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 ( t eff ). The magnitude of the Dzyaloshinskii-Moriya exchange constant ( | D | ) increases with decreasing t eff until t eff ∼ 1 nm, below which | D | decreases. The MCA and | D | scale with 1 / t eff for t eff larger than ∼ 1 nm, indicating an interfacial origin. The increase of MCA with decreasing t eff continues below t eff ∼ 1 nm, but with a slower rate. To clarify the cause of the t eff 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 t eff is smaller than ∼ 0.8 nm. The OMA increases with decreasing t eff more rapidly than what is expected from the MCA, indicating that factors other than OMA contribute to the MCA at small t eff . The t eff dependence of the OMA also suggests that | D | at t eff 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 t eff .