Relationship between orbital moment anisotropy, magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya interaction in W/Co/Pt trilayers

We have studied the Co layer thickness dependences of magnetocrystalline anisotropy (MCA), Dzyaloshinskii-Moriya interaction (DMI), and orbital moment anisotropy (OMA) in W/Co/Pt trilayers, in order to clarify their correlations with each other. We find that the MCA favors magnetization along the film normal and monotonically increases with decreasing effective magnetic layer thickness (𝑡eff). The magnitude of the Dzyaloshinskii-Moriya exchange constant (|𝐷|) increases with decreasing 𝑡eff until 𝑡eff∼1 nm, below which |𝐷| decreases. The MCA and |𝐷| scale with 1/𝑡eff for 𝑡eff larger than ∼1 nm, indicating an interfacial origin. The increase of MCA with decreasing 𝑡eff continues below 𝑡eff∼ 1 nm, but with a slower rate. To clarify the cause of the 𝑡eff dependences of MCA and DMI, the OMA of Co in W/Co/Pt trilayers is studied using x-ray magnetic circular dichroism. We find nonzero OMA when 𝑡eff is smaller than ∼0.8 nm. The OMA increases with decreasing 𝑡eff more rapidly than what is expected from the MCA, indicating that factors other than OMA contribute to the MCA at small 𝑡eff. The 𝑡eff dependence of the OMA also suggests that |𝐷| at 𝑡eff smaller than ∼1 nm is not related to the OMA at the interface. We propose that the growth of Co on W results in a strain and/or texture that reduces the interfacial DMI, and, to some extent, MCA at small 𝑡eff.