Magnetization temperature dependence and freezing of surface spins in magnetic fluids based on ferrite nanoparticles

Manganese and copper ferrite nanoparticles, in the size range 3.3-10.4 nm, are prepared by a hydrothermal coprecipitation process and peptized in aqueous solution. The thermal dependence of the high field magnetization is investigated in the dilute regime and the observed properties can be attributed to individual particles. Our results show that, at low temperatures, the structure of our nanoparticles can be seen as being made of a monodomain ordered core and a surface shell of disordered spins, which can fluctuate freely at high temperatures. Finite sizes effects have implications on the temperature dependence of the saturation magnetization m(S). Its variations are well accounted for by an effective Bloch law with an exponent larger than the bulk value for very small mean diameter (3.5 nm) and a Bloch constant slightly size decreasing for larger ones. A sharp increase of the high field magnetization, more marked as the size decreases, is evidenced at low temperature. It is related to a freezing of surface spins in a disordered state below a temperature of the order of 70 K and adjusted to a reduced exponential behavior.