Thickness dependence of exchange coupling in epitaxial Fe3 O4/ CoFe2 O4 soft/hard magnetic bilayers

Abstract

Epitaxial magnetic heterostructures of ( soft − ) Fe 3 O 4 / ( hard − ) CoFe 2 O 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 3 O 4 layer. We show that the symmetry and epitaxial matching between the spinel structures of CoFe 2 O 4 and Fe 3 O 4 at the interface stabilize the Verwey transition close to the bulk value even for 5-nm-thick Fe 3 O 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