Higher-order perpendicular magnetic anisotropy and interfacial damping of Co/Ni multilayers

We report on the experimental investigation of the perpendicular magnetic anisotropy in $\mathrm{Pt}/{[\mathrm{Co}/\mathrm{Ni}]}_{\mathrm{N}}/\mathrm{Ir}$ multilayers. Broadband ferromagnetic resonance (FMR) and polar angle dependent FMR measurements reveal the presence of a strong fourth-order contribution to the perpendicular anisotropy. By numerically solving the Smit-Beljers relation, we are able to fit the polar angle dependence of the resonance field and extract both the second and fourth-order anisotropy constants. While this model provides a qualitative description for all samples, we note that, for samples near or in the easy cone state of the phase diagram, systematic deviations emerge in the polar angle dependent data. The exchange stiffness of the multilayers is found to be lower than the weighted average of the exchange stiffness reported for Co and Ni in the literature. We find that the effective Gilbert damping scales with the inverse multilayer thickness, indicating a significant interfacial contribution to the damping process.