R. Schad

Giant magnetoresistance in Fe/Cr superlattices with very thin Fe layers

Carefully tailored Fe/Cr epitaxial superlattices with extremely thin Fe layers have been grown on MgO(100) by molecular beam epitaxy. The low‐angle x‐ray spectra reveal the presence of sharp interfaces down to an Fe layer thickness of a few monolayers. An [Fe(4.5 A)/Cr(12 A)]50 superlattice shows a 220% magnetoresistance at 1.5 K, and a saturation field of 110 kOe. A further decrease of the Fe layer thickness produces a drastic decrease in the magnetoresistance.

Tunneling magnetoresistance observed in La0.67Sr0.33MnO3/organic molecule/Co junctions

Tunneling magnetoresistance has been observed in organic based spintronic devices using the organic semiconductors tetraphenyl porphyrin (TPP) and aluminum tris(8-hyroxyquinoline) (Alq3) as the spacer layer between La0.67Sr0.33MnO3 (LSMO) and Co films. The evidence for tunneling is twofold: (1) nonlinear current and conductance versus voltage curves and (2) an increasing junction resistance with decreasing temperature. In general, the magnetoresistance is found to decrease with increasing bias voltage and increasing temperature in both Alq3 and TPP junctions. These results demonstrate that organic molecules can form tunnel barriers that perform as well as most inorganic barrier materials on LSMO.

Selective metal electrodeposition through doping modulation of semiconductor surfaces

We demonstrate selective electrodeposition of magnetic layers on doped semiconductors resulting in a self-aligned pattern which replicates the doping pattern in the semiconductor surface. A Schottky barrier forms at the interface between a semiconductor substrate and the electrolyte, which upon application of a cathodic potential is biased in the forward (reverse) direction for n- or p-type semiconductors, respectively. Electron transfer from an n-type semiconductor is thus possible, while breakdown of the Schottky barrier would be necessary for deposition on a p-type substrate. The process will thus be spatially selective on a lateral modulation of the substrate doping. As an example we demonstrate the deposition of Co on GaAs.

Pinhole analysis in magnetic tunnel junctions

Pinholes in the insulating layer of magnetic tunnel junctions are local shortcuts and cause malfunction of such devices. The need for reduction of the tunnel resistance by reduction of the insulator thickness will make this problem even more severe. Therefore, the development of low-resistance magnetic tunnel junctions requires analyzing the pinhole density. We developed a method for pinhole imaging using electrodeposition of copper. Selective nucleation at pinholes produces characteristic structures that can be visualized by conventional microscopy techniques. The experimental conditions were carefully chosen in order to avoid uncontrolled damage of the insulator layer.

Growth temperature dependence of the magnetic and structural properties of epitaxial Fe layers on MgO(001)

We have studied the growth and magnetic properties of molecular beam epitaxy grown layers of bcc Fe(001) on MgO(001) substrates at a wide range of temperatures. For growth temperatures in the range 80−595 K, the iron forms islands which increase in lateral size with increasing temperature. Completed films in the same temperature range show the magnetic properties expected for a system with biaxial anisotropy, and a coercivity of <10 Oe. The value of the first cubic anisotropy constant divided by the magnetization (K1/M) remained constant. No evidence for uniaxial magnetic anisotropy in the films was found. Above 595 K, the films’ structure and magnetic properties changed dramatically to those characteristic of a particulate system.

Structure of epitaxial Fe films on MgO(100)

Abstract We investigated the epitaxial growth of bcc Fe on MgO(100) using STM and LEED as a function of growth temperature. We found the island size and shape to vary with the deposition temperature and post-deposition annealing. Relatively smooth island surfaces can be obtained by deposition around 160°C, however, the island heights distribution is rather irregular then. The rms roughness is lowest for deposition at 110°C. At lower deposition temperatures the islands become more round-shaped, likely due to a reduction of step edge diffusion. Post-deposition annealing to temperatures around 210°C increases the overall variation of the island height distribution.

Sharp ferromagnet/semiconductor interfaces by electrodeposition of Ni thin films onto n-GaAs(001) substrates

We report on the chemical, electrical, and magnetic properties of Ni/GaAs(001) interfaces prepared using electrodeposition. Electrodeposition is an equilibrium process which thus releases much less energy per absorbed atom than other deposition techniques. This allows preparation of chemically sharp interfaces which otherwise show a high degree of reactivity and interdiffusion. This is demonstrated by the example of Ni grown on GaAs(001). Photoelectron spectroscopy shows the absence of surface segregation of substrate material or diffusion into the Ni layer. This is confirmed by the electrical and magnetic properties of the films.

Analysis of climbing accidents.

The fall of a climber is analyzed by realistic and comprehensive model calculations. Various parameters that define such a fall and the action of the belaying system to stop it are included in an equation describing the balance between the energy gained by the fall and the various channels of dissipation. The result is a representative overview about the interplay between the various parameters. From this understanding, important consequences and recommendations for safety in climbing are deduced.

Electrodeposition of Metastable Au-Ni Alloys

The electrodeposition of Au-Ni alloys from near-neutral, sulfite-based electrolytes derived from a commercial bath for soft gold plating is investigated. Alloy compositions ranging from 0 to 90 atom % Ni were obtained by varying the deposition potential, with Ni content increasing with overpotential. Cathodic efficiency was lower than 50% due to concurrent parasitic reactions, including the reduction of products from the decomposition of sulfites and the hydrogen evolution reaction. As-deposited films form a continuous series of metastable solid solutions and exhibit a nanocrystalline morphology, with grain size decreasing with increasing Ni content and a possible Ni enrichment at the grain boundaries. Thermal annealing at 200°C was sufficient to start the relaxation of the metastable solid solution toward the thermodynamically stable biphasic configuration of pure Au and Ni phases; however, 400°C was necessary to complete the phase separation process within ~ 1 h. The formation of a metastable structure is interpreted in terms of the limited surface diffusivities of adatoms at the growing interface and atomic volume differences. The excess free energy of the as-deposited alloys with respect to the stable, phase separated configuration is estimated between 6 and 18 kJ/mol, consistent with what can be expected in electrochemical processing.

Preparation and characterization of epitaxial ilmenite-hematite films

The solid solution system ilmenite-hematite [(FeTiO3)1−x–(Fe2O3)x] is a potential candidate for applications in spintronics due to its intrinsic ferrimagnetic and semiconducting properties. Epitaxial ilmenite-hematite films with x=0.33, which have the highest room temperature magnetic moment, were grown on α-Al2O3 (0001) substrates using pulsed laser deposition technique with varying oxygen partial pressure in the ambient gas. Structural, magnetic, electrical, and optical properties are found to be largely dependent on the oxygen content of the films which is controlled by substrate temperature and ambient gas composition. The highest crystalline and magnetic ordering and the lowest resistivity values could be obtained for growth at high temperatures and under low oxygen pressure. A narrowing of the band gap (to around 2.4eV) was observed for films grown under high oxygen pressure in comparison with films grown in vacuum or argon (around 3.3eV).