Co₂MnX (X=Si, Ge, Sn) Heusler compounds: An ab initio study of their structural, electronic, and magnetic properties at zero and elevated pressure

The structural, electronic, and magnetic properties of Co₂MnX (X = Si, Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focus on the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be halfmetallic. In contrast, Co₂MnSn is found to be a "nearly half-metallic" compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of 5 μ_B is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about 0.008 μ_B for Mn and about 0.02 μ_B for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and elevated pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn 2p exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.