Surface spin-glass freezing in interacting core-shell NiO nanoparticles.

Magnetization and AC susceptibility measurements have been performed on ∼3 nm NiO nanoparticles in powder form. The results indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered surface shell. The core magnetic moments block progressively with decreasing temperature, according to the distribution of their anisotropy energy barriers, as shown by a broad maximum of the low field zero-field-cooled magnetization (M(ZFC)) and in the in-phase component χ' of the AC susceptibility, centred at ∼70 K. On the other hand, surface spins thermally fluctuate and freeze in a disordered spin-glass-like state at much lower temperature, as shown by a peak in M(ZFC) (at 17 K, for H = 50 Oe) and in χ'. The temperature of the high temperature χ' peak changes with frequency according to the Arrhenius law; instead, for the low temperature maximum a power law dependence of the relaxation time was found, τ = τ(0)(T(g)/(T(ν)-T(g)))(α), where α = 8, like in spin glasses, τ(0) = 10(-12) s and T(g) = 15.9 K. The low temperature surface spin freezing is accompanied by a strong enhancement of magnetic anisotropy, as shown by the rapid increase of coercivity and high field susceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with an antiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of the temperature dependence of the coercivity. Interparticle interactions lead to a shift to a high temperature of the distribution of the core moment blocking temperature and to a reduction of magnetization dynamics.