Annealing Effect on the Magnetic Properties of Polyol-made Ni−Zn Ferrite Nanoparticles

The effects of annealing on the magnetic properties of nanocrystalline Ni 1− x Zn x Fe 2 O 4 ( x = 0.2, 0.4) have been studied. The as-prepared x = 0.2 and 0.4 samples with the particle size of about 6 nm present a relatively low blocking temperature of about 47 and 53 K, respectively, as derived from the peak of the zero-field-cooled direct current (dc) susceptibility measurements. The particle size significantly increases after annealing. Up to the annealing temperature of 800 °C, the blocking temperature is found to increase with increasing particle size, whereas the effective anisotropy constant exhibits a maximum for an annealing temperature of 400 °C. The latter effect is attributed to two antagonist contributions: the first one is related to the partial densification of the particles which introduces canting in the crystal interfaces while the second one is related to the size increase of the particles which reduces the surface anisotropy contribution.