Magnetic field dependent microwave absorption studies on a MgB2 superconductor

Polycrystalline MgB2, synthesized using a conventional solid-state reaction route, has been investigated for its magnetic field dependent microwave ab sorption using a standard X-band EPR spectrometer. The changes in microwave absorption as a function of temperature and/or magnetic field were recorde d by following the microwave reflectivity of a sample-loaded cavity. The mo dulated low-field microwave absorption signal was found to be similar to th at observed in high-temperature superconducting materials. The field depend ent direct microwave absorption in the 0-10 kG range was used to determine H-c1 values at different temperatures, and the value of H-c1 (0) was found to be 250 G. The absorbed microwave power has been found to obey a rootH de pendence with a change of slope indicating a transition from a strongly pin ned flux lattice to the flux flow regime.