Memory effect versus exchange bias for maghemite nanoparticles

Abstract

We studied the temperature dependence of memory and exchange bias effects and their dependence on each other in maghemite (γ-Fe2O3) nanoparticles by using magnetization studies. Memory effect in zero field cooled process in nanoparticles is a fingerprint of spin-glass behavior which can be due to i) surface disordered spins (surface spin-glass) and/or ii) randomly frozen and interacting nanoparticles core spins (super spin-glass). Temperature region (25–70K) for measurements has been chosen just below the average blocking temperature (TB=75K) of the nanoparticles. Memory effect (ME) shows a non-monotonous behavior with temperature. It shows a decreasing trend with decreasing temperature and nearly vanishes below 30K. However it also decreased again near the blocking temperature of the nanoparticles e.g., 70K. Exchange bias (EB) in these nanoparticles arises due to core/shell interface interactions. The EB increases sharply below 30K due to increase in core/shell interactions, while ME starts vanishing below 30K. We conclude that the core/shell interface interactions or EB have not enhanced the ME but may reduce it in these nanoparticles.