Half-Metallic Ferromagnetism in the Heusler Compound Co\(_2\)FeSi revealed by Resistivity, Magnetoresistance, and Anomalous Hall Effect measurements

We present electrical transport data for single-crystalline Co\(_2\)FeSi which provide clear-cut evidence that this Heusler compound is truly a half-metallic ferromagnet, i.e. it possesses perfect spin-polarization. More specifically, the temperature dependence of \(\rho\) is governed by electron scattering off magnons which are thermally excited over a sizeable gap \(\Delta\approx 100 K\) (\(\sim 9 meV\)) separating the electronic majority states at the Fermi level from the unoccupied minority states. As a consequence, electron-magnon scattering is only relevant at \(T\gtrsim\Delta\) but freezes out at lower temperatures, i.e., the spin-polarization of the electrons at the Fermi level remains practically perfect for \(T\lesssim\Delta\). The gapped magnon population has a decisive influence on the magnetoresistance and the anomalous Hall effect (AHE): i) The magnetoresistance changes its sign at \(T\sim 100 K\), ii) the anomalous Hall coefficient is strongly temperature dependent at \(T\gtrsim 100 K\) and compatible with Berry phase related and/or side-jump electronic deflection, whereas it is practically temperature-independent at lower temperatures.