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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 Mn3-xYxZ (Y=Co, Fe, Ni and Z=Ga ). All series exhibit thermally stable magnetic properties. As we demonstrate, Mn3-xFexGa series, which are tetragonal over the whole range of compositions, are good as hard magnets, whereas magnetically more weak Mn3-xNixGa 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) Mn3-xCoxGa 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.