N. N. Garif’yanov

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.

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.

Erratum to: Possible reconstruction of the ferromagnetic state under the influence of superconductivity in epitaxial V/Pd1−xFex bilayers

Epitaxial V/Pd1−xFex (001) bilayers with a V thickness of the order of 40 nm and with a Pd1−xFex thickness in the range from 0.8 nm to 4.4 nm were prepared by molecular beam epitaxy techniques. The Curie temperature of the Pd1−xFex layers varies between 90 and 250 K. For a bilayer with a Pd1−xFex thickness of 1.2 nm the ferromagnetic resonance measurements revealed a decrease of the effective magnetization 4πMeff of the ferromagnetic layer below the superconducting transition temperature of V. As a possible explanation for this decrease we suggest a spatial modulation of the ferromagnetic order in the Pd1−xFex layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.

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.

Superconducting and magnetic properties of epitaxial high-quality Fe/Nb bilayers

Single crystal Fe/Nb (110) bilayers with an Nb thickness dNb in the range from 140 to 650 A and with an Fe thickness dFe in the range from 5 to 100 A were prepared using molecular beam epitaxy (MBE) techniques. For dFe⩾20 A a decrease of the superconducting transition temperature Tc with decreasing dNb was observed. For dFe=20 A the FMR data revealed a decrease of the effective magnetization of the Fe layer below the superconducting transition. This magnetization behavior is attributed to a spatial modulation of ferromagnetic order due to a modification of the RKKY interaction in the superconducting state.

Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures

When studying the nuclear magnetic resonance (NMR) of 51V nuclei in superconductor/ferromagnet thin film heterostructures, anomalous behavior of the NMR signal was observed. This effect was manifested as a systematic distortion of the high-field wing of the resonance line. We consider this to be the first experimental evidence for penetration of ferromagnetism into a superconductor.

Knight shift in superconducting vanadium

The Knight shift in metallic vanadium in the normal and superconducting states has been measured. In contrast to the previously obtained results, this shift appears to change after the transition to the superconducting state. The behavior of the Knight shift in the superconducting state in vanadium samples doped with iron impurities has been found to be different from that in the “pure” samples. As a possible explanation of the effect, the broadening of the peak of the density of states near the Fermi level due to the scattering of conduction electrons on the iron impurities and the earlier predicted impurity polarization shift of the NMR line are discussed.

Structure of interfaces in thin-film metallic multilayer heterostructures

It is demonstrated that the structure of interfaces in thin-film metallic multilayers can be predicted from the phase diagrams of binary alloys of metals constituting the multilayers. Experimental data on the small-angle X-ray reflectivity and magnetization of the samples are used for characterizing the structure of the interfaces.

Direct observation of ferromagnetic order in electrochemically oxygenated La2Cuo4+δ

The magnetic properties of electrochemically oxygenated La2Cuo4+δ are investigated for different values of δ. It is found that ferromagnetism appears after excess oxygen is introduced. It is shown that the ferromagnetic order vector is directed perpendicular to the Cu-O plane. The evolution of the magnetic-field dependence of the magnetization is investigated as the oxygen index is increased. The results are explained with allowance for the presence of phase separation in the oxygen-doped La2Cuo4 and the possibility that a redistribution of the excess oxygen occurs as the degree of doping changes.

Experimental Investigation of the Role of the Triplet Pairing in the Superconducting Spin-Valve Effect

An important role of the morphology of a superconducting layer in the superconducting spin-valve effect has been established. The triplet pairing induced by the superconductor/ferromagnet proximity effect has been experimentally investigated for samples CoOx/Py1/Cu/Py2/Cu/Pb (where Py = Ni0.81Fe0.19) with a smooth superconducting layer. The optimization of the parameters of this structure has demonstrated a complete switching between the normal and superconducting states with a change in the relative orientation of magnetizations of the ferromagnetic layers from the antiparallel to orthogonal orientation. A pure triplet contribution has been observed for the sample with a permalloy layer thickness at which the superconducting spin-valve effect vanishes. A direct comparison of the experimental data with the theoretical calculation of the temperature of the transition to the superconducting state has been performed for the first time.