Investigations of Crossed Andreev Reflection in Hybrid Superconductor-Ferromagnet Structures

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Colci O'Hara, Madalina

Contributors dc:contributor

• Eckstein, James N.

• Physics, Condensed Matter

Language dc:language

eng

Identifier

(MiAaPQ)AAI3391915

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/80606