Role of the Heavy Metal's Crystal Phase in Oscillations of Perpendicular Magnetic Anisotropy and the Interfacial Dzyaloshinskii-Moriya Interaction in W/Co-Fe- B/MgO Films

The interfacial Dzyaloshinskii-Moriya interaction (IDMI) generally arises in "heavy-metal-ferromagnet" (HM-FM) heterostructures due the strong spin-orbit coupling in HMs and plays a crucial role in the nucleation of Dzyaloshinskii domain walls and Skyrmions that can be efficiently moved by spin-orbit torques. We study the IDMI in W/Co-Fe-B/MgO/Ta films with perpendicular magnetic anisotropy for different crystal structures and thicknesses of W layers treated by postsputtering annealing at 300 °C and 400 °C. We employ Brillouin light scattering spectroscopy to precisely measure the energy density and sign of the IDMI. We reveal two peaks associated with the IDMI: one for the amorphouslike phase of W, and another for α-W. The samples with pure β-W underlayers show the smallest values of IDMI. The sign of the IDMI is positive for all samples with the observed maximum effective and surface values of Deff=0.88 erg/cm2 and Ds=0.51×10-7 erg/cm, respectively. We report on the oscillating behavior of the IDMI as a function of thickness, which correlates with the surface anisotropy energy demonstrating common spin-orbit coupling and a robust dependence on the structural properties of the materials used. We ascribe the oscillations of the surface anisotropy and the IDMI mainly to the near inversely proportional dependence on the squared interatomic distance between the HM and FM atoms promoted by the thickness-driven crystal phase formation of W.