaxial magnetic anisotropy of L10-FeNi films with island structures on LaAlO3(110) substrates by nitrogen insertion and topotactic extraction

Tetrataenite (L10-FeNi) has potential for application in next-generation permanent magnets. This compound is a rare-earth-free material comprising Fe and Ni, which are abundant elements. In addition, an extremely high uniaxial magnetocrystalline anisotropy (Ku) is expected for L10-FeNi because of the ordered arrangement of the elements. This study reports the magnetic properties of uniaxially oriented L10-FeNi films with island structures fabricated by a novel method involving the radio-frequency sputter deposition of FeNi alloy films onto a LaAlO3 (LAO)(110) substrate, followed by nitrogen insertion and topotactic extraction. FeNiN(110) films are epitaxially grown on the LAO(110) plane by the nitriding and additional annealing process of the FeNi alloy films at high temperatures under ammonia gas to reduce the volume fraction of crystallographic variants. Uniaxially oriented L10-FeNi alloy films with island structures are obtained after the topotactic denitriding of the FeNiN films. The uniaxial magnetic anisotropy of the L10-FeNi films is determined from the magnetization curves and via magnetic torque measurements of the films. The obtained L10-FeNi film exhibiting a relatively high superlattice order parameter of 0.74 yielded a uniaxial magnetocrystalline anisotropy energy of ∼0.63 MJ/m3 at 300 K, as well as coercivities of 319 and 358 kA/m at 300 and 10 K, respectively. This coercivity is the highest value reported for artificial L10-FeNi to date.