Fabrication and characterization of L10-ordered FeNi thin films

Abstract

L10-ordered FeNi, showing high uniaxial magnetic anisotropy (Ku), is promising as a ‘rare metal-free’ high Ku material. We have worked on L10-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 Ku and the long-range order parameter S estimated by synchrotron x-ray diffraction (XRD) was found with maximum values of S = 0.48 and Ku = 7.0 × 106 erg cm−3. The composition dependence of Ku 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 L10-ordered FeNi was investigated, suggesting that a small amount (<10 at.%) of Co substitution for Ni would enhance Ku 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 L10-FeNi is a candidate material with high Ku 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 L10-ordered phase after RTA was definitely confirmed by XRD. Transmission electron microscopy observations indicated that nanometer-sized L10-ordered clusters were dispersed in a disordered phase, in contrast to that of MBE films showing the homogeneous formation of L10-ordered phase. The enhancement of coercivity (Hc) and residual magnetization (Mr/Ms) was observed associated with the appearance of L10-ordered phase. The maxima of Hc and Mr/Ms were obtained to be 1.35 kOe and 0.22, respectively.