Sol-gel synthesis and magnetization study of Mn1−xCuxFe2O4 (x=0, 0.2) nanocrystallites

The magnetic nanoparticles of Mn_1−xCu_xFe₂O₄ (x=0, 0.2) were prepared by using a sol-gel method. It is proved that both the MnFe₂O₄ and Mn_0.8Cu_0.2Fe₂O₄ nanoparticle samples have superparamagnetic feature. Although the particle sizes are the same, substitution of a small fraction Cu for Mn results in the increase of magnetocrystallite anisotropy energy, thus enhances the blocking temperature from 130 K for MnFe₂O₄ to 260 K for Mn_0.8Cu_0.2Fe₂O₄. Mössbauer spectroscopy confirms that the anisotropy constant K of the Mn_0.8Cu_0.2Fe₂O₄ material is distinctly higher than that of the MnFe₂O₄ compound. Increase of the blocking temperature suggests that the approach we employed is effective to tackle the ‘superparamagnetic limit’ problem.