J. Schumann

Evidence for triplet superconductivity in a superconductor-ferromagnet spin valve.

We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO(x)/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

Thermal conductivity in strain symmetrized Si/Ge superlattices on Si(111)

We have presented systematic cross-plane thermal conductivity (λ) data for the undoped strain-symmetrized Si/Ge superlattices grown on Si(111) with superlattice (SL) period thickness varying from 3.6 to 16 nm. In thin SL period (L⩽7 nm) samples, the data have shown considerable reductions of λ, by more than 50% and 30% compared to the SiGe alloy and to the earlier reported values in (100)-oriented Si/Ge superlattice structures (SLS), respectively. For the thick SL period samples (L>10 nm), λ has shown a tendency to saturate at the SiGe alloy value. This is understood as, with increasing L, the SLS breaks and the SiGe alloying starts to grow. This structural behavior is clearly observed in the cross-plane transmission electron microscope images as well. In addition to these, for the thin SL period (L⩽7 nm) samples, the data have shown a shallow minimum which is attributed to the competing behavior of the wave nature and the classical particle nature of the localized phonons. Nevertheless, the present study...

Transport and structural properties of binary skutterudite CoSb3 thin films grown by dc magnetron sputtering technique

The results of the studies of structural, electrical, and thermoelectric properties of binary skutterudite CoSb3 thin films are presented. The n-type films were prepared without intentional doping on oxidized Si(100) and ceramic Al2O3 substrates using magnetron dc sputtering technique. The electron probe microanalysis has shown that the composition of the films is strongly affected by the preparation conditions. The x-ray measurements indicate the films to be amorphous if they are deposited on substrates kept at room temperature. On the basis of the differential thermal and x-ray analyses data, the temperature of the transition from the amorphous to crystalline state was established to be (153±1)°C. The films are polycrystalline with a preferential orientation of the growth along the (310) direction of the CoSb3 lattice if grown on substrates kept at 200 °C. The electrical resistivity ρ(T) and the Seebeck coefficient S(T) were measured from room temperature up to about 700 K. The value of ρ(T) decreases w...

Full spin switch effect for the superconducting current in a superconductor/ferromagnet thin film heterostructure

Superconductor/ferromagnet (S/F) proximity effect theory predicts that the superconducting critical temperature of the F1/F2/S or F1/S/F2 trilayers for the parallel orientation of the F1 and F2 magnetizations is smaller than for the antiparallel one. This suggests a possibility of a controlled switching between the superconducting and normal states in the S layer. Here, using the spin switch design F1/F2/S theoretically proposed by Oh et al. [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer separated by a non-magnetic metallic spacer layer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have successfully realized a full spin switch effect for the superconducting current.

Manifestation of New Interference Effects in a Superconductor-Ferromagnet Spin Valve

Superconductor-ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1 and F2 layers (T(c)(P)) are smaller than for the antiparallel orientations (T(c)(AP)). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayers with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔT(c) = T(c)(AP) - T(c)(P). We observe the full direct effect with T(c)(AP) > T(c)(P) for Fe2-layer thickness d(Fe2) < 1 nm and the full inverse (T(c)(AP) < T(c((P)) effect for d(Fe2) ≥ 1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2 layer appear as the most probable reason for the observed behavior of ΔT(c).

Effect of Néel coupling on magnetic tunnel junctions

We have studied the effect of the dipolar magnetic coupling (also known as Neel coupling or “orange-peel” coupling) in tunneling magnetoresistive (TMR) elements. With an in situ scanning tunneling microscope we directly accessed the roughness of the films and found a close correspondence between the values for the coupling fields determined by the magneto-optical Kerr effect and the ones computed on the basis of the measured morphology parameters. We confirm an increase of the dipole coupling between the magnetic layers with decreasing barrier thickness as predicted by the model. Deviations from the theoretical predictions are observed for the case of thinner soft magnetic layers, which can be explained by reduced magnetization in very thin films. We demonstrate the importance of dipolar coupling for understanding the magnetic behavior of TMR elements by comparing TMR curves for optimized and nonoptimized structures.

Superconducting spin-valve effect and triplet superconductivity in CoOx /Fe1/Cu/Fe2/Cu/Pb multilayer

We report magnetic and superconducting properties of the modified spin valve system CoOx/Fe1/Cu/Fe2/Cu/Pb. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin valve effect and of the manifestation of the long-range triplet component of the superconducting condensate.

Crystallization behavior of CoSb3 and (Co,Fe)Sb3 thin films

The crystallization behavior of CoSb3 and Fe-doped (Co,Fe)Sb3 thin films prepared by d.c.-magnetron sputtering was studied via ex situ and in situ X-ray diffraction (XRD) measurements. The as-deposited layers are amorphous. Crystallization takes place at temperatures of approximately 160±16 °C. Decomposition occurs in the temperature ranges of 575–610 °C for CoSb3 and of 640–670 °C for (Co,Fe)Sb3. The decomposition products are CoSb and (Co,Fe)Sb, respectively. Both ex situ XRD measurements on annealed layers and in situ measurements on as-deposited samples show a rare dependence of the cubic lattice parameter of the skutterudite type structure on the temperature. Electron probe microanalysis investigations indicate that the composition varies from Co–Sb 1:4 at low temperatures to Co–Sb 1:3 after heating cycles.

Peculiarities of performance of the spin valve for the superconducting current

The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoOx/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔTc = TcAP − TcP, where TcP and TcAP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer dFe1. The obtained dependence of the effect on dFe1 reveals that ΔTc can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at dFe1 ∼ dFe2. At the optimal value of dFe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated.

Boosting the superconducting spin valve effect in a metallic superconductor/ferromagnet heterostructure

Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical for the S/F proximity effect, as regards the suppression of the critical temperature of the S layer. However, it appears to be of paramount importance in the performance of the S/F1/F2 spin valve. As the morphology of the S layer changes from the form of overlapping islands to a smooth case, the magnitude of the conventional superconducting spin valve effect significantly increases. We attribute this dramatic effect to a homogenization of the Green function of the superconducting condensate over the S/F interface in the S/F1/F2 valve with a smooth surface of the S layer.