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

Takahiro Nishio,Hideto Yanagihara,Koki Takanashi,Hiroaki Kura,Keita Ito

doi:10.1063/5.0062692

Abstract

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.

Highlights

Ferromagnetic materials for permanent magnets have been investigated extensively owing to the increasing demand for highperformance magnets in high-efficiency motors for electric vehicles as well as other magnetic devices.1,2 both high-saturation magnetization (Ms) and coercivity (Hc) are desired for permanent magnets, typical permanent magnets with large Ms and Hc contain rare earth elements or noble metals, which renders them economically unsustainable.1 To eliminate the use of rare earth elements or noble metals, the L10-ordered FeNi alloy has garnered substantial attention as a material for ferromagnets.3–15 L10-FeNi possesses a face-centered tetragonal structure with alternately stacked monoatomic layers of Fe and Ni along the c-axis
Assuming that the islands are isolated for tn = 10 and 20 nm, the average aspect ratio deq/tp was calculated to be 2.6 ± 0.6, which indicates that the islands are flat and oval shaped in the in-plane direction of the substrate
Island structures of the L10-FeNi alloy were formed using the nitrogen insertion and topotactic extraction (NITE) method with sputter-deposited FeNi alloy films in the VW growth mode

Summary

INTRODUCTION

Ferromagnetic materials for permanent magnets have been investigated extensively owing to the increasing demand for highperformance magnets in high-efficiency motors for electric vehicles as well as other magnetic devices. both high-saturation magnetization (Ms) and coercivity (Hc) are desired for permanent magnets, typical permanent magnets with large Ms and Hc contain rare earth elements or noble metals, which renders them economically unsustainable. To eliminate the use of rare earth elements or noble metals, the L10-ordered FeNi alloy has garnered substantial attention as a material for ferromagnets. L10-FeNi possesses a face-centered tetragonal structure with alternately stacked monoatomic layers of Fe and Ni along the c-axis. In the case of noncontinuous films, for instance, when FeNi was sputter-deposited on an amorphous substrate and island-grown, a maximum coercivity of ∼64 kA/m was obtained.. The utilization of the NITE method, which facilitates the formation of highly ordered L10structures, with island-grown FeNi films is expected to enhance their coercivities. The third advantage is the effect of stress and strain relaxation on the volume change owing to the phase transitions during the NITE process: FeNiN is ∼30% larger than FeNi per unit cell volume.. The third advantage is the effect of stress and strain relaxation on the volume change owing to the phase transitions during the NITE process: FeNiN is ∼30% larger than FeNi per unit cell volume.10 This results in volume expansion and contraction during nitriding and denitriding, respectively. It is expected to reduce the effect of distortion or disordering associated with the reactions

Synthesis

Material characterization

RESULTS AND DISCUSSION

CONCLUSION