Tuning the coercivity and exchange bias by controlling the interface coupling in bimagnetic core/shell nanoparticles.

In order to explore an alternative strategy to design exchange-biased magnetic nanostructures, bimagnetic core/shell nanoparticles have been fabricated by a thermal decomposition method and systematically studied as a function of the interface exchange coupling. The nanoparticles are constituted by a ∼3 nm antiferromagnetic (AFM) CoO core encapsulated in a ∼4 nm-thick CoZnFeO (x = 0-1) ferrimagnetic (FiM) shell. The system presents an enhancement of the coercivity (H) as compared to its FiM single-phase counterpart and exchange bias fields (H). While H decreases monotonically with the Zn concentration from ∼21.5 kOe for x = 0, to ∼7.1 kOe for x = 1, H exhibits a non-monotonous behavior being maximum, H ∼ 1.4 kOe, for intermediate concentrations. We found that the relationship between the AFM anisotropy energy and the exchange coupling energy can be tuned by replacing Co with Zn ions in the shell. As a consequence, the magnetization reversal mechanism of the system is changed from an AFM/FiM rigid-coupling regime to an exchange-biased regime, providing a new approach to tune the magnetic properties and to design novel hybrid nanostructures.