Magnetism of ion-implanted nickel hydrides

Nickel hydrides $\mathrm{Ni}{\mathrm{H}}_x$ in the composition range $0<x<\mathrm{}2.00\mathrm{}$ were produced by low-temperature ion implantation. Their magnetic properties were studied via time-differential perturbed angular correlation (TDPAC) experiments on ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ implanted into the Ni host before hydrogenation. At low H concentration ($x\lesssim 0.10$) beats are observed in the TDPAC pattern, due to the reduction in hyperfine interaction (HFI) at the probe site by screening from neighboring H atoms. Excellent fits to the TDPAC spectra over the whole concentration range $0<x<\mathrm{}0.70\mathrm{}$ are obtained under the assumption that the H atoms randomly occupy the interstitial octahedral sites in the Ni lattice and that the probe HFI reductions due to H atoms in the first-, second-, and third-neighbor shell are simply additive (albeit different for each shell). The absolute magnitudes of the HFI reduction corresponding to each shell are the only free parameters in the analysis. We conclude that a supersaturated $\alpha$ phase is formed by implantation at least up to $x\sim 0.85$. The magnetization H concentration dependence of the implanted hydride is deduced from that of the average HFI. It is linear, and falls to zero at $x=(0.83\mathrm{}\pm 0.10)$. This result is identical to the one found in electrolytically charged samples, in spite of the fact that the latter's magnetism is determined by a combination of two phases (the magnetic $\alpha$ phase and the non-magnetic $\beta$ phase). The TDPAC of ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ in electrolytically charged and implanted samples at $x=0.70\mathrm{}$ are entirely different, as expected from the structural differences. The magnetization result for the implanted sample is discussed in terms of recent band-structure calculations.