Non-Exchange Bias in Binary Nanoparticle Systems

Abstract

Exchange bias has been extensively studied both in exchange-coupled thin films and nanoparticle systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias are far from being understood. Here, dense soft-hard binary nanoparticle systems are used as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift, as well as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft g-Fe2O3 nanoparticles (no bias – symmetric reversal) and hard cobalt doped g-Fe2O3 nanoparticles (large exchange bias – asymmetric reversal) reveal that the loop shift of the mixture can be significantly enhanced depending on the fraction of soft particles. Simple calculations indicate how this effect can be further enhanced by optimizing the parameters of the constituents (coercivity and loop asymmetry). In addition, the existence of a dipolar induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias can be induced in the initially unbiased g-Fe2O3 nanoparticles due to the dipolar interaction with the exchange biased cobalt doped g-Fe2O3 nanoparticles. These results pave the way for novel approaches to tune the loop shift in magnetic systems beyond interface exchange coupling.