Thickness dependence of the triplet spin-valve effect in superconductor-ferromagnet-ferromagnet heterostructures

Daniel Lenk; Vladimir I Zdravkov; Jan-Michael Kehrle; Günter Obermeier; Aladin Ullrich; Roman Morari; Hans-Albrecht Krug von Nidda; Claus Müller; Mikhail Yu Kupriyanov; Anatolie S Sidorenko; Siegfried Horn; Rafael G Deminov; Lenar R Tagirov; Reinhard Tidecks

doi:10.3762/bjnano.7.88

Thickness dependence of the triplet spin-valve effect in superconductor-ferromagnet-ferromagnet heterostructures

Beilstein J Nanotechnol. 2016 Jul 4:7:957-69. doi: 10.3762/bjnano.7.88. eCollection 2016.

Authors

Affiliations

¹ Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany.
² Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany; D. Ghitsu Institute of Electronic Engineering and Nanotechnologies ASM, Academiei Str. 3/3, MD2028 Kishinev, Moldova.
³ Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany; D. Ghitsu Institute of Electronic Engineering and Nanotechnologies ASM, Academiei Str. 3/3, MD2028 Kishinev, Moldova; Solid State Physics Department, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russian Federation.
⁴ Solid State Physics Department, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russian Federation; Skobeltsyn Institute of Nuclear Physics, Moscow State University, Leninskie gory, GSP-1, Moscow 119992, Russia.
⁵ D. Ghitsu Institute of Electronic Engineering and Nanotechnologies ASM, Academiei Str. 3/3, MD2028 Kishinev, Moldova.
⁶ Solid State Physics Department, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russian Federation.
⁷ Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany; Solid State Physics Department, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russian Federation.

Abstract

Background: In nanoscale layered S/F1/N/F2/AF heterostructures, the generation of a long-range, odd-in-frequency spin-projection one triplet component of superconductivity, arising at non-collinear alignment of the magnetizations of F1 and F2, exhausts the singlet state. This yields the possibility of a global minimum of the superconducting transition temperature T c, i.e., a superconducting triplet spin-valve effect, around mutually perpendicular alignment.

Results: The superconducting triplet spin valve is realized with S = Nb a singlet superconductor, F1 = Cu41Ni59 and F2 = Co ferromagnetic metals, AF = CoO x an antiferromagnetic oxide, and N = nc-Nb a normal conducting (nc) non-magnetic metal, which serves to decouple F1 and F2. The non-collinear alignment of the magnetizations is obtained by applying an external magnetic field parallel to the layers of the heterostructure and exploiting the intrinsic perpendicular easy-axis of the magnetization of the Cu41Ni59 thin film in conjunction with the exchange bias between CoO x and Co. The magnetic configurations are confirmed by superconducting quantum interference device (SQUID) magnetic moment measurements. The triplet spin-valve effect has been investigated for different layer thicknesses, d F1, of F1 and was found to decay with increasing d F1. The data is described by an empirical model and, moreover, by calculations using the microscopic theory.

Conclusion: The long-range triplet component of superconducting pairing is generated from the singlet component mainly at the N/F2 interface, where the amplitude of the singlet component is suppressed exponentially with increasing distance d F1. The decay length of the empirical model is found to be comparable to twice the electron mean free path of F1 and, thus, to the decay length of the singlet component in F1. Moreover, the obtained data is in qualitative agreement with the microscopic theory, which, however, predicts a (not investigated) breakdown of the triplet spin-valve effect for d F1 smaller than 0.3 to 0.4 times the magnetic coherence length, ξF1.

Keywords: heterostructures; superconducting spin valve; thin films; triplet superconductivity.