Ferromagnetic resonance in bulk nanocrystalline Ni

A detailed lineshape analysis of the ferromagnetic resonance (FMR) spectra taken on pulse electrodeposited nanocrystalline (nc-) Ni sheets (with the average crystallite size, d, varying from 10 nm to 40 nm) at temperatures ranging from 113 K to 325 K yield accurate values for saturation magnetization, Ms(T), Landé splitting factor, g, anisotropy field, Hk(T), resonance field, Hres, and FMR linewidth, ΔHpp(T). Thermally-excited spin-wave (SW) excitations completely account for Ms(T) and the SW description of Ms(T) gives the values for the saturation magnetization and spin-wave stiffness at absolute zero of temperature, i.e., Ms(0) and D0, for nc-Ni samples of different d that are in excellent agreement with the corresponding values deduced previously from an elaborate SW analysis of the bulk magnetization data. While Ms(0) varies with d as Ms(0)~d-3/2,D0 follows the power law D0∼d4/3. The angular variations of Hres in the 'in-plane' as well as 'out-of-plane' sample configurations, demonstrate that the main contribution to Hk(T) comes from the cubic magnetocrystalline anisotropy. The exchange-conductivity mechanism describes the observed thermal decline of ΔHpp reasonably well but fails to explain the very large magnitude of ΔHpp at any given temperature. By comparison, the Landau-Lifshitz-Gilbert (LLG) damping gives a much greater contribution to ΔHpp but the LLG contribution is relatively insensitive to temperature.