Ac magnetic susceptibility measurements in nanoparticulate powders of iron-doped ZnO

Low temperature (T) ac magnetic susceptibility (χ_ac) measurements were performed in powder samples of the Zn_1−xFe_xO (x=0-0.078) prepared by a combustion reaction method. The amplitude of the ac magnetic field was kept constant (1 mT) while its frequency (f) varied in the range 10-10⁴ Hz for 5≤T≤300 K. The diluted samples presented cusps with maxima distributed around T_f=17.5±0.5 K while no such feature was observed for the pure sample (x=0). The cusp was found to become more pronounced with the increasing Fe content. Also it decreased and shifted to higher values of T by increasing f. The shift in T_f was found to follow a Vogel-Fulcher law with E_a/k_B=317.6 K, T₀=4.65 K and τ₀=10⁻¹⁴ s, for the activation energy, critical temperature and characteristic time-constant, respectively. The energy gap of the sample was measured and they were found to vary in the range 3.75-3.80 eV. The overall χ_ac-data resembles those obtained for cluster spin-glass state.