On beating the superparamagnetic limit with exchange bias

¹  Department of Physics, University of York - York, YO10 5DD, UK, EU
²  Instituto de Ciencia de Materiales de Madrid, CSIC - Cantoblanco, 28049 Madrid, Spain, EU
³  Center for Materials and Information Technology, University of Alabama - Tuscaloosa, AL, USA

Corresponding author: rfle500@york.ac.uk

Received: 2 September 2009
Accepted: 12 November 2009

Abstract

In order to investigate the possibility of “beating the superparamagnetic limit with exchange bias” (Skumryev V. et al., Nature, 423 (2003) 19) we perform atomistic modeling on CoCoO magnetic nanoparticles varying the strength of the ferromagnetantiferromagnet interfacial coupling. Our results show that exchange-biased systems exhibit an increased energy barrier along the bias direction, with a corresponding decrease of the barrier in the opposite direction. For systems with large values of the interfacial coupling, the ferromagnetic core is found to be unconditionally stable in the bias direction. Such a system is ideal for thermal stability since there is no energetically stable reversed state, providing the antiferromagnetic shell is unchanged. In order to permit magnetic recording, which essentially requires a bi-stable system, we propose a heat-assisted recording method, whereby both the ferromagnet (FM) and antiferromagnet (AF) are switched. Upon heating to the Néel temperature of CoO, the AF magnetic order is destroyed allowing switching of the FM core with a small applied field, with thermal stability reappearing on cooling through T_N.