Observation of fractional vortices and π phases in Josephson junctions involving periodic magnetic layers

Characteristics of $\mathrm{SN}{(\mathrm{F}/\mathrm{N})}_{n}\mathrm{IN}{(\mathrm{F}/\mathrm{N})}_{m}\mathrm{S}$ Josephson junctions with $n, m=1$ to 3 are reported. Here, S is a superconductor (Nb), F is a magnetic material (nickel), nitrogen is a nonmagnetic metal (aluminum), and I is an insulator $(\mathrm{Al}/\mathrm{Al}{\mathrm{O}}_{\mathrm{x}})$. The devices with $n=m=1$ and $n=1, m=2$ display critical current versus magnetic field $[{I}_{\mathrm{c}}(H)]$ dependences that imply the appearance of fractional magnetic vortices in the junction, corresponding to a half of the flux quantum. Stochastic switching between the two half-vortex polarities with emission of an integer vortex was observed. In the devices with $n=m=3$, a typical ${I}_{\mathrm{c}}(H)$ dependence consists of a background current with a large modulation period, and a Fraunhofer-like pattern with a small modulation period on top of the background current. The Fraunhofer-like pattern may be completely or partially inverted; in these states, there is a component of the Josephson current flowing against the bias current, indicating the presence of the $\ensuremath{\pi}$ phase in the junction. The experiments give evidence that switching between the zero and $\ensuremath{\pi}$ states can, potentially, be magnetically controlled.