The magnetization of a magnet is only stable when the energy barrier for its reversal is large compared to the thermal energy.  As the volume of a magnet is decreased, often to the nanometer scale, this stability is eventually lost, and the magnet enters what is known as a superparamagnetic state where the magnetization is constantly flipping back and forth.  This behavior limits the size of grains that can be used in magnetic data storage, but is useful, for instance, for medical imaging and treatment techniques.

To describe the behavior of a superparamagnetic particle in the presence of a magnetic field, it was assumed that it behaves like a single giant spin, and therefore the fundamental equation first proposed by the French physicist Paul Langevin about a century ago to describe paramagnetism was used.  The Langevin equation has been widely used to analyze experiments performed on ensembles of magnetic nanoparticles where assumptions concerning the volume and shape distribution of the nanoparticles in the ensemble are required, and without the ability to observe the underlying dynamics.

In a research conducted in our group, we have monitored directly the effect of an applied magnetic field on the superparamagnetic fluctuations of an individual volume.  The experiment was performed on a patterned nanostructure of the compound SrRuO₃ and confirmed directly the Langevin equation and its underlying dynamics.

This result opens novel routes to the study of superparamagnetism, whose importance increases with the shrinking size of the magnets used in nanoscale storage and electronics technologies.

Omer Sinwani, James W. Reiner, and Lior Klein, Monitoring superparamagnetic Langevin Behavior of individual SrRuO3 nanostructures, Phys. Rev. B 89, 020404(R) (2014).