Construction of FeN alloy films with ultra-strong magnetism and tunable magnetic anisotropy for spintronic application

FeN alloy film is a promising spintronic material with the theoretically ultra-strong magnetism (saturation magnetization MS and magnetic anisotropy Keff) and high spin polarization, which relies on the degree of N ordering interstice occupancy (S). However, due to the high activation energy for N ordering, the S value of an actual FeN film is mostly lower than 35% and this restricts the achievable magnetism and transportation property. Thus, the construction of a FeN alloy film with well-controlled magnetism and efficient electronic transportation remains a long-standing challenge. Here, we tackle the problem by strain engineering. Using an Fe/Cr underlayer, we introduced a considerable epitaxial strain in the FeN lattice. The strain is proven to effectively promote the S value to over 60%, resulting in remarkable enhancement of MS value from 2.18T to 2.81T (30% increment) and effective tunability of Keff value ranging 1.3∼2.2 × 106 J/m3. Besides, the matched energy band symmetry (Δ5) between Cr and Fe16N2 facilitates the efficient electronic transportation for spintronic applications. By simulating interstice distribution with the first-principles calculations, the lattice strain is found to decrease the activation energy for N interstitial migration, which serves as a thermodynamic driving force for the magnetism tunability.