Anomalous polarization-dependent transport in nanoscale double-barrier superconductor/ferromagnet/superconductor junctions

We study the transport properties of nanoscale superconducting (S) devices in which two superconducting electrodes are bridged by two parallel ferromagnetic (F) wires, forming an SFFS junction with a separation between the two wires less than the superconducting coherence length. This allows crossed Andreev reflection to take place. We find that the resistance as a function of temperature exhibits behavior reminiscent of the re-entrant effect and, at low temperatures and excitation energies below the superconducting gap, the resistance corresponding to antiparallel alignment of the magnetization of the ferromagnetic wires is higher than that of parallel alignment, in contrast to the behavior expected from crossed Andreev reflection. We present a model based on spin-dependent interface scattering that explains this surprising result and demonstrates the sensitivity of the junction transport properties to interfacial parameters.Comment: 5 pages, 3 figure

Investigations of Crossed Andreev Reflection in Hybrid Superconductor-Ferromagnet Structures

107 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.Cooper pair splitting is predicted to occur in hybrid devices where a superconductor is coupled to two ferromagnetic wires placed at a distance less than the superconducting coherence length. This thesis searches for signatures of this process, called crossed Andreev reflection (CAR), in three device geometries. The first devices studied are lateral spin valves. In these structures, when electrons with energies less than the superconducting energy gap are injected from one ferromagnetic wire into the superconductor, nonlocal transport processes involving the second ferromagnetic wire are predicted to take place. We measure a negative nonlocal voltage in the antiparallel magnetization alignment of the ferromagnetic wires, which is the theoretically predicted signature of CAR. The second type of hybrid devices we measured consist of two superconducting electrodes connected by two ferromagnetic nanowires placed within a superconducting coherence length of each other, forming an S-FF-S junction. We find that below the critical temperature of the superconductor, the resistance versus temperature curves show re-entrant behavior, with the signal corresponding to antiparallel alignment of the magnetization of ferromagnetic wires distinctly larger than that of the parallel case. We discuss one possible explanation of this result in terms of Cooper pair splitting. We also report the first observation of multiple Andreev reflection peaks in the differential resistance of these devices. The third line of investigation briefly examines superconductor-ferromagnet dc SQUID-type devices to which we apply an external magnetic field to modulate the phase drop across the junctions. We do not observe coherent effects such as supercurrent or resistance oscillations, but suggest future directions of research.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD