Coexistence of Ferromagnetism and Superconductivity in $\mathrm{N}\mathrm{i}/\mathrm{B}\mathrm{i}$ Bilayers

In spite of a lack of superconductivity in bulk crystalline Bi, thin film Bi deposited on thin Ni underlayers are strong-coupled superconductors below $\sim 4\mathrm{K}$. We unambiguously demonstrate that by tuning the Ni thickness the competition between ferromagnetism and superconductivity in the $\mathrm{N}\mathrm{i}/\mathrm{B}\mathrm{i}$ can be tailored. For a narrow range of Ni thicknesses, the coexistence of both a superconducting energy gap and conduction electron spin polarization are visible within the Ni side of the $\mathrm{N}\mathrm{i}/\mathrm{B}\mathrm{i}$ bilayers, independent of any particular theoretical model. We believe that this represents one of the clearest observations of superconductivity and ferromagnetism coexisting.

Figure 1

Conductance ($dI/dV$) versus voltage for a $\mathrm{A}\mathrm{l}4.2\mathrm{n}\mathrm{m}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}{d}_{\mathrm{N}\mathrm{i}}\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ junction at $T=1\mathrm{K}$ for $d_{\mathrm{N}\mathrm{i}}=1.6,2.4,3.2\mathrm{n}\mathrm{m}$. The appearance of both sum and difference gap features clearly indicates the presence of superconductivity in the Ni layer. Inset: Resistance versus temperature for several Ni thicknesses ($d_{\mathrm{B}\mathrm{i}}=40\mathrm{n}\mathrm{m}$), showing a clear superconducting transition in all cases.

Figure 2

Magnetization versus field for $\mathrm{N}\mathrm{i}{d}_{\mathrm{N}\mathrm{i}}\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ samples at 10 (open circles) and 2 K (solid circles). Clear remanance and hysteresis are observed, indicating ferromagnetic behavior. Left: $d_{\mathrm{N}\mathrm{i}}=3.6\mathrm{n}\mathrm{m}$. Right: $d_{\mathrm{N}\mathrm{i}}=4.2\mathrm{n}\mathrm{m}$.

Figure 3

Left: Spin-polarized tunneling curves in $H=0$ (solid circles) and 2.92 T (open circles) for an $\mathrm{A}\mathrm{l}4.2\mathrm{n}\mathrm{m}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}3.6\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ junction at $T=0.5\mathrm{K}$, clearly showing spin polarization in the Ni layer ($\mathrm{P}=+11.6\%$ in this case). Right: $dI/dV-V$ versus voltage for a $\mathrm{C}\mathrm{o}5\mathrm{n}\mathrm{m}/\mathrm{A}\mathrm{l}4.2\mathrm{n}\mathrm{m}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}{d}_{\mathrm{N}\mathrm{i}}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ junction at $T=1\mathrm{K}$ for $d_{\mathrm{N}\mathrm{i}}=3.6,4.2,5.4\mathrm{n}\mathrm{m}$. The curve for $d_{\mathrm{N}\mathrm{i}}=3.6\mathrm{n}\mathrm{m}$ (solid line) clearly shows the presence of an energy gap, as do the curves for $d_{\mathrm{N}\mathrm{i}}=4.2,5.4\mathrm{n}\mathrm{m}$ (open, closed circles, shifted vertically).

Figure 4

(a) Measured P as a function of Ni thickness for $d_{\mathrm{N}\mathrm{i}}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ bilayers. (b) $dI/V$ at $H=0$ for $\mathrm{C}\mathrm{o}/\mathrm{A}\mathrm{l}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}5.4\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ and $\mathrm{A}\mathrm{l}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}5.4\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ junctions. The former sample shows a superconducting energy gap on the Ni surface (expanded vertically), whereas the latter sample shows S-I-S tunneling (Al is superconducting), both indicating superconductivity on the Ni face. (c) Tunneling magnetoresistance in a $\mathrm{C}\mathrm{o}/\mathrm{A}\mathrm{l}/{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{N}\mathrm{i}3.0\mathrm{n}\mathrm{m}/\mathrm{B}\mathrm{i}40\mathrm{n}\mathrm{m}$ junction at 0.5 K.