Simultaneous tuning of the magnetic anisotropy and thermal stability of alpha ^{''}-phase Fe_{16}N_{2}

D Odkhuu,S C Hong

doi:10.1038/s41598-021-87077-2

Abstract

Simultaneously enhancing the uniaxial magnetic anisotropy (K_u) and thermal stability of alpha ^{''}-phase Fe_{16}N_{2} without inclusion of heavy-metal or rare-earth (RE) elements has been a challenge over the years. Herein, through first-principles calculations and rigid-band analysis, significant enhancement of K_u is proposed to be achievable through excess valence electrons in the Fe_{16}N_{2} unit cell. We demonstrate a persistent increase in K_u up to 1.8 MJ m^{text {-}3}, a value three times that of 0.6 MJ m^{text {-}3} in alpha ^{''}-Fe_{16}N_{2}, by simply replacing Fe with metal elements with more valence electrons (Co to Ga in the periodic table). A similar rigid-band argument is further adopted to reveal an extremely large K_u up to 2.4 MJ m^{text {-}3} in (Fe_{0.5}Co_{0.5})_{16}N_{2} obtained by replacing Co with Ni to Ga. Such a strong K_u can also be achieved with the replacement by Al, which is isoelectronic to Ga, with simultaneous improvement of the phase stability. These results provide an instructive guideline for simultaneous manipulation of K_u and the thermal stability in 3d-only metals for RE-free permanent magnet applications.

Highlights

Alpha-phase iron has been known for its extraordinary magnetic properties, including high saturation magnetization ( μ0Ms ) and Curie temperature (T c ), in addition to its relatively simple fabrication and low price
We propose a possible mechanism of tuning the number of valence electrons to simultaneously enhance the thermal stability and Ku by a few times in Fe16N2 and (Fe0.5Co0.5)16N2 apart from the aforementioned approaches (1) and (2), using first-principles calculations and rigid-band model analysis
We further investigate the structural stability at an elevated temperature using ab initio molecular dynamic (AIMD) simulation

Summary

Introduction

Alpha-phase iron has been known for its extraordinary magnetic properties, including high saturation magnetization ( μ0Ms ) and Curie temperature (T c ), in addition to its relatively simple fabrication and low price. These intriguing features make it a potential champion ever for high-performance permanent magnet applications[1,2,3,4]. The tetragonal phase of c/a = 1 is accessible in epitaxial Fe films with a diverse choice of lattice-mismatched substrates. Numerous efforts have been made to improve the thermal stability of α

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Conclusion