[en] The theoretically predicted high spin polarization of half-metallic ferromagnets like CrO₂ and NiMnSb make them to promising materials for magnetoelectronic applications. Highly textured CrO₂ films have been prepared by chemical vapor deposition and molecular beam epitaxy. The temperature-dependent magnetotransport data has been correlated with electronic properties of CrO₂. Over a wide temperature range (150-330 K) the resistivity follows a T² behaviour, consistent with electron-electron scattering. Near the Curie temperature of CrO₂ no metal-insulator transition, a magnetoresistance of -7% (B=9 T) and an enhancement of the resistance due to electron-magnon scattering are observed

[en] We investigated the magnetoresistance of Permalloy (Ni₈₀Fe₂₀) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al₂O₃, MgO and SiO₂ substrates. Growth and transport measurements were carried out at 80 K in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties were ascertained during growth. With increasing thickness the films exhibited a gradual transition from tunnelling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains, as well as the film's transition from superparamagnetic to ferromagnetic behaviour. Using an analysis based on a theoretical model of island growth, we found that the observed evolution of the magnetoresistance in the tunnelling regime originated from changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunnelling magnetoresistance and anisotropic magnetoresistance were found. We attributed this to an increasingly pronounced island growth, and to a slower percolation process of Permalloy when comparing growth on SiO₂, MgO and Al₂O₃ substrates. The different growth characteristics resulted in a markedly earlier onset of both tunnelling magnetoresistance and anisotropic magnetoresistance for SiO₂. For Al₂O₃ in particular the growth mode results in a structure of the film containing two different contributions to ferromagnetism, which lead to two distinct coercive fields in the high thickness regime.

[en] Highly a-axis-textured CrO₂ films have been deposited on Al₂O₃(0001) and on isostructural TiO₂(100) substrates by a chemical vapour deposition technique. For Al₂O₃ substrates a columnar growth of CrO₂(010) on an initial Cr₂O₃(0001) layer has been found in transmission electron microscopy as well as in x-ray diffraction investigations. The sixfold in-plane symmetry of a (0001)-oriented Cr₂O₃ initial layer leads to three equivalent in-plane orientations of the CrO₂ unit cell as confirmed by electron diffraction and scanning electron microscopy. The growth can be understood by a simple model of the in-plane symmetries of the Al₂O₃(0001), Cr₂O₃(0001), and CrO₂(010) lattices. The growth on TiO₂(100) substrates leads to (100)-oriented CrO₂ films of higher crystalline quality than the ones grown on Al₂O₃(0001). Transmission electron microscope images show growth of CrO₂(100) directly on the TiO₂(100) substrates and no significant Cr₂O₃ inclusions within the CrO₂(100) layer. All contributions to the magnetoresistance (MR) due to anisotropic MR, Lorentz MR, spin disorder, and intergrain tunnelling MR have been determined and partly correlated with the crystalline properties of the samples investigated. For films of both types the intrinsic linear contribution to the high-field MR does not depend on the crystalline quality of the films and supports the suggested intrinsic double-exchange mechanism for CrO₂. (author)

[en] The thermal stability of an iron/molybdenum bilayer was determined using Rutherford backscattering spectrometry and X-ray diffraction. Substantial interdiffusion (3%) was observed upon annealing at 973 K for one hour. High temperature annealing of Fe/Mo layers in oxygen ambient is therefore not suitable for the formation of magnetite layers

[en] We report on the electronic properties of Mn₁₂ molecules chemically grafted on the functionalized Au(111) surface studied by means of scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from Mn₁₂ molecules showing a large region of low conductance around the Fermi energy. In agreement with the tunneling spectroscopy results the bias voltage variation upon scanning leads to apparent height changes of the Mn₁₂ clusters. We discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn₁₂ molecules

[en] We present a detailed study on the structural, magnetic, and optical properties, as well as the electronic structure of epitaxial Co-doped ZnO films prepared by magnetron sputtering. Different preparation conditions were implemented in order to control the concentration of oxygen vacancies in the ZnO host lattice. Magnetization measurements indicate ferromagnetic behavior at low temperature for samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show extremely small ferromagnetic signal corroborating that ferromagnetism in Zn_1-xCo_xO correlates with the presence of the oxygen-related defects. X-ray absorption spectroscopy (XAS) at the Co L_2,3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co²⁺ (d⁷) state under tetrahedral symmetry indicating a proper incorporation in the ZnO host lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model

[en] Details are presented of a single shot focused magneto-optic Kerr effect (MOKE) magnetometer which is used to capture the movement of single domain walls (DWs) in permalloy (Ni₈₀ Fe₂₀) nanowires (≥400 nm width and ≥20 nm thickness) in real time. By probing the DW motion within the 1 μm diameter laser spot of the instrument, DW velocity and pinning field distributions were obtained. An external field was ramped up linearly, and depinning of a DW from the same start position was observed at three different fields, indicating the stochastic nature of the DW motion

[en] Zinc oxide (ZnO) nanocrystals (NCs) with high crystalline quality were prepared via radio-frequency magnetron sputtering as a SiO₂/ZnO/SiO₂ trilayer on Si(100) and Al₂O₃(0001) substrates with an intermediate in situ annealing step. Transmission electron microscopy reveals a uniform dispersion of ZnO NCs in the amorphous SiO₂ matrix with typical sizes up to 16 nm with a larger fraction of smaller crystals. The size distribution analysis yields a mean grain size of 5 nm for small particles. Individual ZnO NCs show a well-defined hexagonal close packed wurtzite structure and lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Mapping of the Zn distribution by means of energy-filtered transmission electron microscopy reveals a strongly non-uniform distribution of Zn within the SiO₂ matrix, corroborating the chemical separation of ZnO NCs from surrounding SiO₂. Optical transmittance measurements confirm the findings of the electron microscopy analysis. The fabrication technique described opens up new possibilities in the preparation of ZnO NCs with high crystalline quality, including growth in monolithic optical cavities without intermediate ex situ fabrication steps.

[en] We present a study on the structural, magnetic, and optical properties, as well as the electronic structure of Co-doped ZnO films prepared by magnetron sputtering. Magnetization measurements performed at different temperatures indicate ferromagnetic and paramagnetic behavior for the samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show only paramagnetic behavior corroborating that the presence of oxygen-related defects is essential for ferromagnetism in Zn_1-xCo_xO. X-ray absorption spectroscopy (XAS) at the Co L_2,3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co²⁺ (d⁷) state under tetrahedral symmetry indicating a proper incorporation in the ZnO host lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model

[en] Recently Tobin et al (2007 J. Phys.: Condens. Matter 19 315218) reported on the spin-resolved photoemission study of Fe₃O₄(001) films, claiming magnetite being a case against half-metallicity. In the present communication we re-examine recent spin-resolved photoemission experiments on Fe₃O₄ and explain why their criticism is unfounded. (fast track communication)