Superconducting state of very thin Pd films deposited on a diluted insulating Eu${}_x$Sr${}_{1-x}$S ferromagnet

The electronic transport in very thin Pd films deposited on insulating Eu${}_x$Sr${}_{1-x}$S ($0⩽x⩽1$) is investigated. The temperature dependent resistance $R(T)$ of films with $d_{\mathrm{Pd}}⩾14$ nm shows metallic behavior and a logarithmic increase toward low temperatures characteristic of (anti-)localization and electron-electron interaction effects also observed for Pd films on Si(111). Films with $d_{\mathrm{Pd}}=7$ nm are superconducting below a transition temperature $T_c$ which decreases with increasing Eu concentration from $T_c=0.9$ K for $x=0$ to $T_c<50$ mK for $x=1$. The origin of superconductivity in the thin Pd films is presumably due to the formation of an interfacial Pd-S alloy and/or to a charge transfer at the Pd/Eu${}_x$Sr${}_{1-x}$S interface. The decrease of $T_c$ vs $x$ is attributed to the magnetic pair breaking of the ferromagnetic Eu${}_x$Sr${}_{1-x}$S underneath. The $R(T)$ behavior of films with $d_{\mathrm{Pd}}⩾10$ nm suggests that these are not superconducting or have a strongly reduced $T_c$ presumably due to the proximity effect of the nonsuperconducting S-free upper part of the Pd film.

Figure 1

STM image of 7-nm Pd on 50-nm SrS obtained in constant-current mode. Image size 250 nm $\times$ 250 nm, bias voltage 2 V, tunneling current 80 pA.

Figure 2

X-ray $\theta -2\theta$ scan (${\mathrm{\lambda ̄}}_{K\alpha }=1.5418$ Å) of 50 nm Eu${}_{0.55}$Sr${}_{0.45}$S on Si(111). Inset shows the lattice constant $a$ determined from the (111) lattice plane distance $d_{111}$ by $a=\sqrt{3}\hspace{0.5em}{d}_{111}$ vs concentration $x$ (solid symbols). Open circles and triangles indicate data from Refs. 14 and 15, respectively, dashed line serves as guide to the eye.

Figure 3

X-ray scan (${\lambda }_{K\alpha 1}=1.54056$ Å) of Pd films of different thickness $d_{\mathrm{Pd}}$ on SrS(111). The Si substrate was rotated 4${}^{ˆ}$ off from the Si(111) Bragg reflection to reduce the background intensity arising from the strong Si(111) peak. Dashed lines indicate the positions of (111) and (200) Bragg peaks for bulk Pd.

Figure 4

(a) Auger electron spectrum of 7-nm Pd on SrS. (b) Auger depth profile obtained by sputtering the sample with 1 keV Ar${}^{+}$ ions while recording the Auger spectra (a).

Figure 5

XPS spectra of Pd films of different thickness $d_{\mathrm{Pd}}$ on SrS. Solid lines show a fit to the data for the 5-nm film and the fitted background. The bottom part shows the individual contributions of various oxides and sulfides to the total spectrum derived from the XPS database.[18] Arrow indicates a Pd-S contribution of unknown composition which is not found in the database.

Figure 6

(a) Magnetic phase diagram of Eu${}_x$Sr${}_{1-x}$S. PM: paramagnet; FM: ferromagnet; SG: spin glass. Open circles for Eu${}_x$Sr${}_{1-x}$S bulk samples from Ref. 21, open squares for EuS/SrS multilayers from Ref. 23, closed circles from this work. (b) Magnetic hysteresis $M(B)$ of 50-nm EuS on Si(111) at $T=2.5$ K. (c) Reciprocal magnetic susceptibility ${\chi }^{-1}(T)$ of 50-nm Eu${}_x$Sr${}_{1-x}$S for different concentrations $x$. $T_{\mathrm{C}}$ indicates the Curie temperature determined from the extrapolation of the linear $T$ dependence to ${\chi }^{-1}=0$ (dashed lines). (d) Imaginary part ${\chi }^{″}(T)$ of the ac susceptibility. $T_f$ indicates the spin-glass freezing temperature determined from the maximum of ${\chi }^{″}(T)$.

