Positive exchange bias observed in Pt-inserted Cr₂O₃/Co exchange coupled bilayers

We investigated the effect of Pt insertion on a Cr₂O₃/Co exchange coupling system. The perpendicular exchange bias μ₀H_ex decreased with increasing Pt insertion layer thickness, and we observed positive μ₀H_ex for samples with relatively thick Pt insertion layers. We also examined the cooling field μ₀H_fc dependence of μ₀H_ex for the samples. At small μ₀H_fc, all samples exhibited negative μ₀H_ex. With increasing μ₀H_fc, a shift of μ₀H_ex from negative to positive was observed. In the past, similar behaviors were observed for FeF₂/Fe systems exhibiting positive μ₀H_ex. In addition, the μ₀H_fc dependence of μ₀H_ex was well fitted by an equation taking into account the Zeeman energy at the surface of an antiferromagnet as well as an antiferromagnetic exchange coupling. The results strongly suggest that (1) Cr₂O₃ surface spin is affected by the external magnetic field and (2) the coupling at the Cr₂O₃/Pt/Co interface is antiferromagnetic.