High L21-atomic ordering and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films with low-temperature annealing process

Although an enhanced magnetoresistance (MR) has been observed in many Co-2-based Heusler alloys by promoting their structural ordering from B2 to L2(1) by post-annealing at higher temperatures (T-ann > 500 degrees C), it is desirable to search for other Heusler alloys that crystallize in L2(1)-order below 300 degrees C, as the maximum T-ann is restricted for processing devices. For Co(2)MnZ (Z = Ge, Sn) Heusler alloys, an L2(1)-order is expected to appear even in the as-deposited state or by a low-temperature annealing process due to their very high L2(1) to B2-order transition temperature (>1500 K). Here, epitaxial Co(2)MnZ films were grown on MgO (001) substrate at room temperature (RT) and post-annealed at Tann = 200-500 degrees C. Interestingly, as-sputtered films exhibit L2(1)-ordering, which improves systematically upon increasing Tann. The spin-polarization of electric current (beta) was estimated at RT using nonlocal spin-valve (NLSV) devices by measuring the spin-accumulation signal in a copper (Cu) channel. It was found that at T-ann = 300 degrees C, the beta value of Co2MnGe films is higher (similar to 0.65) than that of Co2FeGe0.5Ga0.5 films due to a higher degree of L2(1)-order, which makes the Co2MnGe alloy a promising ferromagnetic electrode for spintronic device applications. (C) 2022 Author(s).