Measurement of Hc1 in MgB2 thin films and multilayer structures by a microwave absorption technique

For superconducting RF applications, Gurevich suggested a route to enhance the vortex penetration field, Hc1, and thermal breakdown field by a multilayer structure consisting of alternating insulator and superconductor layers with thicknesses smaller than the penetration depth. We have measured Hc1 of MgB2 thin films and multilayer structures by measuring the microwave absorption of the sample at 9.3 GHz in a TE102 rectangular cavity under an applied magnetic field. The magnetic fluxon penetration into the sample as the applied field is increased to greater than Hc1 leads to an increase in the microwave absorption. Preliminary results indicate an enhancement of Hc1 in the MgB2 thin films from the bulk value, consistent with Gurevich's thickness effect model, which is very promising for RF applications of MgB2.