Magnetotransport properties of Cu2MnAl, Co2MnGe, and Co2MnSi Heusler alloy thin films: From nanocrystalline disordered state to long-range-ordered crystalline state

We have studied the magnetotransport properties of thin films of the Heusler compounds Cu2MnAl, Co2MnGe, and Co2MnSi prepared by sputtering on substrates at room temperature. By stepwise annealing at high temperatures, we transform the as-prepared, weakly magnetic, nanocrystalline state to the fully ordered, crystalline state via several intermediate steps. At the phase boundary between the nanocrystalline state and the long-range-ordered crystalline state, we observe a change of the electrical resistivity from that of a strongly disordered metal with negative d rho/dT to that of a normal metal with positive d rho/dT. The high-field magnetoresistance (MR) for Cu2MnAl is large, negative, and isotropic and is mainly due to static spin disorder scattering. For Co2MnGe and Co2MnSi a corresponding sizable spin disorder MR is missing. At low fields Co2MnGe and Co2MnSi exhibit a conventional anisotropic MR, whereas the low-field MR for Cu2MnAl is negative and isotropic. In the nanocrystalline phase of Co2MnGe and Co2MnSi we find a large anomalous Hall effect (AHE), which can be attributed to very effective skew scattering at the nanocrystalline grain boundaries. For Cu2MnAl, the AHE behavior is very unusual, indicating a situation with the contributions of spin-up and spin-down electrons to the anomalous Hall voltage nearly compensating each other.