Ultrafast magnetism at the nanoscale

Single-domain magnetic nanoparticles constitute an attractive system for fundamental research as well as for advanced technological applications. The use of single-domainmagnetic nanoparticles is expected to increase data storage density to several petabits per square inch in the near future. However, when the size of the nanoparticles is reduced, the superparamagnetic regime can be attained and the magnetization fluctuates under the effect of thermal excitations. This effect is a major drawback for technological developments and it is therefore important to investigate and understand the thermally induced magnetization reversal in these systems.

In isolated single-domain magnetic nanoparticles, the magnetization reversal by thermal activation is well described by the Neel-Brown model. The thermal fluctuations cause the magnetic moment to undergo a Brownian-like motion about the axis of easy magnetization, with a finite probability of flipping from one equilibrium direction to another.

In our group, we study the magnetization reversal using either the Landau-Lifschitz-Gilbert equation or an equivalent approach based on the Fokker-Planck equation. We have investigated the effect of the dipolar interactions between an assembly of magnetic nanoparticles in the mean-field approximation. We are currently studying how to control the magnetization reversal by making use of oscillating (microwave) magnetic field.