| Increased interest in the field of magnetoelectronics or spin electronics during the last decade has intensified research on the so-called half-metallic materials which are metallic for one spin direction while at the same time semiconducting for the other spin direction and thus exhibit a complete spin polarization at the Fermi level and the realistic applications for spintronic devices. Thus, the investigation of these half-metallic materials is significant for the development of spintronics.In this paper, we present a systemic study the structure, electronic and magnetic properties of a series Ti2MnZ (Z=Al, Ga, In, Si, Ge, Sn) alloys with the Hg2CuTi-type structure by using the first-principles projector augmented wave potential within the generalized gradient approximation. It is found these Ti-based Heusler alloys with the Hg2CuTi-type structure, the neighbor Ti atoms occupy A(0,0,0) and B(1/4,1/4,1/4) sites, and the remaining Y enters C(1/2,1/2,1/2) and Z at D(3/4,3/4,3/4). The cubic lattice parameters for these Heusler alloys with the Hg2CuTi-type structure are optimized by minimizing the total energy with respect to the lattice parameter variation. For all studied alloys, the ferromagnetic state was found to be more stable than the paramagnetic state. The optimized lattice constants of the TiiMnZ alloys are6.138,6.199,6.248,5.997,6.076,6.317A for Z=Al, Ga, In, Si, Ge, Sn, respectively. We can also see that with the same Z (Z=Al, Ga, In, Si, Ge, Sn) atom, the optimized lattice constant increases according to the order of Ti2MnZ, Ti2Z and Ti2TiZ. The Ti2MnZ (Z=A1, Ga, In, Si, Ge, Sn) alloys are found to be half-metallic ferrimagnets and potential applications in spintronic devices. The total magnetic moments of the Heusler alloys Ti2MnZ follow theμr=Zr18rule and agree with the Slater-Pauling curve quite well. For Ti2MnZ (Z=A1, Ga, In) alloys the total magnetic moment per unit cell is0, together with the spin-down band gaps of0.538,0.592,0.526eV for Z=A1, Ga, In, respectively. While for Ti2MnZ (Z=Si, Ge, Sn) alloys, the total magnetic moment per unit cell is1μB and their spin-down band gaps are0.455,0.504,0.538eV, respectively. After a Ti or vacancy substituting Mn atom at C site, the half-metallic character disappear in Ti2TiZ and Ti2Z (Z=A1, Ga, In, Si, Ge, Sn) alloys, and below the Fermi level the occupied DOS in both channels shifts to the high energy region compared with Ti2MnZ (Z=Al, Ga, In, Si, Ge, Sn). Due to different surroundings of the neighbor Ti atoms, in T12YZ alloy the d states of the neighbor Ti atoms should be hybridized firstly, then the hybridized Ti1-Ti2orbitals continue to couple with d states of Y atom, finally resulting in a similar band structure and magnetic behavior to the half-Heusler alloy. The band gaps are mainly determined by the bonding and antibonding states created from the hybridizations of the d states between the Ti1-Ti2coupling and Mn atom. The s-p elements play an important role in the half-metallicity of these Heusler alloys. In addition, for all Ti2-based Heusler alloys, the localized magnetic moment carried by the Ti and Mn atoms is restricted by the s-p element even though they have a small contribution to the total spin magnetic moment.The half-metallicity observed in bulk can be lost near the surface or interface because the surface or interface states exist. We also investigate the magnetic and electronic properties in (001) surface of the Heusler alloy TiaMnAl as a example. Owing to the surface effects, the half-metallicity is destroyed in five terminations. The high spin polarization only maintains in TiMn-termination. Further studies on the atomic magnetic moments, we find that the moment of surface Til is enhanced obviously, as well as the absolute value of surface Mn atomic moment. |
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