Norman O. Birge

We report large π-state critical current densities, Jc(π), in Nb/Pd89Ni11/Nb Josephson junctions at Pd89Ni11 thicknesses near the first π-state. We observe oscillations in the critical current with ferromagnetic barrier thickness consistent with a 0–π transition. For a junction with a 9.4 nm Pd89Ni11 barrier, we obtain Jc(π)=410 kA/cm2 at 4.2 K, exceeding values reported in prior PdNi-based studies. Magnetization measurements on continuous films, together with coercivity tests on patterned arrays, confirm that Pd89Ni11 exhibits perpendicular magnetic anisotropy, enabling zero-field operation without magnetic initialization. The combination of large Jc(π) and intrinsic anisotropy establishes Pd89Ni11 as a promising barrier material for passive π-shifters in superconducting digital logic and qubit architectures.

Josephson junctions containing ferromagnetic materials are under consideration for applications in digital superconducting logic and memory. Some memory applications rely on the ability to reverse the magnetization direction of a “soft” magnetic layer within the junction using a small local magnetic field generated on the chip. It is crucial, therefore, to find a suitable soft magnetic material with a low switching field and low switching energy. A popular magnetic material for such applications is Ni80Fe20, also known as Permalloy; however, Permalloy has a rather large magnetization, leading to large magnetic switching energies. In this work, we explore Cr-doped Permalloy, specifically Ni73Fe18Cr9, which has a saturation magnetization just under two-thirds that of Permalloy. Josephson junctions containing this NiFeCr alloy undergo a 0-π transition at a NiFeCr thickness of 2.3 nm, and the critical supercurrent decays in the alloy over a short characteristic length of 0.36 nm. Switching fields of a few millitesla are promising, but the short decay length and overall small values of the critical current in the Josephson junctions may preclude the use of NiFeCr in current cryogenic memory technologies.

Josephson junctions containing ferromagnetic (F) materials have been the subject of intense study over the past two decades. The ground state of such junctions oscillates between 0 and π as the thickness of the ferromagnetic layer increases. For some applications, it might be beneficial to replace a very thin F layer with an unbalanced synthetic antiferromagnet (SAF) consisting of two F layers of different thicknesses whose magnetizations are coupled antiparallel to each other. According to theory, such a system should behave similarly to a single F layer whose thickness is equal to the difference of the two F-layer thicknesses in the SAF. We test that theoretical prediction with Josephson junctions containing unbalanced Ni/Ru/Ni SAFs, keeping the thickness of one layer fixed at 2.0 nm and varying the thickness of the other layer between 2.0 and 5.0 nm. We observe the first 0−π transition at a thickness difference of 0.86 nm, which closely matches the position of the transition observed previously using single Ni layers.

The past two decades have seen an explosion of work on Josephson junctions containing ferromagnetic materials. Such junctions are under consideration for applications in digital superconducting logic and memory. In the presence of the exchange field, spin–singlet Cooper pairs from conventional superconductors undergo rapid phase oscillations as they propagate through a ferromagnetic material. As a result, the ground-state phase difference across a ferromagnetic Josephson junction oscillates between 0 and π as a function of the thickness of the ferromagnetic material. π-junctions have been proposed as circuit elements in superconducting digital logic and in certain qubit designs for quantum computing. If a junction contains two or more ferromagnetic layers whose relative magnetization directions can be controlled by a small applied magnetic field, then the junction can serve as the foundation for a memory cell. Success in all of those applications requires careful choices of ferromagnetic materials. Often, materials that optimize magnetic properties do not optimize supercurrent propagation, and vice versa. In this review, we discuss the significant progress that has been made in identifying and testing a wide range of ferromagnetic materials in Josephson junctions over the past two decades. The review concentrates on ferromagnetic metals, partly because eventual industrial applications of ferromagnetic Josephson junctions will most likely start with metallic ferromagnets (either in all metal junctions or junctions containing an insulating layer). We will briefly mention work on non-metallic barriers, including ferromagnetic insulators, and some of the exciting work on spin–triplet supercurrent in junctions containing non-collinear magnetic inhomogeneity.

Ferromagnetic Josephson junctions exhibit fascinating physics and the potential for applications in superconducting logic and memory. The junctions in a demonstrated superconducting memory prototype contain a magnetic spin-valve structure with Ni as the fixed layer and NiFe (Permalloy) as the free layer. However, NiFe exhibits poor supercurrent transmission, which limits the efficiency of the Josephson junction. We have previously shown that the supercurrent transmission through a Cu/NiFe/Cu trilayer can be improved by adding thin layers of Ni between the Cu and NiFe – possibly due to the advantageous spin-dependent transport properties of the Cu/Ni interfaces. In this work we explore this idea further by replacing the Cu/NiFe interfaces with Pd/NiFe, which also have more desirable transport properties. Compared to the reference junctions containing Cu/NiFe interfaces, the new junctions exhibit an increase in the $\pi$ -state supercurrent by a factor of 2 along with a change in the position of the first $0-\pi$ transition.

Studies of ferromagnet-superconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings. The electromagnetic (EM) proximity effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet. In this work, we use a Nb superconducting layer and Ni ferromagnetic layer to test for such magnetic interactions. We use the complementary, but independent, techniques of polarized neutron reflectometry and detection Josephson junctions to probe the magnetic response inside the superconducting layer at close to zero applied field. In this condition, Meissner screening is negligible, so our measurements examine only additional magnetic and screening contributions from proximity effects. We report the absence of any signals originating from EM proximity effect in zero applied field. Our observations indicate that either EM proximity effect is below the detection resolution of both of our experiments or may indicate a new phenomenon that requires extension of current theory. From our measurements, we estimate a limit of the size of the zero field EM proximity effect in our Ni–Nb samples to be ±0.27 mT.

Josephson junctions containing magnetic materials may be useful in superconducting electronics and memory. One design for a memory cell uses a junction containing an S/F/F'/S spin valve where the two ferromagnetic layers (F, F') have different switching fields. Such a spin valve can be made more robust if the hard layer is pinned by exchange bias with an adjacent antiferromagnetic (AF) layer. We report on the magnetic characterization of NiFe/NiMn and Ni/NiMn bilayer sheet films to confirm the pinning behavior of Ni ₄₁ Mn ₅₉ . We then report on the fabrication and measurement of S/N/AF/N/S Josephson junctions where NiMn is used as the AF layer, Nb as the S layer, and Cu as the N layer. From measurement of critical current in samples with NiMn thicknesses in the range from 1.2 nm to 6.0 nm, we measure the characteristic supercurrent decay length within NiMn to be 1.0 nm.