Determination of the Temperature Dependence of the Magnetic Anisotropy Constant in Magnetite Nanoparticles

The temperature dependence of the effective magnetic anisotropy constant, K(T ), of Fe3O4 (magnetite) nanoparticles is obtained based on SQUID (superconducting quantum interference device) magnetometry. 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 from 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 5.9 × 10 J/m at 5 K to 1.1 × 10 J/m at 280 K.