Magnetic properties of nanostructured CuFe2O4

The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by X-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mossbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1)*10(5)erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.