Disentangling Distribution Effects and Nature of the Dynamics in Relaxation Measurements: the RMR Method

We discuss here the nature of the low temperature magnetic relaxation in samples of magnetic nanoparticles. In addition to usual magnetic viscosity measurement, we have used the Residual Memory Ratio (RMR) method. This procedure enables us to overcome the uncertainties usually associated with the energy barrier distribution, thus giving a more detailed insight on the nature of the observed dynamics. A custom made apparatus coupling dilution refrigeration and SQUID magnetometry allowed measurements of very diluted samples at temperatures ranging between 60mK and 7K. Two types of particles have been studied: γ-Fe2O3 of moderate anisotropy, and CoFe2O4 of higher anisotropy where quantum effects are more likely to occur. In both cases, the data cannot simply be interpreted in terms of mere thermally activated dynamics of independent particles. The deviation from thermal activation seems to go opposite of what is expected from the possible effect of particle interactions. We therefore believe that it suggests the occurrence of quantum dynamics at very low temperatures.