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

TL;DR

This study investigates the structural, magnetic, and superconducting properties of Nb/Gd/Nb trilayers, revealing oscillatory $T_c$ behavior linked to phase transitions, with implications for superconducting spintronics.

Contribution

It provides the first detailed analysis of the magnetic and superconducting phase diagram of Nb/Gd/Nb trilayers, including the observation of a $0$ to $\pi$ phase transition in highly transparent S/F interfaces.

Findings

Superconducting transition temperature $T_c$ exhibits damped oscillations with Gd thickness.

No magnetic dead layers found for Gd thickness > 1 monolayer.

Penetration length in Gd is approximately 4nm, larger than in other ferromagnets.

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; the behavior, which is in qualitative agreement with the prior work (J.S. Jiang et al, PRB 54, 6119). The analysis of the $T_c(d_f)$ dependence based on Usadel equations showed that the observed minimum at $d_f$=3nm can be described by the so called "$0$" to "$\pi$" phase transition of highly transparent S/F interfaces with 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, simplifying thus preparation of S/F structures with $d_f \sim \xi_f$ which are of topical interest in superconducting spintronics.