$0\mathrm{\text{−}}\pi$ Josephson Tunnel Junctions with Ferromagnetic Barrier

We fabricated high quality $\mathrm{Nb}/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{Ni}}_{0.6}{\mathrm{Cu}}_{0.4}/\mathrm{Nb}$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a $0\mathrm{\text{−}}\pi$ junction, with equal lengths and critical currents of 0 and $\pi$ parts. The ground state of our $330\mu \mathrm{m}$ ($1.3{\lambda }_J$) long junction corresponds to a spontaneous vortex of supercurrent pinned at the $0\mathrm{\text{−}}\pi$ step and carrying $\sim 6.7\%$ of the magnetic flux quantum ${\Phi }_0$. The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.

Figure 1

Dependence of critical current $I_c$ for SIFS reference JJs that were not etched (filled circles) and etched uniformly (open stars) on the thickness of the $F$-layer $d_F$. Fit of the experimental data for nonetched samples using Eq. (1) is shown by continuous line.

Figure 2

Picture of the $330\times 30\mu {\mathrm{m}}^2$ JJs (top view). The $0\mathrm{\text{−}}\pi$ boundary or step in the $F$-layer (if any) is indicated by a dashed line.

Figure 3

$I_c(B)$ of 0 JJ (red filled triangles), $\pi$ JJ (blue open triangles), and $0\mathrm{\text{−}}\pi$ JJ (black spheres) measured at (a) $T\approx 4.2\mathrm{K}$ and (b) $T\approx 2.65\mathrm{K}$.

Figure 4

Numerically calculated magnetic field of spontaneous fractional flux in $0\mathrm{\text{−}}\pi$ JJ of $\ell =1.3$. The field of semifluxon in an infinite LJJ is shown for comparison.