Influence of the capping layer material on the interfacial Dzyaloshinskii–Moriya interaction in Pt/Co/capping layer structures probed by Brillouin light scattering

Co ultrathin films, of various thicknesses (0.8 nm ≤ tCo ≤ 2.5 nm), have been grown by sputtering on Si substrates, using Pt buffer layers and different capping layers (Cu, Ir, MgO and Pt). The x-ray diffraction revealed that our films have a (1 1 1) out-of-plane texture with various degrees of strains. Their magnetic properties have been studied by vibrating sample magnetometry (VSM) and Brillouin light scattering (BLS) in the Damon–Eshbach geometry. VSM characterizations revealed that films with Co thickness below (above) the spin reorientation transition thickness, which is capping layer dependent, are perpendicularly (in-plane) magnetized, suggesting the existence of an interface anisotropy. The surface anisotropy constant was found to be 1.42 ± 0.02 erg cm−2 and of 1.33 ± 0.02 erg cm−2 for the Pt/Co/Cu and Pt/Co/Ir samples, respectively, suggesting that it is due to the Pt/Co interface and that the top Co/Cu, Co/Pt or Co/Ir interfaces have a minor contribution. A lower value of 1.07 ± 0.02 erg cm−2 has been obtained for Pt/Co/MgO most probably due to over-oxidation of Co at the Co/MgO interface. The BLS measurements revealed a pronounced nonreciprocal spin waves propagation, due to the interfacial Dzyaloshinskii–Moriya interaction (iDMI) induced by Pt interface with Co, which increases with decreasing Co thickness. The magnetic dead layer has been taken into account to precisely determine the surface iDMI constant Ds estimated at −0.8 pJ m−1, −1.05 pJ m−1 and −0.95 pJ m−1, respectively for Pt/Co/Ir, Pt/Co/Cu and Pt/Co/MgO for sample thicknesses where a linear thickness dependence of the effective iDMI constant has been observed.