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 Cu${}_{2}$MnAl, Co${}_{2}$MnGe, and Co${}_{2}$MnSi 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 $d\ensuremath{\rho}/dT$ to that of a normal metal with positive $d\ensuremath{\rho}/dT$. The high-field magnetoresistance (MR) for Cu${}_{2}$MnAl is large, negative, and isotropic and is mainly due to static spin disorder scattering. For Co${}_{2}$MnGe and Co${}_{2}$MnSi a corresponding sizable spin disorder MR is missing. At low fields Co${}_{2}$MnGe and Co${}_{2}$MnSi exhibit a conventional anisotropic MR, whereas the low-field MR for Cu${}_{2}$MnAl is negative and isotropic. In the nanocrystalline phase of Co${}_{2}$MnGe and Co${}_{2}$MnSi we find a large anomalous Hall effect (AHE), which can be attributed to very effective skew scattering at the nanocrystalline grain boundaries. For Cu${}_{2}$MnAl, 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.