Observation of thickness dependence of magnetic surface anisotropy in ultrathin amorphous films

Ferromagnetic resonance (FMR) and SQUID magnetometry measurements have been made on multilayers of amorphous ${\mathrm{Fe}}_{70}$${\mathrm{B}}_{30}$/Ag. The dependence of the magnetic surface anisotropy constant ${\mathit{K}}_{\mathit{s}}$ on the magnetic layer thickness 2L has been determined in the range 1.6 Å<2L<90 Å using more than twenty samples. It is found that ${\mathit{K}}_{\mathit{s}}$ is constant for 2L>16.5 Å, but decreases monotonically towards zero as 2L decreases from 16.5 Å towards zero. The FMR results can be well described by a theory developed for ultrathin amorphous ferromagnetic layers.