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 sa mples of magnetic nanoparticles. In addition to usual magnetic viscosity me asurement, we have used the residual memory ratio (RMR) method. This proced ure enables us to overcome the uncertainties usually associated with the en ergy barrier distribution, thus giving a more detailed insight on the natur e of the observed dynamics. A custom-made apparatus coupling dilution refri geration and SQUID magnetometry allowed measurements of very diluted sample s at temperatures ranging between 60 mK and 7 K. Two types of particles hav e been studied: gamma -Fe2O3 of moderate anisotropy, and CoFe2O4 of higher anisotropy where quantum effects are more likely to occur. In both cases, t he data cannot simply be interpreted in terms of mere thermally activated d ynamics of independent particles. The deviation from thermal activation see ms to go opposite of what is expected from the possible effect of particle interactions. We therefore believe that it suggests the occurrence of quant um dynamics at very low temperatures. (C) 2000 Elsevier Science B.V. All ri ghts reserved.