Magnetoresistance in polycrystalline and epitaxial Fe1-xCoxSi thin films

Thin films of Fe1-xCoxSi were grown using molecular beam epitaxy on Si(111). These 20-nm-thick films, with compositions x = 0 or 0.5, were produced by two methods: the first produced large (111)-textured crystallites of the B20 phase; the second produced phase-pure B20 (111) epilayers. The lattice mismatch with the substrate causes biaxial tensile strain in the layers, greater in the epilayers, that distorts the (111)-oriented material to a rhombohedral form. Magnetotransport measurements show that a combination of additional scattering arising from crystal grain boundaries and strain-free polycrystalline films results in a higher resistivity than for the epitaxial films. Magnetometry for x = 0.5 suggests an increase in the ordering temperature in strained films relative to the polycrystalline films of 15 +/- 4 K. Moreover, the characteristic linear magnetoresistance, typical of bulk single-crystal material of this composition, is retained in the polycrystalline film but reduced in the epitaxial film. While the bulk properties of these materials are reproduced qualitatively, there are small quantitative modifications, due to the strain, to properties such as band gap, Curie temperature, and magnetoresistance.