M. Karpovski

Oscillations of the Superconducting Critical Current in Nb-Cu-Ni-Cu-Nb Junctions

We report on experimental studies of superconductor-ferromagnet layered structures. Strong oscillations of the critical supercurrent were observed with the thickness variation of the ferromagnet. Using known microscopic parameters of Ni, we found reasonable agreement between the period of oscillations and the decay of the measured critical current, and theoretical calculations.

Observation of periodic pi-phase shifts in ferromagnet-superconductor multilayers

We report complementary studies of the critical temperature and the critical current in ferromagnet (Ni) - superconductor (Nb) multilayers. The observed oscillatory behavior of both quantities upon variation of the thickness of the ferromagnetic layer is found to be in good agreement with theory. The length scale of oscillations is identical for both quantities and is set by the magnetic length corresponding to an exchange field of 200 meV in Ni. The consistency between the behavior of the two quantities provides strong evidence for periodic pi phase shifts in these devices.

Hillock formation during electromigration in Cu and Al thin films: Three‐dimensional grain growth

The evolution of microstructure in Al and Cu thin film lines during electromigration has been studied using a transmission electron microscopy. Grain boundary migration was found to be critically involved in the electromigration induced hillock formation that can be described as a three‐dimensional growth of a single grain.

Reversal of the extraordinary Hall effect polarity in thin Co/Pd multilayers

Thin Co/Pd multilayers, with room temperature perpendicular anisotropy and an enhanced surface scattering, were studied for the possible use in the extraordinary Hall effect (EHE)-based magnetic memory devices. Polarity of the EHE signal was found to change from negative in thick samples to positive in thin ones. Reversal of EHE sign was also observed in thick samples with aging. The effect is argued to be related to the dominance of surface scattering having the EHE polarity opposite to that of the bulk.

Correlation between the extraordinary Hall effect and resistivity

We propose to reconsider the correlation between the extraordinary Hall effect and resistivity by using the skew scattering model and Matthiesens rule to separate contributions of different scattering sources. The model has been experimentally tested for the cases of scattering by magnetic nanoparticles embedded in normal-metal matrix, insulating impurities in magnetic matrix, surface scattering, and temperature-dependent scattering.

Luttinger-liquid behavior in weakly disordered quantum wires

We have measured the temperature dependence of the conductance in long V-groove quantum wires fabricated in GaAs/AlGaAs heterostructures. Our data are consistent with recent theories developed within the framework of the Luttinger-liquid model, in the limit of weakly disordered wires. We show that, for the relatively low level of disorder in our quantum wires, the value of the interaction parameter g congruent with 0.66, which is the expected value for GaAs. However, samples with a higher level of disorder show conductance with stronger temperature dependence, which does not allow their treatment in the framework of perturbation theory. Fitting such data with perturbation-theory models leads inevitably to wrong (lower) values of g.

Effect of microstructure on electromigration kinetics in Cu lines

Strong correlation of the modes of electromigration damage and microstructure is reported for Cu films. It is found that changes in the microstructure lead to qualitative variation in electromigration damage kinetics - from the traditional open circuit due to void growth across the line, to damage growing along the line, and not leading to failure. Some of our findings are consistent with the theoretical model based on interplay between surface and grain boundary diffusion. The activation energy eV of electromigration mass transport was measured using a modified electrical resistance method.

Critical oxygen content in porous anodes of solid tantalum capacitors

X-ray diffraction analysis of sintered porous anodes of solid tantalum capacitors and the current-voltage (I–V) characteristics of Ta2O5 amorphous layers formed on the anode surface have been performed. A strong correlation between a sharp increase of direct current in the I–V characteristics at some critical oxygen content and the creation of a saturated solid-phase solution of oxygen in tantalum was found. The appearance of a crystalline oxide Ta2O5 phase was detected in porous anodes at oxygen contents above the critical oxygen level. The decrease of effective radius below 1 μm of Ta powder particles used in sintering leads to the size effect: the oxygen content in the porous anode after sintering exceeds the solubility limit.

Electrical, thermoelectric and thermophysical properties of hornet cuticle

Seebeck effect (thermo-emf), thermal conductivity and electrical conductivity of social hornet cuticle were measured in a direction perpendicular to the cuticular surface. The obtained value of the Seebeck coefficient (S) was about 3 ± 0.5 mV K−1 and its sign corresponded to an n-type (electronic) conductivity. Hornet cuticle is shown to be a fairly good heat insulator, with recorded values of the heat conductivity as low as 0.1–0.2 W m−1 K−1. The measured value of the electrical conductivity in the linear regime is σ = 8.5 × 10−5 Ω−1 cm−1. The thermoelectric figure of merit is computed. Implications for possible exploitation as a natural thermoelectric heat pump are discussed.

Asymmetric field dependence of magnetoresistance in magnetic films

We study an asymmetric in field magnetoresistance that is frequently observed in magnetic films and, in particular, the odd longitudinal voltage peaks that appear during magnetization reversal in ferromagnetic films, with out-of-plane magnetic anisotropy. We argue that the anomalous signals result from small variation of magnetization and Hall resistivity along the sample. Experimental data can be well described by a simple circuit model, the latter being supported by analytic and numerical calculations of current and electric field distribution in films with a gradual variation of the magnetization and Hall resistance.