Аннотация:The theory of superconductor-ferromagnet (S-F) heterostructures with two and more ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment of magnetizations of the F-layers ([1] and references therein). As a consequence of the unconventional triplet pairing the triplet spin-valve e ect has been predicted in [2]. To observe this e ect experimentally we realized Nb/Cu41Ni59/Nb/Co/CoOx spin-valve type proximity-e ect heterostructure [3]. A weak ferromagnet-Cu41Ni59 alloy with an equilibrium magnetic moment perpendicular to the lm plane was used as a propagator layer adjacent to the bottom niobium layer -a conventional S-wave superconductor.
A second very thin Nb lm between the Cu41Ni59 and metallic cobalt layers served as a normal conducting spacer. An antiferromagnetic cobalt oxide layer exchange biased the in-plane magnetization of the underlying cobalt layer, which plays a role of mixer of the triplet and singlet pairing channels. Our FMR and SQUID measurements con rmed that the Cu41Ni59 layer has easy magnetization axis perpendicular to the lm plane. The magnetoresistance measurements at temperatures close to the superconducting (SC) transition temperature Tc, and magnetic eld applied in the in-plane direction, have shown a transition from the normal to the SC state when decreasing the magnetic eld. Then, upon changing the eld polarity and passing the coercive eld of the Cu41Ni59 layer (i.e. near the crossed magnetization con guration of the Cu41Ni59and Co layers) the system returned back to the resistive state
followed with subsequent SC transition when the system is driven towards the antiparallel alignment of the Cu41Ni59 and Co layers magnetic moments. We refer this unusual magnetoresistance behavior to the indication of the triplet pairing generation at the non-collinear alignment of magnetizations in the Nb/Cu41Ni59/Nb/Co/CoOxheterostructure. The superconducting Tc also shows pronounced minimum close to the crossed magnetic con gurations giving further support towards triplet spin-valve interpretation of the observed phenomena.
The support by DFG, grant No. GZ: HO 955/6-2, and RFBR, grants No. 11-02-00848-a (L.R.T.) and 11-02-12065-o m (M.Yu.K.), is gratefully acknowledged.
References
[1] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Rev. Mod. Phys. 77, 1321 (2005). [2] Ya. V. Fominov, et al., JETP Lett. 91, 308 (2010). [3] V. I. Zdravkov, et al. (submitted to PRL).
