Effect of crystallinity of Co layer on perpendicular exchange bias in Au-capped ultrathin Co film on Cr2O3(0 0 0 1) thin film

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr2O3 thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr2O3(0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr2O3(0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr2O3(0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr2O3. The valence state of the Cr2O3(0 0 0 1) layer is similar to that of bulk Cr2O3. The ultrathin Co film directly grown on the α-Cr2O3(0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Néel temperature of Cr2O3 for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr3+ in Cr2O3(0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.

Research Highlights
► Crystalline orientation of ferromagnetic layer affects the high perpendicular exchange bias.
► Influence of the perpendicular magnetic anisotropy on the perpendicular exchange bias is weak.
► Direct exchange coupling mechanism explains the results well.