Magnetotransport properties of Cu_2MnAl, Co_2MnGe, and Co_2MnSi Heusler alloy thin films: From nanocrystalline disordered state to long-range-ordered crystalline state

摘要

We have studied the magnetotransport properties of thin films of the Heusler compounds Cu_2MnAl, Co_2MnGe, and Co_2MnSi 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 dp/dT to that of a normal metal with positive dp/dT. The high-field magnetoresistance (MR) for Cu_2MnAl is large, negative, and isotropic and is mainly due to static spin disorder scattering. For Co_2MnGe and Co_2MnSi a corresponding sizable spin disorder MR is missing. At low fields Co_2MnGe and Co_2MnSi exhibit a conventional anisotropic MR, whereas the low-field MR for Cu_2MnAl is negative and isotropic. In the nanocrystalline phase of Co_2MnGe and Co_2MnSi we find a large anomalous Hall effect (AHE), which can be attributed to very effective skew scattering at the nanocrystalline grain boundaries. For Cu_2MnAl, 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.