Size effect on the time dependence of magnetization of iron oxide particles

The dependence of the thermal stability of the magnetization on the size of iron oxide particles is experimentally investigated, with measurements of the variation of the magnetization in presence of a reverse field. It is found that when the size of the particles decreases, the thermal stability of the magnetization is progressively weakened, with increase of magnetization decay, magnetic viscosity and fluctuation field and decrease of activation volume. The ratio between activation volume and mean physical volume of the particles increases when the particle size decreases, approaching unity for the smallest particles, which suggests an evolution of the magnetization switching mode with the size of the particles.