Tetragonal Heusler Compounds for Spintronics

With respect to the requirements of spin torque transfer (STT) materials, one the most promising materials families are the tunable tetragonal Heusler compounds based on Mn₂YZ (Y=Co,Fe,Ni,Rh,...; Z=Al, Ga, Sn). They form the inverse cubic Heusler structure with three distinct magnetic sublattices, which allows a fine tuning of the magnetic properties. Starting with the stoichiometric Mn₃Ga compound, we explored the complete phase diagram of Mn_3-xY_xZ (Y=Co, Fe, Ni and Z=Ga ). All series exhibit thermally stable magnetic properties. As we demonstrate, Mn_3-xFe_xGa series, which are tetragonal over the whole range of compositions, are good as hard magnets, whereas magnetically more weak Mn_3-xNi_xGa series exhibit the shape memory phenomenon. Even more surprising properties are found in the Co-based series. On the one hand, the Co-poor (x <; 0.5) Mn_3-xCo_xGa tetragonal alloys are well-suited for the STT. At the same time, the Co-rich cubic series (0.5 <; x <; 1) are half-metallic ferromagnets, following the Slater-Pauling rule, similar to Co₂YZ cubic Heusler compounds.