Anomalous magnetotransport properties of epitaxial full Heusler alloys

We report the magnetotransport properties of epitaxial films of the full Heusler alloys ${\mathrm{Ni}}_2\mathrm{MnGa},$ ${\mathrm{Ni}}_2\mathrm{MnGe},$ and ${\mathrm{Ni}}_2\mathrm{MnAl},$ grown by molecular beam epitaxy on (001) GaAs. The ferromagnetic alloys $({\mathrm{Ni}}_2\mathrm{MnGa},{\mathrm{Ni}}_2\mathrm{MnGe})$ exhibit an anomalous temperature dependence of the resistivity and a negative magnetoresistance peaking near the Curie temperature due to spin disorder scattering. Considering the absolute values of the resistivity, the anomalous high temperature behavior and an upturn in the resistivity below 20 K, we suggest that these Heusler alloys rather than being conventional metals are in fact strongly disordered electronic systems.