Conductance of superconductor -ferromagnet -superconductor structures

Miles Daniel Lawrence, Purdue University

Abstract

We have studied electron transport in superconductor-ferromagnet-superconductor structures, where the ferromagnet was nickel. The superconductors were relatively large thick films (∼1000 Å thick) that were joined by a narrow Ni films or wires. The two-dimensional Ni films were ∼2 − 5 μm wide and ∼2 − 50 μm long. The quasi-one-dimensional wires were ∼400 Å wide and ∼200 nm −10 μm long. In the two-dimensional structures, measurements of the resistance as a function of temperature and magnetic field were performed. The superconducting films were made from various different metals, as well as a similar system with non-superconducting films for comparison. The low field magnetoresistance has a general appearance similar to that expected for weak localization, but the magnitude is much too large to be accounted for in terms of this mechanism. In zero field the resistance exhibits behavior well below the critical temperature of the superconductor which we attribute to a large proximity effect. In the one-dimensional structures, measurements of the resistance as a function of temperature and magnetic field were also performed. Sn was used exclusively for the superconductor. A more careful study on the role of the superconductor-ferromagnet interface was performed for this geometry. The behavior of the system is very sensitive to the quality of this interface. With a clean interface, there appears to be a significant proximity effect in the ferromagnet, and the results imply an unexpectedly long proximity length. However, when an oxide layer is present at the superconductor-ferromagnet interface, the effective resistance of the ferromagnet increases as the temperature is reduced below the Tc of the superconductor. It does not appear that these observations can be explained by current theories.

Degree

Ph.D.

Advisors

Giordano, Purdue University.

Subject Area

Condensation

COinS