Eric Gingrich

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.

S/F/S Josephson junctions with single-domain ferromagnets for memory applications

Josephson junctions containing ferromagnetic materials are being considered for applications in cryogenic random access memory. The road to such applications requires thorough characterization of junction properties, including critical current and ground-state phase shift, as a function of the thickness of a single ferromagnetic layer. We carried out such a study for elliptically-shaped submicron Josephson junctions containing a Ni0.73Fe0.21Mo0.06 alloy similar to commercial Supermalloy. From the field dependence of the critical current, we conclude that the ferromagnets in our junctions are primarily single-domain. These measurements also produce pertinent information about the switching properties of the nanomagnet. We observe a 0–π transition occurring at a NiFeMo thickness of 2.25 ± 0.10 nm, while the critical current decays exponentially with a characteristic length scale of 0.48 ± 0.04 nm.

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thicknesses. We present studies of Nb based micron-scale elliptically shaped Josephson junctions containing ferromagnetic barriers of Ni81Fe19 or Ni65Fe15Co20. By applying an external magnetic field, the critical current of the junctions is found to follow characteristic Fraunhofer patterns and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across ...

Phase control in a spin-triplet SQUID

The phase state of a spin-triplet Josephson junction containing three ferromagnetic layers is magnetically controlled. It is now well established that a Josephson junction made from conventional spin-singlet superconductors containing ferromagnetic layers can carry spin-triplet supercurrent under certain conditions. The first experimental signature of that fact is the propagation of such supercurrent over long distances through strong ferromagnetic materials. Surprisingly, one of the most salient predictions of the theory has yet to be verified experimentally—namely, that a Josephson junction containing three magnetic layers with coplanar magnetizations should exhibit a ground-state phase shift of either zero or π depending on the relative orientations of those magnetizations. We demonstrate this property using Josephson junctions containing three different types of magnetic layers, chosen so that the magnetization of one layer can be switched by 180° without disturbing the other two. Phase-sensitive detection is accomplished using a superconducting quantum interference device, or SQUID. Such a phase-controllable junction could be used as the memory element in a fully superconducting computer.