Microwave losses of MgB2 thin films in dc magnetic field

The microwave surface impedance (Zs = Rs+ iXs) of in situ MgB2 thin films was measured as a function of temperature and parallel dc magnetic field at several frequencies between 5.7 GHz and 18.5 GHz using a dielectric resonator technique. The results are consistent with the expectations for a classical type-II superconductor and, consequently, quite different from those of the high-Tc cuprates. The films cooled in zero-field revealed a clear indication of the lower critical field, Bc1. At higher fields (up to 2 T at 3 K) both Rs and Xs showed a linear increase with field in good agreement with the vortex state model introduced for type-II superconductors by Coffey, Clem and Brandt. The curves measured in sweeping fields (e.g. -2 T < B < 2 T at 3 K) revealed a narrow hysteresis loop in the range |B| < (3 ÷ 4) Bc1and were perfectly symmetrical with respect to the ordinate axis.