Critical Current Oscillations in Strong Ferromagnetic $\pi$ Junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni, and ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py). All these materials show multiple oscillations of critical current with a barrier thickness implying repeated 0-$\pi$ phase transitions in the superconducting order parameter. We show, in particular, that the Co barrier devices can be accurately modeled using existing clean limit theories and that, despite the high exchange energy (309 meV), the large $I_c{R}_N$ value in the $\pi$ state means Co barriers are ideally suited to the practical development of superconducting $\pi$-shift devices.

Figure 1

Magnetic moment per unit area vs Co, Py, and Ni thickness at 300 K. Inset: hysteresis loops for Co and Ni.

Figure 2

$I_c{R}_N$ as a function of Co, Py, and Ni thickness at 4.2 K. Inset (a) an FIB micrograph of a typical Nb-Co-Nb Josephson junction. Inset (b) $V(I)$ and $dV/dI(I)$ plotted for a Nb/Py/Nb Josephson junction. Inset (c) $M(T)$ as a function of warming and cooling temperature for a Nb/Ni/Nb trilayer where $d_{\mathrm{Ni}}\simeq 9\mathrm{nm}$.

Figure 3

The dependence of ${\xi }_2/{\xi }_1$ with inverse magnetic length, $L/{\xi }_H$, calculated for different ratios of $L/{\xi }_0$. Inset: inverse decay length, $L/{\xi }_2=f(L/{\xi }_0)$, for when $L/{\xi }_H\simeq 0.1$.