Magnetization-Dependent Critical Current in S-(S/F)-S Junctions : Experimental Realization and Micromagnetic Simulation

Abstract

The antagonistic properties of superconductors and ferromagnets open up an attractive area of research for energy-efficient computing technologies. The remanent magnetization of ferromagnets, in particular, offers potential for nonvolatile storage solutions. This thesis presents the fabrication and investigation of ferromagnetically constricted superconducting junctions composed of aluminum and cobalt in an S-(S/F)-S geometry. Low-temperature transport measurements reveal that the critical current of these junctions is strongly dependent on the magnetization state of the ferromagnet, enabling the realization of a field-trainable, nonvolatile superconducting switch. The persistence of the controllability in samples with oxidized S/F interface suggests that the control mechanism is predominantly mediated by stray fields. The inverse proximity effect, driven by interfacial diffusion of Cooper pairs, manifests itself in a strong influence on the critical current amplitude of the junction. Micromagnetic simulations conducted in this thesis can replicate the measured phenomenology semi-quantitatively using three-dimensional micromagnetics and a purely stray-field dependent critical current model. The simulations provide microscopic insight into the magnetization dependence of the critical current, successfully replicating critical current jumps in the experiment caused by Barkhausen jumps in the ferromagnet. The results also demonstrate that the grain size of the polycrystalline ferromagnet strongly impacts the observed phenomenology.