Exchange spring based superconducting heterostructures and Josephson junctions are studied to search for evidence of odd-triplet superconductivity. Cooper pairs from a superconductor can leak into a nonhomogeneous ferromagnet a much greater distance than they leak into a homogeneous ferromagnet. This is a result of a conversion of the superconducting condensate at the superconductor-nonhomogeneous ferromagnet interface from the singlet and triplet states to the odd-triplet state. The odd-triplet state is insensitive to the exchange field of the ferromagnet.
To generate the nonhomogeneous magnetic region, an exchange spring is used. The exchange spring consists of coupled hard and soft magnetic layers that are used to produce a nonhomogeneous magnetization. The system studied consists of superconducting Niobium (Nb) and a Samarium-Cobalt/Permalloy (SmCo/Py) exchange spring.
Initial samples of Niobium had a critical temperature lower than that obtainable in our laboratory (< 1.8 K). Preliminary work was done to find the cause of the suppressed critical temperature of Nb and to increase it. This work resulted in obtaining Niobium thin films with critical temperatures as high as 6 K.
Indirect evidence of the odd-triplet component is searched for by looking at the critical temperature of superconductor/exchange spring bi-layers. As the nonhomogeneity of the magnetization is increased, it is expected that the critical temperature will decrease as the condensate leaks further into the exchange spring. In Nb/Py/SmCo systems, this behavior was observed, along with a modulation in the resistance that is attributed to the anisotropic magnetoresistance of the permalloy layer. A decrease in the critical temperature with increasing nonhomogeneity of the exchange spring was also observed in Nb/SmCo/Py layers, provided the SmCo layer is not too thick.
Direct evidence of the odd-triplet component is searched for by looking at the modulation of the critical current through exchange spring based Josephson junctions as exchange spring magnetization becomes more nonhomogeneous. As the nonhomogeneity of the magnetization increases, the critical current through the junction should increase as well. Fabrication of Josephson junctions with exchange spring interlayers was performed at Oak Ridge National Laboratory, and the procedure is presented here. The critical current through these junctions was observed to increase with increasing nonhomogeneity of the exchange spring magnetization, although more tests are needed to verify this is due to the odd-triplet component of the superconducting condensate.
|Commitee:||Bill, Andreas, Kwon, Chuhee|
|School:||California State University, Long Beach|
|Department:||Physics and Astronomy|
|School Location:||United States -- California|
|Source:||MAI 55/01M(E), Masters Abstracts International|
|Subjects:||Condensed matter physics|
|Keywords:||Exchange spring, Josephson junctions, Magnetism, Odd triplet superconductivity, Superconductivity, Thin films|
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