Magnetic resonance study of exchange-biased Ni/NiO nanoparticles

Finite sized Ni/NiO nanoparticles (NPs) are prepared by oxidizing pure Ni-NPs in an ambient atmosphere with varying annealing time (t(A)). A synchrotron radiation x-ray diffraction technique was used to estimate the grain size, weight fraction and lattice parameters of Ni and NiO. The temperature (T) dependencies of effective g-factor and line-width of ferromagnetic resonance (FMR) spectra for a series of Ni/NiO NPs are determined. Three T-regions with different FMR behaviors T > 200 K, 200 > T > 130 K and T < 130 K are identified. In particular, for T < 200 K, the T-dependency of the g-factor reveals an evolution of exchange coupling between Ni and NiO due to the gradual oxidation of Ni NPs.