Exchange bias studies of CoFe2O4 coated BiFeO3 nanoparticles

Abstract

The study of exchange bias effects in inverted core-shell nanoparticles has unearthed rich and varied physical phenomena. In this paper, we have studied the exchange bias effects in inverted BFO/CFO core-shell nanoparticles by varying the crystallinity of the CFO shell. The M-T measurements have shown distinct features of the BFO core and CFO shell. AC susceptibility and memory effect measurements indicated that the core-shell nanoparticles with the less crystalline CFO shell displayed spin-glass characteristics. The training effect displayed by the nanoparticles with the spin-glass shell can be explained using the spin-glass and Binek’s models. Exponential decay of coercivity and exchange bias with temperature is in conformance with the spin-glass behavior of the shell, and the cooling-field dependence of the exchange bias field is explained based on the Niebieskikwiat model. In the case of core-shell nanoparticles with the more crystalline CFO shell, the anisotropy field is greater than the maximum applied field. The exchange bias value has the contributions from the minor loop effects, as evident from the forced minor loop measurements. The extrapolated value of the exchange bias increases moderately compared with similar inverted core-shell structures, even though the coercivity increased considerably. The out-diffusion of Bi ions from the BFO core changed the pinning strength of the BFO core, affecting the exchange bias and coercivity.