Superconductivity in layered Nb/Gd films

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 ${\mathit{H}}_{\mathit{c}2\mathrm{\perp }}$ 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 ${\mathit{H}}_{\mathit{c}\mathrm{\parallel }}$ 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 ${\mathit{d}}_{\mathrm{Gd}}$≊20 Å. 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 ${\mathit{d}}_{\mathrm{Gd}}$≊20 Å, which is attributed to the formation of a discontinuous film below this thickness. These results indicate a change in the underlying pair-breaking mechanism.