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.
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Sakô et al. (1996) studied this question.