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Magnetic and Superconducting Phase Diagram of Nb/Gd/Nb trilayers

Research paper by Yu. N. Khaydukov, A. S. Vasenko, E. A. Kravtsov, V. V. Progliado, V. D. Zhaketov, A. Csik, Yu. V. Nikitenko, A. V. Petrenko, T. Keller, A. A. Golubov, M. Yu. Kupriyanov, V. V. Ustinov, V. L. Aksenov, B. Keimer

Indexed on: 15 Jan '18Published on: 15 Jan '18Published in: arXiv - Physics - Superconductivity



Abstract

We report on a study of the structural, magnetic and superconducting properties of Nb(25nm)/Gd($d_f$)/Nb(25nm) hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and X-ray scattering with the aid of depth sensitive mass-spectrometry. The magnetization of the samples was determined by SQUID magnetometry and polarized neutron reflectometry and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature $T_c(d_f)$ has a damped oscillatory behavior with well defined positions of the minimum at $d_f$=3nm and the following maximum at $d_f$=4nm. Similar behavior, albeit with a 1.5nm off-set in $d_f$, was already observed by Jiang et al. (PRB 54, 6119) in analogous Nb/Gd/Nb trilayers. The analysis of the $T_c(d_f)$ dependence based on Usadel equations allowed us to describe our experimental data. Our structures are characterized by highly transparent S/F interfaces and the superconducting correlation length $\xi_f \approx 4$nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co or Ni. This simplifies preparation of S/F structures with $d_f \sim \xi_f$ which are of topical interest in superconducting spintronics.