Josephson junctions containing ferromagnetic layers are being studied for their interesting physics and potential applications in energy-efficient superconducting electronics. These ferromagnetic Josephson junctions exhibit ground state phase shifts of either 0 or $\\pi$, and controlling the phase is crucial to their applications as the memory elements for a superconducting computer. Phase control in ferromagnetic Josephson junctions has been demonstrated in a pseudo-spin valve structure containing Ni as the fixed layer and NiFe as the free layer. The same magnetic layers were also used in a prototype of the cryogenic memory called Josephson Magnetic Random Access Memory (JMRAM). However, the magnetic layers currently being used need to be optimized to improve their reliability and efficiency.The fixed layer Ni has a multidomain magnetic structure and can interfere with the free layer switching. In this work, we propose replacing Ni with an unbalanced Ni/Ru/Ni synthetic antiferromagnet (SAF) where one Ni layer is thicker than the other, in hopes of achieving better magnetic properties and reducing the interference with the free layer switching. We first characterize the magnetic properties of the synthetic antiferromagnets as a function of Ni and Ru thicknesses to find the first antiferromagnetic coupling peak at a Ru thickness of 0.9 nm. We then study the magnetic properties of balanced Ni/Ru/Ni SAFs where both Ni layers have identical thickness. We then study the supercurrent transmission through these balanced Ni/Ru/Ni SAFs and find that the decay of supercurrent with Ni thickness is very slow with a decay length of 7.5 $\\pm$ 0.8 nm. Finally, we study the magnetic properties of unbalanced Ni/Ru/Ni SAFs and find that in some cases the coercivity of the free layer switching is smaller than Ni, which could potentially lead to it being a viable replacement for the Ni fixed layer.The free layer NiFe has better switching properties than Ni, however it exhibits poor supercurrent transmission. In this work, we add thin layers of Ni at the interface between NiFe and the Cu spacer layers in our junctions with the hope of improving their critical currents. The idea behind this is based on lessons learned from Giant Magnetoresistance studies which showed that Ni/Cu interfaces have better spin-dependent properties for supercurrent transmission than NiFe/Cu interfaces. We characterize the magnetic properties and critical currents of Ni/NiFe/Ni trilayers as a function of Ni and NiFe layer thickness. We find that the magnetic properties of these trilayers are not severely degraded compared to NiFe. For a Ni thickness of 0.4 nm, we find that the maximum supercurrent in the $\\pi$-state of these trilayer junctions is increased by a factor of four relative to the NiFe junctions. We explore this idea further by replacing Cu/NiFe interfaces with Pd/NiFe interfaces, and find an enhancement of supercurrent by a factor of two. These results seem to indicate that the supercurrent through ferromagnets can be enhanced by ``engineering'' the interface for better spin-dependent transport properties at the interfaces.
Read
