Intrinsically frustrated superconducting array of superconductor-ferromagnet-superconductor pi junctions

We report a direct observation of the transition of the superconductor-ferromagnet-superconductor (SFS) junctions to the pi state. This manifests itself in the half-period shift of the external magnetic field dependence of the transport critical current in the triangular SFS arrays. The shift is associated with the appearance of spontaneous supercurrents in the array at zero external field, with the ground state degenerated with respect to the two possible current flow directions. In conventional Josephson arrays this state can be observed only by imposing a frustrating external field equal to a half-integer number of magnetic flux quanta per array cell.