[en] Growth of high-quality dilute magnetic semiconductor (DMS) material is often compromised by the low solubility of magnetic dopants, leading to formation of precipitates. Here, we explore the feasibility of growing precipitate-free Mn-doped Ge at doping levels near the kinetic solubility limit. Ge:Mn DMS films were grown at low temperature so as to minimize precipitate formation. Meanwhile, epitaxial quality was maintained by employing a very low growth rate. The magnetic properties of these lightly doped films exhibit both interesting contrasts and similarities with those of heavily-doped DMS reported in the literature, indicating that the substitutional Mn contents are very similar. Films grown at 95 degree C are free of intermetallic precipitates, offering useful opportunities for studying the fundamentals of carrier mediated exchange and metal insulator transitions without complications arising from precipitate formation.

[en] We show that the growth, composition, and superconductive properties of ultrathin Pb_1-xGa_x alloy films depend strongly on the atomic structure of the substrate interface. Whereas alloy films on the Si(111)-(7 x 7) surface grow in multilayers, reminiscent of the quantum growth phenomenon observed in pure Pb layers, alloy films on the Si(111)-(√3 x √3)R30^o-Pb and Si(111)-(√3 x √3)R30^o-Ga interfaces gradually switch to classical layer-by-layer growth. The (7 x 7) interface is unique in that it enables formation of atomically smooth alloy films containing up to 6% of Ga, which is far beyond the solid solubility limit. In contrast, the (√3 x √3) interfaces promote phase separation. The contrasting influences of these interfaces are also reflected by their superconductive properties, most notably by the differences in the critical current density, exceeding more than one order of magnitude.

[en] We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in⁻² or higher. (paper)

[en] We report predictions for the energy eigenstates and inelastic neutron scattering excitations of an isotropic Heisenberg hexamer consisting of general spin S and S(prime) trimers. Specializing to spin-1/2 ions, we give analytic results for the energy excitations, magnetic susceptibility, and inelastic neutron scattering intensities for this hexamer system. To examine this model further, we compare these calculations to the measured magnetic susceptibility of a vanadium material, which is considered to be well defined magnetically as an isolated S = 1/2 V⁴⁺ trimer model. Using our model, we determine the amount of inter-trimer coupling that can be accommodated by the measured susceptibility, and predict the inelastic neutron scattering spectrum for comparison with future measurements.

[en] Practical applications of second generation (2G) high temperature superconductor (HTS) wires require high critical current density, Jc, at high temperatures and magnetic fields. It has been well established that Jc can be increased via nanostructural engineering of artificial pinning centers within the HTS matrix. In the present work, composite LaMnO3:MgO (LMO:MgO) cap buffer layers with varying MgO contents 5 vol% up to 75 vol% have been grown on homo-epi MgO/IBAD(MgO) substrates to enhance the performance of YBa2Cu3O7-x (YBCO) films. Results showed formation of phase separated MgO nanocolumns within the LMO matrix. The impact of these nanocolumns on the superconducting properties of YBCO films deposited on the nanostructured layer was investigated by electrical transport measurements. Such YBCO films showed better in-field performance compared to that of YBCO films on standard LMO cap films. In particular, measurements of the field-angle dependence revealed c-axis correlated pinning for YBCO films on these composite cap layers. The present results demonstrate a practical approach to obtain high performance superconducting wires.

[en] We have investigated the electronic, magnetic, and optical properties of two Fe-based superconductors and related parent compounds via three powerful techniques: scanning tunneling microscopy/spectroscopy (STM/S), high-temperature vibrating sample magnetometer (VSM), and optical transmission spectroscopy (OTS). Below the superconducting transition temperature Tc ∼ 48 K, the STM/S of polycrystalline NdFeAsO0.86F0.14 reveals a single-gap feature. The quantitative fitting of STS data results in BCS like temperature dependence of the energy gap (T), with 2 (0)/kBTc ∼ 4.3. Surprisingly, the tunneling spectra of BaFe2As2 single crystals show no evidence for the opening of a gap below its magnetic/structural transition temperature TMS ∼ 140 K. This transition also had little impact on the transmission spectra in the wavelength range between 400 and 2400 nm. But its effect to the magnetic properties is dramatic, as reflected by the unusual magnetic susceptibility in a wide temperature range.

[en] The magnetic properties of zinc ferrite (Zn-substituted magnetite, Zn_yFe_1-yFe₂O₄) formed by a microbial process compared favorably with chemically synthesized materials. A metal reducing bacterium, Thermoanaerobacter, strain TOR-39 was incubated with Zn_xFe_1-xOOH (x=0.01, 0.1, and 0.15) precursors and produced nanoparticulate zinc ferrites. Composition and crystalline structure of the resulting zinc ferrites were verified using X-ray fluorescence, X-ray diffraction, transmission electron microscopy, and neutron diffraction. The average composition from triplicates gave a value for y of 0.02, 0.23, and 0.30 with the greatest standard deviation of 0.02. Average crystallite sizes were determined to be 67, 49, and 25 nm, respectively. While crystallite size decreased with more Zn substitution, the lattice parameter and the unit cell volume showed a gradual increase in agreement with previous literature values. The magnetic properties were characterized using a superconducting quantum interference device magnetometer and were compared with values for the saturation magnetization (M_s) reported in the literature. The averaged M_s values for the triplicates with the largest amount of zinc (y=0.30) gave values of 100.1, 96.5, and 69.7 emu/g at temperatures of 5, 80, and 300 K, respectively indicating increased magnetic properties of the bacterially synthesized zinc ferrites. - Highlights: → The magnetism of bio-zinc ferrite compared with chemically synthesized materials. → A metal reducing Thermoanaerobacter, TOR-39 produced nanoparticulate zinc ferrites. → Crystallite size decreased with more Zn substitution. → Lattice parameter and the unit cell volume increased with more Zn substitution. → Saturation magnetization of the bacterially synthesized zinc ferrites was increased.