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 Co_1-xZn_xFe₂O₄ (x = 0-1) ferrimagnetic (FiM) shell. The system presents an enhancement of the coercivity (H_C) as compared to its FiM single-phase counterpart and exchange bias fields (H_EB). While H_C decreases monotonically with the Zn concentration from ∼21.5 kOe for x = 0, to ∼7.1 kOe for x = 1, H_EB exhibits a non-monotonous behavior being maximum, H_EB ∼ 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 Co²⁺ with Zn²⁺ 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.