Thickness dependence of exchange coupling in epitaxial Fe_3O_4/CoFe_2O_4 soft/hard magnetic bilayers

摘要

Epitaxial magnetic heterostructures of (soft-)Fe_3O_4/(hard-)CoFe_2O_4 (001) have been fabricated with a varying thicknesses of soft ferrite from 5 to 25 nm. We report a change in the regime of magnetic interaction between the layers from rigid-coupling to exchange-spring behavior, above a critical thickness of the soft magnetic Fe_3O_4 layer. We show that the symmetry and epitaxial matching between the spinel structures of CoFe_2O_4 and Fe_3O_4 at the interface stabilize the Verwey transition close to the bulk value even for 5-nm-thick Fe_3O_4. The large interface exchange-coupling constant estimated from low-temperature M(H) data confirmed the good quality of the ferrite-ferrite interface and the major role played by the interface in the magnetization dynamics. The results presented here constitute a model system for understanding the magnetic behavior of interfaces in core/shell nanoparticles and magnetic oxide-based spintronic devices.