Walther Schwarzacher

Single crystal superconductor nanowires by electrodeposition

Superconducting Pb wires (diameter∼50 nm) have been prepared by pulse electrodeposition in nanoporous membranes. Single crystal or polycrystalline nanowires may be grown selectively and reproducibly depending on the pulse parameters. Unexpectedly, the growth of single crystal wires requires a greater departure from equilibrium conditions (greater overpotential) than the growth of polycrystalline ones. The importance of controlling the crystal texture is demonstrated by measurements of the superconducting transition temperature Tc which give significantly different results for polycrystalline and single crystal nanowires.

Giant magnetoresistance in electrodeposited superlattices

We have observed ‘‘giant magnetoresistance’’ in short‐period Cu/Co‐Ni‐Cu alloy superlattices electrodeposited from a single electrolyte under potentiostatic control. The superlattices were grown on polycrystalline Cu substrates which were removed before transport measurements were made. Room‐temperature magnetoresistances of over 15% in applied magnetic fields of up to 8 kOe were observed in superlattices having Cu layer thicknesses of less than 10 A.

Current perpendicular to plane giant magnetoresistance of multilayered nanowires electrodeposited in anodic aluminum oxide membranes

Co–Ni–Cu/Cu multilayered nanowires were prepared by electrodeposition using nanoporous aluminum oxide membranes rather than the more usual track-etched polycarbonate membranes as templates. Very large values of the current perpendicular to plane giant magnetoresistance (CPP-GMR) were recorded: 55% at room temperature and 115% at 77 K. The use of aluminum oxide membranes also made possible a study of the effects of annealing on the CPP-GMR.

Preparation and Magnetoresistance Characteristics of Electrodeposited Ni‐Cu Alloys and Ni‐Cu/Cu Multilayers

Galvanostatic electrodeposition was used to produce Ni-Cu alloys and Ni 81 Cu 19 /Cu multilayers by direct current (dc) plating and two-pulse plating, respectively, from a sulfate/citrate electrolyte. For the dc-plated Ni-Cu alloys, the deposition rate and the alloy composition were established as a function of the deposition current density, from which the appropriate deposition parameters for the constituent sublayers of the multilayers could be established. By measuring the resistivity at room temperature in magnetic fields up to H = 7 kOe, anisotropic magnetoresistance (AMR) was found for Ni 81 Cu 19 electrodeposits, whereas both giant magnetoresistance (GMR) and AMR contributions were observed for most Ni 81 Cu 19 /Cu multilayers. Finally, Ni-Cu alloys were also prepared by conventional pulse plating, varying the length of the deposition pulse (on-time) with constant separation (off-time) between the pulses. Clear evidence of a GMR contribution also appeared in these pulse plated Ni-Cu alloys which may be explained by the formation of a Cu enriched layer between the ferromagnetic layers deposited during the cathodic pulses. A quartz crystal microbalance experiment confirmed that an exchange reaction takes place during the off-time. These findings provide useful information on the formation mechanism of multilayers by the two-pulse plating technique.

Electrodeposited p-type magnetic metal-base transistor

In this work the development of a magnetic metal-base transistor that operates by hole transport is reported. The transistor is constructed using p-type silicon as the collector, Co as the base, and Cu2O as the emitter. Both base and emitter are deposited using electrochemical procedures. The transistor shows a magnetic-field-dependent current gain and a magnetocurrent of ∼40% observed for a low emitter current value of 2 mA.

Electrodeposition of Thin Films and Multilayers on Silicon

Cu, Co and Ni thin films and Co-Ni-Cu/Cu multilayers have been electrodeposited directly on n-type silicon substrates. This removes the need of using a seed-layer deposited by some other methods as a part of the growth process and integrates an efficient, inexpensive and convenient method for fabricating thin films with silicon technology. The deposits were prepared under potentiostatic conditions from different aqueous solutions, containing basically: (i) sulphates of the metallic ions plus sodium sulphate and boric acid for thin films and (ii) Ni sulphamate, Co sulphate, Cu sulphate, boric acid and sulphamic acid for multilayers. Aspects related to the deposition process and deposited layers were investigated by cyclic voltammetry, current transients, scanning electron microscopy. Rutherford backscattering and magnetoresistance measurements. Typically, thin compact metallic layers with regular granularity were obtained. Depending on the additives used, different nucleation and growth mechanisms were obseived. Magnetic multilayers with a maximum giant magnetoresistive (GMR) ratio of about 10% were produced.

Magnetic properties of electrodeposited nanowires

Electrodeposited multilayered nanowires grown within a polycarbonate membrane constitute a new medium in which giant magnetoresistance (GMR) perpendicular to the plane of the multilayers can be measured. These structures can exhibit a perpendicular GMR of at least 22% at ambient temperature. We performed detailed studies both of reversible magnetization and of irreversible remanent magnetization curves for CoNiCu/Cu/CoNiCu multilayered and CoNiCu pulse-deposited nanowire systems with Co:Ni ratios of 6:4 and 7:3 respectively in the range 10 - 290 K, allowing the magnetic phases of these structures to be identified. Shape anisotropy in the pulse-deposited nanowire and inter-layer coupling in the multilayered nanowire are shown to make important contributions to the magnetic properties. Dipolar-like interactions are found to predominate in both nanowire systems. Magnetic force microscope (MFM) images of individual multilayered nanowires exhibit a contrast consistent with there being a soft magnetization parallel to the layers. Switching of the magnetic layers in the multilayered structure into the direction of the MFM tips stray field is observed.

Giant magnetoresistance peaks in CoNiCu/Cu multilayers grown by electrodeposition

Giant magnetoresistance (GMR) of CoNiCu/Cu multilayers grown by electrodeposition was measured as a function of the copper layer thickness and effects of the order of 14% were obtained. The copper layer thickness ranged from 0.7 to 3.5 nm. Two peaks in the magnetoresistance were observed. One was centered at a copper thickness of ∼1.0 nm and the second was centered at ∼2.3 nm. Comparison of the field dependence of the magnetoresistance with the field dependence of the magnetization, as determined by vibrating‐sample magnetometer, suggests that the saturation field for GMR and the magnetization are similar for the larger copper thicknesses, but are strikingly different near 1.0 nm copper thickness. This observation suggests that the GMR is affected by different factors depending on the thickness of the copper layer.

Organic-metal-semiconductor transistor with high gain

We use evaporated C60 as the emitter in a vertical transistor structure with Au base and Si collector. The proportion of emitted electrons that overcome the barrier is measured as at least 0.99. Our metal-base transistor is easy to fabricate as it does not involve wafer bonding or require perfect semiconductor-on-metal growth.

Giant magnetoresistance in electrodeposited Co–Ni–Cu/Cu superlattices

We have electrodeposited a series of Co–Ni–Cu/Cu superlattices in which the Cu layer thickness was varied between 7 and 35 A and the Co–Ni–Cu alloy layer thickness held constant. ‘‘Giant magnetoresistance’’ was observed for all films, with the magnitude of the effect decreasing with increasing Cu spacer layer thickness.