Preparation and magnetic properties of Ba2Co2Fe28O46 nanocrystals

Abstract

Ba 2 Co 2 Fe 28 O 46 nanocrystals were synthesized through a stearic acid sol–gel method. The reaction temperatures were notably lower than that of the conventional ceramic method. The nanocrystalline particles obtained at 750 °C were spherical in shape with grain sizes ranging from 10 to 25 nm. Their morphology became more cubic as the heat-treatment temperature was increased. The surface composition of the nanocrystals was found to be different from that of the bulk Ba2Co2Fe28O46, exhibiting a higher content of Ba and Co. The magnetic properties of these samples are clearly size dependent. The specific saturation magnetization is lower than that of bulk Ba2Co2Fe28O46 and inversely proportional to the grain size D. This can be explained by assuming the existence of a nonmagnetic layer on the surface of the particles. The coercivity of Ba2Co2Fe28O46 nanocrystals reaches a maximum when the particle sizes are approximately equal to 70 nm and then decreases by a factor of 3 as the particle size is further reduced. These results are attributed to superparamagnetic effects in a random distribution of single domain ferromagnetic particles.