Tunable magnetism and half-metallicity in bulk and (100) surface of quaternary Co2MnGe1-xGax Heusler alloy

The structural, magnetic and half-metallic properties of the bulk and (1 0 0) surface of quaternary Heusler alloy Co2MnGe1-xGax are investigated from the first-principles calculations. For the bulk, the lattice constant and total magnetic moment follow the Vegard law and Slater-Pauling rule well, respectively. Except for Co2MnGa, the Co2MnGe1-xGax series are half-metallic. Because the Fermi level of Co2MnGe0.5Ga0.5 is just located at the middle of the minority-spin gap, we predict that it bears the most robust half-metallicity as against remnant doped alloys. As for the Co2MnGe1-xGax (1 0 0) surface, the analyses on relaxed atomic positions and surface energies reveal that Co-Ge and Co-Ga bonding are more favourable than Co-Mn bonding and the terminations involving surface Mn atoms are more stable than CoCo terminations. By comparing with the bulk values, the surface Co and Mn magnetic moments are enhanced obviously. The calculated PDOS of all accessible 'ideal' surfaces show that the half-metallicity observed in bulk has been destroyed by the surface states, which is a possible reason why the tunnel magnetoresistence steeply drops as temperature increases. However, in the pure atomic terminations the surface properties can be slightly adjusted by the Ga-doped concentrations in bulk through the dipolar interaction. As a result, in the MnMn termination of Co2MnGe0.5Ga0.5(1 0 0) the spin polarization of 100% is detected, indicating that in the pure Mn atomic termination the half-metallicity of the (1 0 0) surface can remain if the corresponding bulk presents excellent half-metallic stability. Thus we predict that this thin film will present a higher potential for applications in ferromagnetic electrodes.