Observation of periodic $\pi$-phase shifts in ferromagnet-superconductor multilayers

We report complementary studies of the critical temperature and the critical current in ferromagnet (Ni) - superconductor (Nb) multilayers. The observed oscillatory behavior of both quantities upon variation of the thickness of the ferromagnetic layer is found to be in good agreement with theory. The length scale of oscillations is identical for both quantities and is set by the magnetic length corresponding to an exchange field of $200\mathrm{meV}$ in Ni. The consistency between the behavior of the two quantities provides strong evidence for periodic $\pi$ phase shifts in these devices.

Figure 1

(Color online) Schematic layout: (a) Of the junctions for $I_c$ experiments, (b) of planar samples for $T_c$ experiments.

Figure 2

(Color online) (a) The critical Josephson current $I_c$ as function of the Ni-layer thickness $d_{\mathsf{Ni}}$. The solid line shows the theoretical curve with $E_{ex}=200\mathrm{meV}$ used as a fit parameter. The dots are the experimental data for all 3 sets. (b) Oscillating part of $I_c$ for set 2.

Figure 3

(Color online) Critical temperature $T_c$ as a function of Ni-layer thickness $x_{\mathsf{Ni}}$ for the trilayer and pentalayer structures. The experimental data are shown as symbols, while the curves are fits to the data with the theory discussed in the text. For the pentalayer, the highest $T_c$ is obtained for a zero junction (full line) for $x_{\mathsf{Ni}}<16\mathrm{\AA }$, and for a $\pi$-junction (dashed line) in the range $16\mathrm{\AA }<x_{\mathsf{Ni}}<40\mathrm{\AA }$.

Figure 4

(Color online) Superconducting transitions for several pentalayer samples. $T_c$ is determined as a temperature at which resistance attains $\frac{1}{2}$ of its normal state value.