Compositional inhomogeneity in electroless-deposited CoNiP films studied by spin-echo ⁵⁹Co nuclear magnetic resonance

A compositionally inhomogeneous state in electroless-deposited CoNiP films with perpendicular magnetic anisotropy is investigated using spin-echo ⁵⁹Co nuclear magnetic resonance (NMR), focusing on its correlation with magnetic properties. By adjusting pH of the electroless-deposition bath, films were prepared which possess almost the same average composition (Co₄₂Ni₅₀P₈) and crystal orientation, but the differing values of coercivity (H_c). By analyzing the shift in peak resonance frequency of NMR spectra of the CoNiP films with respect to that of bulk CoNi alloys, we observed the formation of a compositionally inhomogeneous state in the film which possesses high H_c of 2600 Oe, whereas formation of inhomogeneities was not observed for the low-H_c films. By assuming that a Co-enriched ferromagnetic component detected by NMR was separated from a non-magnetic amorphous Ni-P component, we found that the high-H_c film consists of Co₄₂Ni₂₆ and Ni₂₄P₈. Moreover, the results of high spatial resolution X-ray microanalysis on the high-H_c film showed a tendency that the Co-enriched ferromagnetic component segregates to the center of grains and the Ni-P non-magnetic component segregates to grain boundaries. Such evidence indicates that compositional inhomogeneities could be one of the origins of high-H_c in CoNiP films.