This Letter discusses the possibility of realizing half-metallic antiferromagnets, i.e. , systems with 100% spin polarization of the conduction electrons without showing a net magnetization.The predictions are based on electronic structure calculations using the local density approximation.

Abstract. This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface.Measurements are presented of the circular-polarization-dependent photocurrent (the socalled helicity asymmetry) in thin-film tunnel junctions of Co/Al203/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/r-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of r-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

In order to understand the electronic structure of the mis6t-layer compound (SnS)&»NbS2 we carried out an ab initio band-structure calculation of the closely related commensurate compound (SnS) & ppNbS2.The band structure is compared with calculations for NbS2 and for hypothetical SnS with structure and interatomic distances as in (SnS) i 2pNbS&. The calculations show that the electronic structure is approximately a superposition of the electronic structures of the two components NbS2 and SnS, with a small charge transfer from the SnS layer to the NbS2 layer. The interlayer bonding between SnS and NbS2 is dominated by covalent interactions. X-ray and ultraviolet photoelectron spectra were obtained for the valence bands. The observed spectra are in good agreement with the band-structure calculations.

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