Figure 7

(a) Sheet resistance $R_{\square }$ vs ln $T$ of 7-nm Pd on Si(111). Dashed line shows a behavior $R_{\square }\propto -\ln \hspace{0.5em}T$. Inset shows a linear $R(T)$ plot for higher temperatures. (b) Semilogarithmic plot of the magnetoresistance of 7-nm Pd on Si(111) in parallel magnetic field for different temperatures $T$. Dashed line shows a behavior $R_{\square }\propto \ln \hspace{0.5em}B$.

Figure 8

Scaled resistance $R/R_0(T=8\hspace{0.5em}\mathrm{K})$ vs $T$ for Pd films of different thickness $d_{\mathrm{Pd}}$ on SrS. $R_0=692.85$ $\Omega$ (7 nm), 231.11 $\Omega$ (14 nm), and 42.52 $\Omega$ (30 nm).

Figure 9

(a) Sheet resistance $R_{\square }(T)$ for 7-nm Pd on SrS in perpendicular magnetic fields. Inset shows the field dependence of the temperature $T^{*}$ where the maximum occurs in $R_{\square }(T)$. (b) $R_{\square }(T)$ for 7-nm Pd on Eu${}_{0.55}$Sr${}_{0.45}$S in perpendicular magnetic fields.

Figure 10

Resistive superconductive transitions $R_{\square }(T)$ in (a) parallel and (b) perpendicular magnetic fields for 7-nm Pd on SrS. The superconductive transition temperature $T_c$ is indicated by arrows.

Figure 11

Resistive superconductive transitions $R_{\square }(T)$ in (a) parallel and (b) perpendicular magnetic fields for 7-nm Pd on Eu${}_{0.55}$Sr${}_{0.45}$S. The superconductive transition temperature $T_c$ is indicated by arrows.

Figure 12

Parallel and perpendicular upper critical magnetic fields $B_{c2}(T)$ of 7 nm Pd on (a) SrS and (b) Eu${}_{0.55}$Sr${}_{0.45}$S. Dashed lines indicate a linear and square-root behavior for the perpendicular and parallel field orientation, respectively. Inset (a) shows the behavior of $B_{c2\perp }(T)$ for films prepared at substrate temperatures $T_S=330$ K and $T_S=77$ K.

Figure 13

(a) $T_c(x)$ of 7-nm Pd on Eu${}_x$Sr${}_{1-x}$S. Dashed line serves as guide to the eye. For $x=1$, no superconducting transition was found down to 50 mK. (b) Scaled transition temperature $t=T_c/T_{c0}$ vs scaled Curie temperature. Solid line shows the universal functional behavior obtained from the Abrikosov-Gor’kov theory[38] for a pair-breaking parameter ${\alpha }_{\mathrm{AG}}$ taken as the reduced Curie temperature $T_{\mathrm{C}}(x)/T_{\mathrm{C}}(x=1)$. Dash-dotted curve represents $t({\alpha }_{\mathrm{TSR}})$ indicating pair breaking by the proximity of superconducting Pd to an insulating ferromagnet.[39] Inset shows the spin-mixing angle $\theta$ vs $T_{\mathrm{C}}/T_{\mathrm{C}0}$, see text for details. Dashed line shows a linear behavior that would arise from $t({\alpha }_{\mathrm{TSR}})$ plotted in the main frame (dash-dotted curve).

Figure 14

Source-drain resistance $R_{\mathrm{SD}}(T)$ of 7-nm Pd on Eu${}_{0.55}$Sr${}_{0.45}$S for various gate voltages $U_{\mathrm{G}}$. Inset shows the scaled $R_{\mathrm{SD}}(U_{\mathrm{G}})/R_{\mathrm{SD}}(U_{\mathrm{G}}=-10$ V) in the superconducting state. Dashed line is a guide to the eye.