On the origin of the surface magnetic anisotropy in manganese-ferrite nanoparticles

Magnetic resonance has been used to obtain the surface anisotropy in mangan ese-ferrite nanoparticles from 100 to 250 K. The surface anisotropy data ha s been successfully explained using a model that includes the single-ion (m agnetocrystalline) term plus the magnetoelastic term The single-ion surface -anisotropy contribution has been calculated using the mean-field approxima tion proposed by Millev and Fanhle [Phys. Rev. B 51 (1995) 2937]. The tempe rature dependence of the magnetostriction has been included in the descript ion of the magneto elastic term. We found that the, surface-anisotropy chan ges from negative to positive at a characteristic temperature which is expl ained by the model proposed in this study. (C) 2001 Elsevier Science B.V. A ll rights reserved.