Coexistence of exchange bias and memory effect in nanocrystalline CoCr2O4

Abstract

This article reports exchange bias (EB) and memory effect in nanocrystalline CoCr2O4 with a detailed understanding of the origin of the observed effect. In order to investigate the EB effect in nanocrystalline CoCr2O4, two samples are synthesized with particle sizes 12 and 7 nm via sol-gel route (the second one is embedded in amorphous silica host). For both the samples, despite usual core-shell interaction or ‘surface’ domination, conventional EB effect is observed where the EB field sharply decreases with temperature and vanishes over the spiral ordering temperature (TS) of the samples. A bulk sample of the same composition, with particle size greater than 150 nm, reproducing similar characteristics, rules out the origin of EB due to surface effect. Results indicate that the exchange bias is due to inherent magnetic inhomogeneity: interaction between spiral spin order and collinear ferrimagnetic order below TS. The ac-susceptibility measurement points towards absence of surface glassiness in the sample with 12 nm particle size. However, the sample with particle size 7 nm reveals the characteristics of superspin-glass (SSG), confirmed by magnetic memory effects both in time (t) and temperature (T) dependent magnetization protocols.