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 absorption using a standard X-band EPR spectrometer. The changes in microwave absorption as a function of temperature and/or magnetic field were recorded by following the microwave reflectivity of a sample-loaded cavity. The modulated low-field microwave absorption signal was found to be similar to that observed in high-temperature superconducting materials. The field dependent 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 (H)^1/2 dependence with a change of slope indicating a transition from a strongly pinned flux lattice to the flux flow regime.