Temperature dependence of magnetic anisotropy constant in cobalt ferrite nanoparticles

The temperature dependence of the effective magnetic anisotropy constant K(T) of CoFe2O4 nanoparticles is obtained based on the SQUID magnetometry measurements and Mössbauer spectroscopy. The variation of the blocking temperature TB as a function of particle radius r is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and TB. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 1.1×107 J/m3 at 5 K to 0.6×105 J/m3 at 280 K. The attempt time τ0 is also determined to be 6.1×10−12 s which together with the K(T) best explains the temperature dependence of superparamagnetic fraction in Mössbauer spectra.