Comment on "Memory Effects in an Interacting Magnetic Nanoparticle System" by Sun et. al, prl, 91, 167206 (2003)

TL;DR

This comment challenges previous claims by showing that memory effects in magnetic nanoparticle systems can occur without interactions and are more prominent in non-interacting samples, explained by superimposed relaxation times.

Contribution

It demonstrates that memory effects are not exclusive to interacting particles and can be explained by superposition of relaxation times, contradicting prior assumptions.

Findings

Memory effects observed in non-interacting samples are more prominent.

Memory effects can be explained by superposition of relaxation times.

Interparticle interactions are not necessary for memory effects.

Abstract

In this Comment we report a phenomenon identical to that observed in ({Y. Sun, M. B. Salamon, K. Garnier and R. S. Averback, Phys. Rev. Lett. 91, 167206 (2003)}) for systems of NiFe{$_2$}O{$_4$} particles (mean size $\approx$ 3nm) embedded in a SiO{$_2$} matrix with two different interparticle spacings 4 nm (1) and 15 nm (2), which controls the strength of the dipolar interactions. Not only do we find the memory effect to be present in the non-interacting sample (2), indeed we find it to be {\em more} prominent than in the interacting case (1). We demonstrate that this effect can be simply attributed to a superposition of relaxation times of two sets of particles.