Use of Pd–Fe and Ni–Fe–Nb as Soft Magnetic Layers in Ferromagnetic Josephson Junctions for Nonvolatile Cryogenic Memory

Josephson junctions containing ferromagnetic layers are under consideration as the basic elements for cryogenic random access memory. For memory applications, either the amplitude of the critical current or the phase shift across the junction must be controllable by changing the direction of magnetization of one or more of the ferromagnetic layers in the junction. We have measured the critical currents in large-area Josephson junctions containing three ferromagnetic layers. These junctions carry spin-triplet supercurrent. This work addresses the choice of material and optimum thickness for the one soft magnetic layer in such junctions. We have used either a Pd-Fe or Ni-Fe-Nb alloy for the soft layer, and find hysteresis in the low-field “Fraunhofer patterns” due to magnetic switching of the soft layer. The critical current is one order of magnitude smaller in the junctions containing the Ni-Fe-Nb alloy compared to those containing Pd-Fe alloy, which is probably due to strong spin-memory loss in the former. While the large-area junctions studied here are not suitable for memory applications, these experiments lay the groundwork for future studies of submicron junctions where the magnetic state of the junction can be controlled by shape anisotropy.