Substrate dependent reduction of Gilbert damping in annealed Heusler alloy thin films grown on group IV semiconductors

A structural and FMR study is presented for epitaxial thin films of the Heusler alloy Co2FeAl0.5Si0.5 (CFAS) grown on Ge(111) and Si(111) substrates. All films, as-grown and post-annealed, show B2 ordering; full chemical order (L2(1)) is not obtained over the range of anneal temperatures used in this study. As-grown films show a lower Gilbert damping constant, alpha, when grown on a Si(111) substrate compared to Ge(111). Annealing the films to 450 degrees C significantly reduces alpha for CFAS on Ge while increasing alpha for CFAS on Si. This is related to a substrate dependent competition between improvements in lattice structure and increased interfacial intermixing as a function of anneal temperature. The optimal annealing temperature to minimize alpha is found to differ by similar to 100 K between the two substrates. Above an anneal temperature of 500 degrees C, films grown on both substrates have increased coercivity, decreased saturation magnetization, and show characteristic two-magnon scattering features.