Exchange‑bias and magnetic anisotropy felds in core–shell ferrite nanoparticles

Abstract

Exchange bias properties of MnFe2O4@γ–Fe2O3 core–shell nanoparticles are investigated. The measured feld and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross analyzed by comparison against ferromagnetic resonance experiments at 9GHz. These measurements allow one to discern three types of magnetic anisotropies afecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a feld-cooling process. The rotatable anisotropy is caused by partially pinned spins at the core/shell interface; it manifests itself as an intrinsic feld always parallel to the external applied magnetic feld. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the efect of thermal fuctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.