Surface anisotropy of Ni-Co from ferromagnetic resonance

We report here the comparison between a calculation of ferromagnetic resonance (FMR) position and linewidth using the Landau-Lifshitz equation and experimental values for highly perfect (100) Ni-Co plates. Despite the fact that we have seven parameters at our disposal: composition (which implies values both of magnetization and crystalline anisotropy), thickness, conductivity, gyromagnetic ratio, exchange, magnetic damping, and surface anisotropy, it is possible to find a best fit, using data for three frequencies, and angles between plate normal and parallel. We find that $K_s$ is -0.11±0.02 erg/${\mathrm{cm}}^2$ for ${\mathrm{Ni}}_{0.84}$ ${\mathrm{Co}}_{0.16}$ (normal easy direction). This is in agreement with theory for pure Ni. These results are somewhat ambiguous, as it is known that other effects such as surface roughness or magnetization gradient can simulate a surface anisotropy in FMR. However, scanning electron microscopy and Auger measurements suggest that this is unlikely for our samples.