Superconductivity in layered Nb/Gd films

TLDR

The transition temperature and critical fields of electron‑beam evaporated Nb/Gd/Nb triple layers and Nb/Gd multilayers were determined by electrical resistivity and dc susceptibility measurements, and the ferromagnetic transition of the Gd films was clarified using transverse magneto‑optical Kerr effect to understand the step in Tc versus Gd thickness. Measurements reveal that decreasing Nb layer thickness lowers Tc and Hc2⊥ to a critical thickness where superconductivity vanishes, while Hc∥ shows a nonmonotonic dependence on Nb thickness; Tc also decreases with increasing Gd thickness, exhibiting a steplike drop near 20 Å coinciding with the onset of long‑range magnetic order in Gd films, indicating a change in the underlying pair‑breaking mechanism.

Abstract

The transition temperature and the critical fields of electron-beam evaporated Nb/Gd/Nb triple layers and Nb/Gd multilayers have been determined by measurements of the electrical resistivity and the dc susceptibility. For constant thickness ${\mathit{d}}_{\mathrm{Gd}}$ of the Gd layers, we observe a decrease of ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{H}}_{\mathit{c}2\mathrm{\ensuremath{\perp}}}$ with decreasing thickness ${\mathit{d}}_{\mathrm{Nb}}$ of Nb layers down to a critical thickness ${\mathit{d}}_{\mathit{c}}$, below which superconductivity is completely destroyed. The parallel critical fields mostly show the square-root temperature dependence near ${\mathit{T}}_{\mathit{c}}$, typical for two-dimensional superconductors. As predicted theoretically, competing pair-breaking mechanisms lead to a nonmonotonic dependence of ${\mathit{H}}_{\mathit{c}\mathrm{\ensuremath{\parallel}}}$ on ${\mathit{d}}_{\mathrm{Nb}}$. We have also studied the dependence ${\mathit{T}}_{\mathit{c}}$(${\mathit{d}}_{\mathrm{Gd}}$) with constant ${\mathit{d}}_{\mathrm{Nb}}$ and find a decrease of the ${\mathit{T}}_{\mathit{c}}$(${\mathit{d}}_{\mathrm{Gd}}$) curve with increasing ${\mathit{d}}_{\mathrm{Gd}}$ and a steplike structure at ${\mathit{d}}_{\mathrm{Gd}}$\ensuremath{\approxeq}20 \AA{}. To clarify the nature of this step, the ferromagnetic transition of the Gd films is determined with the transverse magneto-optical Kerr effect. Long-range magnetic order is found only above ${\mathit{d}}_{\mathrm{Gd}}$\ensuremath{\approxeq}20 \AA{}, which is attributed to the formation of a discontinuous film below this thickness. These results indicate a change in the underlying pair-breaking mechanism.