Magnetically Modulated Microwave Absorption Study of Superconducting MgB2 Regions

Experiments on thin MgB2 films synthesized by ion implantation followed by hydrogen-plasma pulse treatment or annealing the sample at a given temperature in a furnace were undertaken. Superconducting properties were examined by magnetically modulated microwave absorption (MMMA) measurements. MMMA was measured as a function of temperature and external magnetic field in differently prepared MgB2 samples. The results show the existence of superconducting regions with critical temperatures dependent on the recrystallization conditions. The MMMA dependence on the magnetic field sweep is very sensitive to the morphology of the sample and reveals variations in the quality of the superconducting material. The samples studied exhibit signals with different widths and hysteresis properties. Such behavior is analyzed in terms of a model of Josephson junctions at narrow regions within an imperfect film of MgB2 and flux pinning in superconducting regions. The amplitude of the MMMA signal at a given T < T c scales with the volume fraction of the superconducting material only if the hysteresis has the same looplike shape.