de Robert Groot

Half-metallic ferromagnets: From band structure to many-body effects

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

Spin-polarization in half-metals (invited)

Half-metals are defined by an electronic structure, which shows conduction by charge carriers of one spin direction exclusively. Consequently, the spin polarization of the conduction electrons should be 100%. In reality this complete spin polarization is not always observed. Since the experimental search for half-metals is tedious and the verification of the expected spin polarization is involved, electronic structure calculations have played an important role in this area. So, an important question is, how the approximations in such calculations influence the resulting spin polarization of the conduction. Another aspect is the well-known fact that bulk properties can be very different from surface and interface properties. Indeed, measurements of the spin polarization in the bulk for, e.g., NiMnSb, show results different from surface sensitive measurements. In this respect it is important to realize that the origin of half-metallic behavior is not unique. Consequently, the deviations from the bulk behavior at the surface/interface can be important. Three different categories of half-metals can be distinguished and their expected surface properties will be discussed. Finally, ways will be described to control the properties at interfaces.

Spin-dependent transport in metal/semiconductor tunnel junctions

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 so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/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/ tau -MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of tau -MnAl along the tunnelling direction; the latter is corroborated by band structure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Amorphous WO3: a first-principles approach

Results of first-principles calculations on the structure and electronic structure of amorphous WO3 are presented. The effect of non-stoichiometry is investigated. In particular, we discuss the pairing of W5+ species in oxygen-deficient films resulting in deep in-gap states and its possible consequences for the electrochromic coloration efficiency. To this end, we make a connection with Raman experiments by calculating the vibrational density from molecular-dynamics simulation. For the W5+-W5+ stretch we find a low frequency mode at ~200 cm-1 which agrees well with the Raman data. We also estimate the stability of isolated W5+ species in stoichiometric and oxygen-deficient tungstentrioxide.

The continuing drama of the half-metal/semiconductor interface

In this article, based on electronic structure calculations, the conditions are discussed under which a genuine half-metallic interface between a heusler C1b half-metal and a semiconductor can exist. An explanation is given why for the III–V semiconductors the double anion terminated (111) interface is the only possible interface. For semiconductors, based on transition metals, a much wider variety of interfaces are found to be possible.

Nonquasiparticle states in the half-metallic ferromagnet NiMnSb

Nonquasiparticle states above the Fermi energy are studied by first-principle dynamical mean field calculations for a prototype half-metallic ferromagnet NiMnSb. We present a quantitative evaluation of the spectral weight of this characteristic feature and discuss the possible experimental investigation (Bremsstrahlung isohromat spectroscopy, nuclear magnetic resonance, scanning tunneling microscopy, and Andreev reflection) to clarify the existence of these states.

Electronic band structure of tetracene-TCNQ and perylene-TCNQ compounds

The relationship between the crystal structures, band structures, and electronic properties of acene-TCNQ complexes has been investigated. We focus on the newly synthesized crystals of the charge-transfer salt tetracene-TCNQ and similar to it perylene-TCNQ, potentially interesting for realization of ambipolar transport. The band structures were calculated from first principles using density-functional theory (DFT). Despite the similarity in the crystal structures of the acene-TCNQ complexes studied here, the band structures are very different. Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCNQ, which has good transport properties for electrons only. The estimated degree of charge transfer for tetracene-TCNQ is 0.13e and for perylene-TCNQ 0.46e.

Fermi surface and DHVA effect in ZrZn2 and TiBe2: Theory

The bandstructure, density of states, Fermi surface and external DHVA areas are presented for ZrZn2 and TiBe2. Arguments are given to suggest the feasibility of DHVA experiments.

The electronic structure of the metastable layer compound 1T-CrSe2

The electronic structure of the metastable compound (a = 3.399 A, c = 5.911 A, space group ) was calculated with and without spin polarization using the LSW method. The energy is 0.29 eV/mol lower for the spin-polarized calculation. The total magnetic moment of on Cr consists of in spin-up and in spin-down states; the total number of 3d electrons on Cr is 4.12, much greater than expected for Cr(IV) . The Cr 3d-based bands overlap the selenium 4p-based valence band which implies strong covalency of the Cr - Se bonding. At the Fermi level there are electrons and holes with Cr 3d character, and holes with Se 4p character. The results clearly indicate the reduction of the cations and the presence of holes in the Se 4p valence band. is a magnetic metal. Similar calculations for showed a very small energy difference between the magnetic and non-magnetic states, indicating that is a non-magnetic metal.

Spintronic materials based on main-group elements

Recently, half-metallic magnetism was predicted for rubidium sesquioxide at relatively high temperatures. Here, we present density-functional calculations on related potassium and caesium oxides as well as hypothetical continuous series. Finally, we consider ammonium sesquioxide and estimate a Curie temperature of 250 K. All these half-metals posses band gaps for the majority spin direction.