J.L. Costa-Krämer

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

We present tunneling experiments on Fe(001)/MgO(20 A)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

Magnetization reversal asymmetry in Fe/MgO(0 0 1) thin films

We study the in-plane magnetization process in 200 A Fe(0 0 1) thin films grown by sputtering at normal incidence. In spite of this growth geometry, a small uniaxial in plane magnetic anisotropy, whose origin is not totally understood, is found superimposed to the expected cubic biaxial one. This has a dramatic effect both on the reversal process and the domain structure. A combined longitudinal and transversal Kerr study shows the different switching processes (180° walls along the main easy axis versus 90° along the secondary easy axis) depending on the relative orientation of the magnetic field with respect to the Fe crystallographic axes. Remarkably, this two- and sometimes three-step switching process appears only when the field is applied along certain crystallographic directions. These findings are corroborated by domain observations.

Epitaxy, magnetic and tunnel properties of transition metal/MgO(001) heterostructures

The main aspects of the fabrication and the structural and magnetic properties of epitaxial metal-insulator heterostructures are reviewed. Continuous and lithographically processed systems are discussed, with special emphasis on their crystalline nature when interpreting their physical properties. The work is placed within the scope of the previous and current activities by different groups on Fe/MgO(001), considered as a model system. The influence of deposition conditions on the epitaxy and morphology are discussed initially, correlating afterwards magnetic properties with crystalline structure. Subsequently, magnetic anisotropies and interactions are treated in both continuous and patterned film structures. Finally, the use of these fully epitaxial transition metal-insulator heterostructures for the development of magnetic tunnel junctions is described.

Evidence of shell structures in Au nanowires at room temperature

Statistical conductance properties of gold elongating nanowires at room temperature are studied experimentally. The measured conductance histogram, built with thousands of consecutive nanocontact breakage experiments, exhibits a rich structure of low-intensity peaks at high-conductance quanta values. Their position as a function of a peak index number suggests two markedly different regimes for electronic and atomic shell structures in these nanowires, as observed previously for alkali metal nanowires.

Size and dimensionality effects in superconducting Mo thin films

Molybdenum is a low Tc, type I superconductor whose fundamental properties are poorly known. Its importance as an essential constituent of new high performance radiation detectors, the so-called transition edge sensors (TESs) calls for better characterization of this superconductor, especially in thin film form. Here we report on a study of the basic superconducting features of Mo thin films as a function of their thickness. The resistivity is found to rise and the critical temperature decreases on decreasing film thickness, as expected. More relevant, the critical fields along and perpendicular to the film plane are markedly different, thickness dependent and much larger than the thermodynamic critical field of Mo bulk. These results are consistent with a picture of type II 2D superconducting films, and allow estimates of the fundamental superconducting lengths of Mo. The role of morphology in determining the 2D and type II character of the otherwise type I molybdenum is discussed. The possible consequences of this behaviour on the performance of radiation detectors are also addressed.

Anisotropy measurements in mesoscopic magnets by magneto-optical torque

The development of a versatile tool to quantify magnetic anisotropies in continuous and patterned magnetic thin films is reported. The technique involves measuring the magnetooptic response to a rotating magnetic field. Similarly to mechanical torque techniques, a single measurement obtains both the anisotropy constants and their symmetry axes distribution. The technique has been applied to analyze arrays of submicrometer stripe-shaped Fe (001) elements with different interelement separations (s). For s larger than 1 μm, the anisotropy associated with the stripes is independent on separation, with a value of the effective uniaxial anisotropy constant very consistent with theoretical estimations for these systems.

Effects of Stress and Morphology on the Resistivity and Critical Temperature of Room-Temperature-Sputtered Mo Thin Films

We report on the structural and electrical characterization of Mo thin films deposited at room temperature by RF magnetron sputtering. The effect of RF power on the morphology and residual stress of the films is analyzed. The films are under compressive stress and consist of densely packed columns with a lateral size on the order of 20 nm. The stress, critical temperature, and resistivity of the films are found to rise when increasing the ejected ion energy during the sputtering process. The changes in critical temperature and resistivity are discussed in terms of the observed morphology and stress changes.

Mo-Based Proximity Bilayers for TES: Microstructure and Properties

We report on the fabrication and characterization of Mo films, Mo/Au and Mo/Cu bilayers for Transition Edge Sensors (TES). The fabrication conditions (at room temperature) have been varied to achieve layers with the required properties for TES applications. The dependence of their functional properties (i.e. electrical resistivity and superconducting critical temperature) on microstructure (grain size, stress) is investigated.

Nanopatterning effects on magnetic anisotropy of epitaxial Fe(001) micrometric squares

Magneto-optic (MO) studies are performed on regular arrays of Fe(100) micrometric squares, where the elements are patterned with different sizes (2.5–10 μm) and separations (0.2–0.6 μm). When a laser beam is focused inside the patterned structure a Bragg diffraction pattern is produced allowing MO studies on both reflected and diffracted spots. The magnetic anisotropy has been analyzed by in-plane MO hysteresis loops finding that, for square sizes below 2.5 μm, it is not consistent with the Fe crystalline cubic anisotropy, presenting a uniaxial-like behavior. The magnitude of the corresponding anisotropy constants has been determined by analyzing the array response (on reflected and diffracted spots) to a rotational magnetic field in a magneto-optical torque setup. For square sizes of 10 μm the anisotropy induced by patterning is negligible whereas for the small squares (2.5 μm) the uniaxial magnetic constant has been found to be roughly 0.2 times the Fe cubic anisotropy constant.

Breakage dynamics and quantized conductance of gold nanowires in the presence of lubricants

The conductance of stretching gold metallic nanowires has been measured when immersed in different lubricants. Very similar conductance values are found for all the lubricants used at room temperature. This is measured as a conductance histogram, built with thousands of consecutive stretching experiments, with well defined peaks close to integer values of the quantum conductance, G0 = 2e2/h. Thus no appreciable evidence of chemical or configurational disorder in the nanowire due to the lubricants presence is found. In addition, nanowires stretch slightly slower when immersed in lubricant than when in air. This is due to a nanowire geometry dictated mainly by the electrode atomic composition and separation, the electrostatics and a somehow larger stabilization due to the presence of the lubricants.