Half-metallic Band Structure Research Articles

Chromium oxides thin films prepared and coated in situ with gold by pulsed laser deposition

Chromium dioxide is a very suitable compound for spintronic devices. It is ferromagnetic at room temperature ( T c = 395 K) and has a half-metallic band structure. CrO 2 has been theoretically predicted to be100% spin polarized at the Fermi level. For some applications in spintronics low deposition temperatures are required. However, the fabrication of single phase thin films of CrO 2 at low temperature is quite difficult due to its metastable character. In this work we report our experiments to deposit thin films of chromium oxides by reactive pulsed laser deposition (RPLD). RPLD experiments were performed with a UV KrF* excimer laser source ( τ FWHM ≥ 20 ns, λ = 248 nm) in different conditions regarding the target, the substrate temperature, the atmosphere inside the deposition chamber and the laser fluence. In order to prevent the reduction of CrO 2 to Cr 2O 3 in thin films preserved in ambient conditions some of the samples were capped with gold in situ using the same set-up. The composition and morphology of the deposited films were characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. A higher and stable content of CrO 2 in the deposited films was evidenced in the case of samples capped with Au. Subsequent analysis performed after 4 months showed no difference in the content of the film.

Microstructures and Magnetic Property of Bulk and Thin Film Co2MnGe Heusler Compound

Half metallic ferromagnets are candidate materials for novel electronic devices, which can control the spin character of the conducting electrons. Electronic band calculation predicted that Co2MnGe Heusler compounds may has the half metallic band structure. In the present study, structure and magnetic properties have been investigated of the Co2MnGe for bulk and thin films. In the bulk sample, plate defects have been found.

Very low chemical disorder in epitaxial NiMnSb films on GaAs(111)B

Single-crystalline NiMnSb(111) films with negligibly low defect levels have been grown epitaxially on GaAs(111)B using molecular beam epitaxy and characterized by nuclear magnetic resonance. In a film with only 1% deviation from stoichiometry, 1.1% of all Mn atoms is involved in planar defects, ∼0.5% of all Sb sites is occupied by AsSb substitutional atoms, and ∼0.2% of all Sb atoms has a modified environment. Both the average concentration of defects and the interface orientation are compatible with maintaining a half-metallic band structure at the ferromagnet/semiconductor interface, making these films a good candidate for spin injection into a semiconductor.

THICKNESS DEPENDENCE OF PHOTOEMISSION SPECTRA IN ZINC-BLENDE CrAs

The electronic structure of CrAs thin films with nominal thicknesses of 2 nm and 30 nm were studied by angle-resolved photoemission spectroscopy using synchrotron radiation. The CrAs film of 2 nm is expected to form a zinc-blende phase which is predicted to have a half-metallic band structure by theoretical calculation. On the other hand, it is considered that the CrAs film of 30 nm is a polycrystalline compound judging from reflection high-energy electron diffraction patterns. Different valence-band photoemission spectra were obtained for the two films. The photoemission spectra of the CrAs film of 30 nm showed no band dispersion, whereas a band dispersion was observed in that of the CrAs film with the nominal thickness of 2 nm.

Coulomb correlation and magnetic ordering in double-layered manganites: LaSr 2Mn 2O 7

A detailed study of the electronic structure and magnetic configurations of the 50% hole-doped double-layered manganite LaSr 2Mn 2O 7 is presented. We demonstrate that the on-site Coulomb correlation ( U) of Mn d electrons (i) significantly modifies the electronic structure, magnetic ordering (from FM to AFM), and interlayer exchange interactions, and (ii) promotes strong anisotropy in electrical transport, reducing the effective hopping parameter along the c-axis for electrically active e g electrons. This finding is consistent with observations of anisotropic transport—a property which sets this manganite apart from conventional 3D systems. A half-metallic band structure is predicted with both the LSDA and LSDA+U methods. The experimentally observed A-type AFM ordering in LaSr 2Mn 2O 7 is found to be energetically more favorable with U⩾7 eV. A simple interpretation of interlayer exchange coupling is given within double and super-exchange mechanisms based on the dependencies on U of the effective exchange parameters and e g state sub-band widths.

Spin Dynamics in the Ferromagnetic Phase of Colossal Magnetoresistive Oxides

A review is given that focuses on the spin dynamics in the ferromagnetic regime of the magnetoresistive oxides. At small wave vectors the quadratic dispersion relation is remarkably isotropic throughout the composition range, with a gap that is too small to measure with conventional neutron scattering techniques. At larger wave vectors the spin waves are strongly damped in the ground state, in contrast to expectations based on a simple half-metallic band structure, while the temperature dependence of the damping and renormalization of the spin waves is anomalous. An unusual spin-diffusion component to the fluctuation spectrum develops as the Curie temperature is approached, which appears to be related to the formation of polarons in the system and the consequent charge localization. Near and above TC the diffuse scattering from polarons is directly observed, as well as the correlations between the polarons.

Transport, optical, and electronic properties of the half-metalCrO2

The electronic structure of ${\mathrm{CrO}}_{2}$ is critically discussed in terms of the relation of existing experimental data and well converged local-spin-density approximation (LSDA) and generalized gradient approximation (GGA) calculations of the electronic structure and transport properties of this half metal magnet, with a particular emphasis on optical properties. We find only moderate manifestations of many-body effects. Renormalization of the density of states is not large and is in the typical range for transition metals. We find substantial deviations from Drude behavior in the far-infrared optical conductivity. These appear because of the unusually low energy of interband optical transitions. The calculated mass renormalization is found to be rather sensitive to the exchange-correlation functional used and varies from 10% (LSDA) to 90% (GGA), using the latest specific-heat data. We also find that dressing of the electrons by spin fluctuations, because of their high energy, renormalizes the interband optical transition at as high as 4 eV by about 20%. Although we find no clear indications of strong correlations of the Hubbard type, strong electron-magnon scattering related to the half metallic band structure is present and this leads to a nontrivial temperature dependence of the resistivity and some renormalization of the electron spectra.

Low-Temperature Growth of NiMnSb Heusler Alloy Thin Films

ABSTRACTFerromagnetic Heusler alloys such as NiMnSb may have unique applications to magnetoresistive devices because of their predicted half-metallic band-structure (i.e., 100% spin-polarized at the Fermi level). However, the high temperatures (>500°C) used to date for the synthesis of the ordered alloy in bulk or thin-film form is incompatible with the reliable fabrication of nanoscale multilayer structures from this material. We report on the growth of high-quality polycrystalline thin films, 200–1000Å thick, of Clb-structured NiMnSb by RF-magnetron sputtering of a composite NiMnSb target onto glass and silicon substrates at temperatures as low as 250°C. We have established that substrate temperature, deposition rate and argon gas pressures all play a critical role in obtaining the Clb-structured phase by direct deposition. Optimal conditions result in films whose properties, including lattice parameter, saturation magnetization, resistivity and magneto-optical Kerr rotation spectrum are identical to those of bulk NiMnSb. Additionally, coercive fields as low as a few oersteds make these films compatible with low-field applications.