Surface magnetic anisotropy of CoFe2O4 nanoparticles with a giant low-temperature hysteresis

Ferrimagnetic nanoparticles of CoFe2O4 about 4–16 nm in diameter were synthesized by pyrolysis of a mixture of Fe and Co acetylacetonates. The field dependences of magnetization and hysteresis loops were investigated in magnetic fields up to 4000 kA/m at various temperatures from 4.2 to 500 K. A considerable contribution, positive or negative depending on the temperature, of “surface” anisotropy to the effective magnetic anisotropy of the nanoparticles was observed. A correlation was found between the magnetic properties that represent the specificity of small particles, namely, between “surface” anisotropy, magnetization, and high-field susceptibility.