Temperature dependence of magnetic anisotropy constant in CoFe₂O₄ nanoparticles examined by Mössbauer spectroscopy

The temperature dependence of the effective magnetic anisotropy constant of CoFe₂O₄ nanoparticles is determined based on the measurements of SQUID magnetometry and Mössbauer spectroscopy. Under an intuitive assumption that the superparamagnetic fraction of the cumulative area in the particle size distribution at a temperature is equal to the doublet fraction in the Mössbauer spectra at that temperature, we are able to get a relation between r and T_B, from which the temperature dependence of the effective magnetic anisotropy constant is determined. The resultant magnetic anisotropy constant increases markedly with decreasing temperature from 2.0 ×10⁵ J/m ³ at 300 K to 8.3 ×10⁶ J/m ³ at 125 K