The effect of the decrease in the coordination number of atoms in surface on the magnetic transition temperature was investigated experimentally and theoretically for CoO ultrafine particles. The decreasing tendency of the Néel temperature T N with decreasing average size was confirmed in the size range from 2.0 to 3.0 nm through the ESR measurement. The similar tendency of T N for size was gained through the calculation of T N in the mean-field theory using the prism model within the Ising thin-film model.
The magnetic nature of a NiO shell and a Ni core were separately investigated in the temperature dependence of the magnetization curve of the samples with the average sizes of 1.6, 2.0, 2.2, 2.5, 2.8, 3.0 and 3.2 nm. The superparamagnetism of the NiO shell and Ni core were observed in specific temperature ranges. The increase of the magnetic moment per atom in the metallic free cluster with decreasing size were not observed in the small Ni core with the NiO shell though the proper saturation magnetization of Ni core was estimated.
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