Fabrication of L1-FeNi films with island structures by nitrogen insertion and topotactic extraction for improved coercivity

Conventional permanent magnets that exhibit high-saturation magnetizations and coercivities contain rare earth elements or noble metals, which renders them economically unsustainable. L10-FeNi alloy, which does not contain such elements, possesses excellent magnetic properties even at high temperatures. However, the coercivity of the fabricated alloy is much lower than its potential coercivity. In this study, to investigate the temperature dependence of L10-FeNi and its magnetic properties in the vicinity of the single magnetic domain size, L10-FeNi island structures were fabricated via sputter deposition of a FeNi alloy and subsequent application of the nitrogen insertion and topotactic extraction method. The crystallinity of FeNiN in the islands improved during the nitriding process, whereas in the denitriding process, the isolation of the islands for magnetic decoupling proceeded owing to volume shrinkage during phase transition. The superlattice structures in each process were confirmed based on transmission electron microscopy diffraction patterns. By varying the nominal film thickness, it was found that a 20 nm film containing island structures, which had undergone a complete denitriding reaction and showed particle sizes close to the single magnetic domain size, exhibited the highest coercivity (188 kA/m at 10 K) reported thus far. Even at 600 K, the coercivity was only reduced by 15% compared to that at 300 K. These results suggest that L10-FeNi with a controlled island structure is a promising magnetic material with good heat resistance.