Controllable 0-π Josephson junctions containing a ferromagnetic spin valve

Superconductivity and ferromagnetism are antagonistic forms of order, and rarely coexist. Many interesting new phenomena occur, however, in hybrid superconducting/ferromagnetic systems. For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of pi in its ground state for certain thicknesses of the material(1). Such 'pi-junctions' were first realized experimentally in 2001 (refs 2,3), and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing(4-10). Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and pi by changing the relative orientation of the two magnetizations. These controllable 0-pi junctions have immediate applications in cryogenic memory, where they serve as a necessary component to an ultralow power superconducting computer(11). Such a fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers(12). Phase-controllable junctions also open up new possibilities for superconducting circuit elements such as superconducting 'programmable logic', where they could function in superconducting analogues to field-programmable gate arrays.