Fabrication and characterization of L 1₀-ordered FeNi thin films

L 1₀-ordered FeNi, showing high uniaxial magnetic anisotropy ( K _u), is promising as a 'rare metal-free' high K _u material. We have worked on L 1₀-ordered FeNi thin films prepared by two methods: one is molecular beam epitaxy (MBE) with alternate deposition of Fe and Ni monatomic layers, and the other is sputtering with co-deposition or multilayer-deposition of Fe and Ni followed by rapid thermal annealing (RTA). For the MBE films prepared by alternate monatomic layer deposition (leading to the stoichiometric composition: Fe 50 at.%– Ni 50 at.%), a clear relationship between K _u and the long-range order parameter S estimated by synchrotron x-ray diffraction (XRD) was found with maximum values of S   =  0.48 and K _u  =  7.0  ×  10⁶ erg cm⁻³. The composition dependence of K _u was also investigated by deviating the thickness from monatomic layer, showing a maximum of 9.3  ×  10⁶ erg cm⁻³ around 60 at.%Fe. In addition, the effect of Co addition to L 1₀-ordered FeNi was investigated, suggesting that a small amount (<10 at.%) of Co substitution for Ni would enhance K _u if S keeps the same. The experiments were in qualitatively good agreement with the first-principles calculations. The magnetic damping constant α was also measured to be approximately 0.01 irrespective of S , suggesting that L 1₀-FeNi is a candidate material with high K _u and low α . For the sputtered films with RTA, no major difference between co-deposition and multilayer-deposition was found: in both cases the formation of L 1₀-ordered phase after RTA was definitely confirmed by XRD. Transmission electron microscopy observations indicated that nanometer-sized L 1₀-ordered clusters were dispersed in a disordered phase, in contrast to that of MBE films showing the homogeneous formation of L 1₀-ordered phase. The enhancement of coercivity ( H _c) and residual magnetization ( M _r/ M _s) was observed associated with the appearance of L 1₀-ordered phase. The maxima of H _c and M _r/ M _s were obtained to be 1.35 kOe and 0.22, respectively. (topical review)