Epitaxial magnetic heterostructures of $(\mathrm{soft}-){\mathrm{Fe}}_3{\mathrm{O}}_4/(\mathrm{hard}-){\mathrm{CoFe}}_2{\mathrm{O}}_4\left(001\right)$ 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 ${\mathrm{Fe}}_3{\mathrm{O}}_4$ layer. We show that the symmetry and epitaxial matching between the spinel structures of ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ at the interface stabilize the Verwey transition close to the bulk value even for 5-nm-thick ${\mathrm{Fe}}_3{\mathrm{O}}_4$. The large interface exchange-coupling constant estimated from low-temperature $M\left(H\right)$ 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.

• Received 29 March 2016
• Revised 20 June 2016

DOI:https://doi.org/10.1103/PhysRevB.94.054405