Aging and Memory Effects in Superparamagnets and Superspin Glasses

TL;DR

This paper investigates aging and memory effects in superparamagnets and superspin glasses, revealing distinct dynamical behaviors and their theoretical explanations through experiments and models.

Contribution

It provides a comparative analysis of slow dynamics in superparamagnets and superspin glasses, highlighting unique phenomena and their theoretical interpretations.

Findings

Superparamagnets exhibit aging and memory effects.

Superspin glasses show unique phenomena like flat field-cooled magnetization.

The observed phenomena are explained by the droplet theory and random energy model.

Abstract

Many dense magnetic nanoparticle systems exhibit slow dynamics which is qualitatively indistinguishable from that observed in atomic spin glasses and its origin is attributed to dipole interactions among particle moments (or superspins). However, even in dilute nanoparticle systems where the dipole interactions are vanishingly small, slow dynamics is observed and is attributed solely to a broad distribution of relaxation times which in turn comes from that of the anisotropy energy barriers. To clarify characteristic differences between the two types of slow dynamics, we study a simple model of a non-interacting nanoparticle system (a superparamagnet) analytically as well as ferritin (a superparamagnet) and a dense Fe-N nanoparticle system (a superspin glass) experimentally. It is found that superparamagnets in fact show aging (a waiting time dependence) of the thermoremanent-magnetization as well as various memory effects. We also find some dynamical phenomena peculiar only to superspin glasses such as the flatness of the field-cooled magnetization below the critical temperature and memory effects in the zero-field-cooled magnetization. These dynamical phenomena are qualitatively reproduced by the random energy model, and are well interpreted by the so-called droplet theory in the field of the spin-glass study.