Study of the Superconducting Proximity Effect Spin-Valve Phenomenon in Superconductor / Ferromagnet Nanolayered Structures
Final Report Abstract
In this project the unconventional superconductor (S) / ferromagnet (F) proximity effect and its application in a superconducting spin valve (SSV) has been investigated in Nb-Cu41Ni59 layered systems. Although superconductivity and ferromagnetism are not expected to coexist (due to antagonistic long-range orderings), Fulde-Ferrell and Larkin-Ovchinnikov (FFLO) predicted a superconducting state in the presence of a ferromagnetic background, established by Cooper pairs of electrons with opposite spins, but pairing with non-vanishing total momentum, yielding a wave function oscillating in space. A FFLO-like state can be realized in nanolayered S-F proximity effect structures. Due to interference effects of the superconducting pairing wave function, oscillations of the superconducting transition temperature Tc and even reentrant superconductivity as a function of the F layer thickness are predicted to appear in S/F, F/S, and F/S/F systems. For the trilayers Tc depends on the mutual orientation of the magnetizations of the F layers. Thus, the system represents a SSV. To deposit sample series with high-quality smooth S layers of constant thickness and with different F layer thicknesses in the same run by magnetron sputtering, we developed a moving target spray technique and a wedge technique, respectively. Rutherford backscattering spectrometry (RBS) was applied to determine thicknesses and composition of the layers. Cross-sectional high resolution transmission electron microscopy (TEM) shows sharp, clean interfaces between the layers and serves as an independent method to measure the layer thicknesses. Magnetic properties were investigated by superconducting quantum interference device (SQUID) magnetometry and ferromagnetic resonance (FMR). The fabricated bilayer and trilayer series show all types of non-monotonous behavior predicted by the theory (even if trilayers were deposited on an antiferromagnetic (AF) layer of CoOx or with such layer on top), including deep Tc oscillations and, especially, reentrant superconductivity with an expressed extinction region, which is a prerequisite for a large SSV effect. The application of an external magnetic field on F/S/F trilayers shifts the Tc behavior from reentrant to oscillating. The predicted large standard (direct) SSV effect (of order 1K) could so far not be realized, although a large exchange bias of one diluted ferromagnetic alloy layer (necessary to form an antiparallel alignment of the magnetizations by an external magnetic field) could be achieved in Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 systems. Instead memory effects (i.e. the resistance at H = 0 depends on the magnetic history) with a resistance ratio of up to 25 appear, probably generated by s-wave triplet superconductivity, which is generated by noncollinear magnetizations due to an exchange spring in the bottom layer. In S/F/F systems of Nb/Cu41Ni59/nc-Nb/Co/CoOx (where nc-Nb is a normal conducting Nb spacer) the recently predicted triplet SSV effect could be detected for the first time. It was observed at non-collinear orientations of the F layers, which is realized at the coercive field of the Cu41Ni59 layer, at which the magnetization aligns with the easy axis perpendicular to the film, while the magnetization of the exchange biased Co layer is in plane.
Publications
- Reentrant Superconductivity in Superconductor/Ferromagnetic-Alloy Bilayers, Phys. Rev. B 82, 054517 (2010)
V. I. Zdravkov, J. Kehrle, G. Obermeier, S. Gsell, M. Schreck, C. Müller, H.-A. Krug von Nidda, J. Lindner, J. Moosburger-Will, E. Nold, R. Morari, V. V. Ryazanov, A. S. Sidorenko, S. Horn, R. Tidecks, and L. R. Tagirov
- Interference Effects of the Pairing Wave Function Due to the Fulde-Ferrell Larkin-Ovchinnikov Like State in Ferromagnet/Superconductor Bilayers, Supercond. Sci. Technol. 24, 095004 (2011)
V. I. Zdravkov, J. Kehrle, G. Obermeier, A. Ulrich, S.Gsell, M. Schreck, C. Müller, R. Morari, A. S. Sidorenko, V. V. Ryazanov, L. R. Tagirov, R. Tidecks, and S. Horn
- Critical Temperature Oscillations and Reentrant Superconductivity Due to the FFLO Like State in F/S/F Trilayers, Ann. Phys. (Berlin) 524, 37 (2012)
J. Kehrle, V. I. Zdravkov, G. Obermeier, J. Garcia-Garcia, A. Ullrich, C. Müller, R. Morari, A. S. Sidorenko, S. Horn, L. R. Tagirov, and R. Tidecks
(See online at https://doi.org/10.1002/andp.201100133) - Experimental Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure, Phys. Rev. B 87, 144507 (2013)
V.I. Zdravkov, J. Kehrle, G. Obermeier, D. Lenk, H.-A. Krug von Nidda, C. Müller, M. Yu. Kupriyanov, A. S. Sidorenko, S. Horn, R. Tidecks, and L. R. Tagirov
(See online at https://doi.org/10.1103/PhysRevB.87.144507) - Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F Trilayers of Cu41Ni59/Nb/ Cu41Ni59 Grown on Cobalt Oxide. J. Appl. Phys. 114, 033903 (2013)
V. I. Zdravkov, J. Kehrle, D. Lenk, G. Obermeier, A. Ullrich, C. Müller, H.-A. Krug von Nidda, R. Morari, A. S. Sidorenko, L. R. Tagirov, S. Horn and R. Tidecks
(See online at https://doi.org/10.1063/1.4813131)
