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For spintronic applications, it is highly desirable to realize 100%-spin-polarized two-dimensional (2D) electron systems in field-controllable epitaxial ultrathin films or 2D materials on semiconductor substrates. Through systematic first-principles investigation, we find that one epitaxial TcO 2 bilayer on rutile TiO 2 (001) substrate is an antiferromagnet-like narrow-gap semiconductor, and its electronic and magnetic properties can be manipulated through electric field. When electric field reaches 0.03 V/A, it transits to a half-metallic ferrimagnet with 100% spin polarization. Our analysis indicates that in both phases the magnetization density and the electronic states near the Fermi level originate mainly from the TcO 2 bilayer, and across the interface the bond lengths and angles quickly converge to the corresponding values of the bulk TiO 2 . Therefore, the heterostructure actually hosts a 2D electron system determined by the TcO 2 bilayer and the TiO 2 substrate. Because the half-metallic phase can be achieved from a nonmetallic phase, such epitaxial 2D electron systems should be usable in spintronic devices.

We use first-principles methods to study double perovskite Pb2CrMO6 (M: some 4d and 5d transition metals), and find that a magnetic I 4 / m structure is lowest in total energy per formula unit for M=Mo, W, and Re. Our electronic structure investigation shows that the three double perovskite compounds are half-metallic ferrimagnets with total magnetic moments of 2 μ B , 2 μ B and 1 μ B per formula unit, respectively. With the spin–orbit coupling included, our calculations show the high magnetocrystalline anisotropy (MCA) in Pb2CrReO6. Fortunately, the spin polarization of Pb2CrMO6 at the Fermi level remains 90% ~ 99 % . The orbital moments of 3d Cr and 4d Mo are small, but the 5d ions of W and Re have large unquenched orbital moments, which can be attributed stronger spin–orbit coupling in the 5d orbitals than in 3d/4d ones. In addition, these results are confirmed by using improved exchange–correlation functionals. Furthermore, we calculate the spin exchange constants of the three Pb2CrMO6, and our Monte Carlo simulated Curie temperatures for them are at least 480 K. These materials, when synthesized, should be useful in spintronics.

We use a modified Becke–Johnson (mBJ) exchange plus local density approximation correlation functional within the density functional theory (DFT) [semi-local, orbital-independent, producing accurate semiconductor gaps, see Tran and Blaha, Phys. Rev. Lett., 102 (2009) 226401] to investigate the electronic structures of CdCr 2 S 4 and CdCr 2 Se 4 as two ferromagnetic semiconductors. Our results show that mBJ makes the empty Cr d-eg character bands higher by about 1.1 eV for CdCr 2 S 4 and 0.7 eV for CdCr 2 Se 4 for the majority-spin channel with respect to the generalized gradient approximation (GGA). For the minority-spin channel, mBJ raises the Cr d character bands by about 0.8 eV for CdCr 2 S 4 and CdCr 2 Se 4 . The mBJ gap is enhanced by about 104% for CdCr 2 S 4 and 170% for CdCr 2 Se 4 compared to GGA results, and is consistent with experiment in both of the cases. Our mBJ dielectric functions and the zero frequency refractive index n(0) for CdCr 2 S 4 and CdCr 2 Se 4 are also consistent with experimental results available. The improved DFT results can be understood in terms of mBJ-enhanced spin exchange splitting and the reduced band width of Cr d character bands.

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.

The spin-s quantum Heisenberg ferromagnetic model is investigated in the physical magnon theory. The effect of the extra unphysical magnon states on every site is completely removed in the magnon Hamiltonian and during approximation procedure so that the condition = 0(n greater than or equal to 2s + 1) is rigorously satisfied. The physical multi-approximation occupancy (1 less than or equal to n less than or equal to 2s) is proportional to T-3n/2 at low temperature and is equivalent to 1/(2s + 1) at the Curie temperature. The magnetization not only unified but also well-behaved from zero temperature to Curie temperature is obtained in the framework of the magnon theory for the spin-s quantum Heisenberg ferromagnetic model. The ill-behaved magnetizations at high temperature in earlier magnon theories are completely corrected. The relation of magnon (spin wave) theory with spin-operator decoupling theory is clearly understood. (C) 2001 Elsevier Science B.V. All rights reserved.

In this paper we study the effect of next-nearest-neighbor exchange on antiferromagnetic order and spin correlation of quasi-two-dimensional antiferromagnets in terms of our anisotropic nonlinear spin wave theory. We emphasize the Neel temperature, antiferromagnetic order parameter, and spin correlation as functions of the next-nearest-neighbor exchange parameter. We apply the theoretical results to the antiferromagnetic properties of insulating cuprates.