Josephson junctions consisting of two superconductors separated by a thin non-superconducting layer are investigated. In conventional superconductors, the supercurrent is due to the flow of singlet state paired electrons where they have opposite spins. A non-zero triplet state can appear by introducing a material with a noncollinear magnetization next to the superconductor. These triplet pairs have a much farther penetration length into a ferromagnet than a singlet pair and should give a larger critical current. To introduce the noncollinear magnetization, a magnetic system called an exchange spring was chosen. Exchange springs consist of a hard magnetic layer in proximity to a soft magnetic layer. When a field is applied to these structures, part of the soft layer can rotate with the field and part remains pinned to the hard layer causing a spiral shaped magnetization. For the soft layer permalloy was used and for the hard layer either SmCo or SmFe. The exchange spring behaviour is sensitive to the ratio of the soft layer thickness to hard layer so work was done in optimizing these layers.
The exact process for building these junctions with the equipment available was not known at the beginning. Using DC magnetron sputtering to make the films, several approaches involving mechanical masks and photolithography were used to find the best method for patterning the junctions. The dimensions for the overlap for the two superconductors ends up being a concern since the critical current through a junction is proportional to the area. Most of the improvements to the fabrication attempted are to minimize this area.
It was found that using a layer of photoresist material to define the junction area gave the best results. Measurements for the critical current vs applied field strength seem to suggest that the method presented in this thesis can be used to produce junctions. There also seems to be a change in the critical current as a noncollinearity is introduced but there is a large amount of noise in the data. The measured voltages involved have to be done in the lowest gain setting for the equipment used which is not quite sensitive enough.
|Commitee:||Bill, Andreas, Gu, Jiyeong, Kwon, Chuhee|
|School:||California State University, Long Beach|
|Department:||Physics and Astronomy|
|School Location:||United States -- California|
|Source:||MAI 51/04M(E), Masters Abstracts International|
|Subjects:||Low Temperature Physics, Condensed matter physics|
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