SHANGHAI-2008

RE-ENTRANT SUPERCONDUCTIVITY IN SUPERCONDUCTOR/FERROMAGNET BILAYERS – A STEP TO SUPERCONDUCTING SPIN-VALVES

L.R. Tagirov¹, V. Zdravkov², R. Morari², J. Kehrle³, G. Obermeier³, S. Gsell³, M. Schreck³, C. Müller³, V.V. Ryazanov⁴, S. Horn³, A. Sidorenko², R. Tidecks³

¹Solid State Physics Department, Kazan State University, Kazan, 420008, Russia

²Institute of Electronic Engineering and Industrial Technologies ASM, Kishinev, MD2028, Moldova

³Institut für Physik, Universität Augsburg, Augsburg, D-86159, Germany

⁴Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, 142432, Russia

In superconductor-ferromagnetic metal (S/F) contacts the superconducting condensate penetrates through the S/F interface into an F-layer, and the pairing wave function not only decays into the F metal, but simultaneously oscillates. Based on this oscillating pairing function a variety of new physical effects were predicted (see reviews [1] and references therein). Some of them were observed experimentally: a non-monotonous superconducting T_c behavior as a function of the F -layer thickness, Josephson junctions with an intrinsic π shift across the junction, and inverted differential I-V characteristics. In this work we report on the first experimental observation of re-entrant and doubly suppressed superconductivity in Nb/Cu_1-xNi_x bilayers as a function of the ferromagnetic layer thickness d_CuNi (see figure). The superconducting T_c drops sharply with increasing d_CuNi till superconductivity is suppressed at d_CuNi » 2.5-4 nm. For further increase of the Cu_1-xNi_x layer thickness, the superconducting state restores at d_CuNi ≈ 14 nm for the re-entrant behavior (d_Nb ≈ 7.3 nm) [2], and d_CuNi ≈ 25 nm for the doubly suppressed re-entrant behavior (d_Nb ≈ 6.2 nm) [3]. Then, with a subsequent increase of d_CuNi, superconductivity either saturates (d_Nb ≈ 7.3 nm) [2] or vanishes again at d_CuNi » 38 nm (d_Nb ≈ 6.2 nm) [3]. To fabricate these samples, an improved thin-film deposition technique has been applied [2]. The experiments represent an important progress towards possible fabrication of the superconducting spin-valve proposed by theory [4].

　

　

　

　

　

　

　

　

　

　

　

　

　

The work was partially supported by BMBF (project MDA01/007), RFBR (project No 07-02-00963) and RFBR-Moldova (project “Cercetarea supraconductibilităţii reversibile în nanostructuri Supraconductor-Feromagnet – element de bază a spintronicii cu supraconductori”).

[1] A.I. Buzdin , Rev. Mod. Phys. 77, 935 (2005).

[2] V.I. Zdravkov, A.S. Sidorenko, G. Obermeier, S. Gsell, M. Schreck, C. Müller, S. Horn, R. Tidecks, L.R. Tagirov, Phys. Rev. Lett. 97, 057004 (2006).

[3] A.S. Sidorenko, V.I. Zdravkov, J. Kehrle, G. Obermeier, S. Gsell, M. Schreck, C. Müller, V.V. Ryazanov, S. Horn, R. Tidecks, L.R. Tagirov, Nature Physics (submitted).

[4] L.R. Tagirov, Phys. Rev. Lett. 83, 2058 (1999).