| Exchange coupling between antiferromagnetic order in multiferroic BiFeO 3 and a fer- romagnetic overlayer has until now only been shown in multidomain BiFeO3 films in which domain walls, either for their ferromagnetic or ferroelastic properties, are considered essential to the coupling. Difficulties controlling domain wall properties, including fatigue and leakage, as well as devices challenges of reproducibility and scaling suggest an alternative approach may be necessary for practical exchange coupled spintronic devices based on BiFeO3. Here we show a room temperature, deterministic and robust "intrinsic" (without domain wall) exchange coupling. In situ ferroelectric switching of monodomain BiFeO3/Co heterostructures performed during photoemission electron microscope (PEEM) and magnetooptic Kerr effect (MOKE) measurements reproducibly show a coherent ∼75° clockwise rotation of the Co magnetization upon poling and counterclockwise ∼75° rotation upon reverse poling, for hundreds of cycles. The Co magnetization rotation is strictly correlated to single, domain 71° ferroelectric switching and reorientation of the antiferromagnetic spin-cycloid plane from in-plane to nearly vertical. The experimentally observed reorientation of the Co magnetization can be explained by competition between the substrate-induced magnetic anisotropy that dominates in the pole "down" state, and exchange coupling between Co and the vertical spin-cycloid plane that dominates in the pole "up" state. Our experimental data demonstrate that the spin-cycloid properties in these strained films are different from bulk BiFeO3, suggesting that the internal magnetoelectric coupling is changed by the strain-induced crystal symmetry lowering that is likely responsible for the strong intrinsic coupling at the BiFeO3/Co interface reported here. |
