Surface anisotropy effects in NiO nanoparticles

Magnetization and ferromagnetic resonance (FMR) experiments have been performed on 3 nm NiO noninteracting nanoparticles. The results indicate that the high temperature behavior is determined by the noncompensated moments of particle core, whose blocking is centered at T approximate to 60 K [field dependent maximum of the zero-field-cooled (ZFC) magnetization]. On the other hand, the low temperature behavior is determined by surface cluster spins whose thermal fluctuations freeze in a cluster-glass-like state at T approximate to 15 K (weakly field dependent maximum of ZFC magnetization), giving the major contribution to the effective anisotropy, as shown by the rapid increase of coercivity, remanent magnetization, and confirmed by the temperature dependence of signal intensity, resonant field, and linewidth of FMR spectra.