Magnetic properties of Ba4Co2Fe36O60 nanocrystals prepared through a sol–gel method

Ba4Co2Fe36O60 (Co2–U) nanocrystals were synthesized through a stearic acid sol–gel method. The reaction temperatures were dramatically lower than those prepared through the conventional ceramic method. The nanocrystalline powders obtained at 750°C were approximately spherical in shape with grain sizes ranging from 10 to 25 nm and became cubic-like when the heat-treatment temperature was increased. The surface composition of the nanocrystalline Ba4Co2Fe36O60 was found to be different from that of the bulk counterpart material, 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 Ba4Co2Fe36O60 and decreases with the grain size D. This phenomenon is explained by assuming the existence of a nonmagnetic layer on the surface of the particles. The coercivity of Ba4Co2Fe36O60 nanocrystals reaches a maximum when the particle sizes are approximately equal to 80 nm and then decreases notably as the particle sizes are further reduced. These results are attributed to the superparamagnetic effects in a random distribution of single domain ferromagnetic particles.