Exchange coupling in single-crystalline spinel-structure (Mn,Zn)${\mathrm{Fe}}_2$${\mathrm{O}}_4$/Co${\mathrm{Fe}}_2$${\mathrm{O}}_4$ bilayers

We have observed strong exchange coupling between single-crystalline spinel-structure ferrite thin films of Co${\mathrm{Fe}}_2$${\mathrm{O}}_4$ and magnetically soft (Mn,Zn)${\mathrm{Fe}}_2$${\mathrm{O}}_4$ at room temperature. The exchange coupling constant of the interface, $A_{\mathrm{int}}$, which is estimated from exchange-biased (Mn,Zn)${\mathrm{Fe}}_2$${\mathrm{O}}_4$ loops is comparable to estimated exchange coupling constants within each individual layer, $A_{\mathrm{MZF}}$ and $A_{\mathrm{CF}}$. This result contrasts with exchange coupling in polycrystalline ${\mathrm{Ni}}_{0.8}$${\mathrm{Fe}}_{0.2}$/${\mathrm{Mn}}_{0.5}$${\mathrm{Fe}}_{0.5}$ and ${\mathrm{Ni}}_{0.81}$${\mathrm{Fe}}_{0.19}$/${\mathrm{Ni}}_{0.53}$${\mathrm{Mn}}_{0.47}$ bilayers where $A_{\mathrm{int}}$ is at best two orders of magnitude smaller than estimates of $A_{\mathrm{NiFe}}$.