Spin excitation gap in epitaxial Co2FeSi thin films revealed by longitudinal resistivity and negative magnetoresistance

Heusler alloys hold great promise for spintronic applications because of their potential half-metallicity, as suggested by electronic structure calculations for certain cases, such as Co$_{\mathrm{2}}$Fe$_{\mathrm{x}}$Mn$_{\mathrm{1-x}}$Si [B. Balke \textit{et al.}, PRB \textbf{74}, 104405 (2006)]. Here we report on signatures of a minority spin gap in Co$_{\mathrm{2}}$FeSi using transport measurements. The 5-nm thick Co$_{\mathrm{2}}$FeSi thin film sample studied in this work is grown epitaxially on a GaAs (100) substrate. In addition to typical phonon and weak-localization contributions, the temperature dependence of the resistivity shows a spin-fluctuation contribution that is suppressed at low temperatures, consistent with the presence of a minority spin gap of approximately 500 K. Most significantly, the Co$_{\mathrm{2}}$FeSi shows a linear and isotropic negative magnetoresistance that increases with increasing temperature, reaching a magnitude of 0.012 $\mu \Omega $ cm T$^{\mathrm{-1}}$ at room temperature. Once the weak localization contribution at low temperature is removed, the temperature dependence of the negative magnetoresistance can be fitted using a simple model that includes a zero-field spin gap obtained from the resistivity measurement and a field-dependent contribution that can be obtained from ferromagnetic resonance measurements.