"Pinning Forces in Superconducting Thin Films"
Timothy Bramfeld, Advisor: Dr. Joseph C. Amato
The pinning forces acting on superconducting vortices in niobium thin films was studied.
Niobium samples were prepared in Colgate's argon ion sputtering system, with negative bias
voltages ranging from zero to 350 volts, to control the grain size in each film. Resistence
measurements were made at different temperatures (4.2 K < T < 8.0 K) in an applied magnetic
field (B < 4 Tesla). These data allowed us to determine the superconducting properties of each
sample, such as its critical temperature (T_c), Ginzberg-Landau prameter, coherence length,
penetration depth, and the elastic mean free path of the normal electrons. Then, the critical
current of the sample was measured for applied magnetic fields (0 < B < B_c2). These measurements
were made at a constant  reduced temperature t = .975, where t is the ratio of the sample's
actual temperature  to its critical temperature (t = T/T_c). The critical current data was used
to plot the volume pinning force of each sample against the applied field, and then a
comprehensive plot was made to compare the magnitude of the pinning forces. It was found
that the maximum pinning force increased with an increase in the negative bias voltage. Since
the grain size of these samples decreases with an increase in the bias voltage's magnitude, it can
then be concluded that the maximum pinning force increases as the grain size of the sample decreases.