Transition from ferromagnetism to superparamagnetism on the nanosecond time scale

Langevin dynamics has been used to study different aspects of the transition from ferromagnetism to superparamagnetism when reducing the size of thin magnetic single-domain nanoparticles with in-plane uniaxial anisotropy for observation times in the order of a few nanoseconds. It is found that nonuniformities in the magnetization, usually ignored for such small particles, favor thermal relaxation. Quantitatively, the size range over which the transition occurs is found to move roughly $1$ nm towards larger sizes due to this contribution. On the other hand, we also find that the thickness-dependent perpendicular demagnetizing field induces thermal switching between the two in-plane energy minima. The results are in good agreement with the Arrhenius law, which is used to characterize this effect. A strong dependence of the preexponential factor τ₀ on the particle thickness is found.