Third harmonic AC susceptibility in superconducting powder

The third harmonic AC susceptibility is theoretically investigated by numerically solving Campbell´s model in which the effect of the reversible fluxoid motion is taken into account. According to the calculated result of the AC field amplitude dependence of real and imaginary parts, their peak values decrease and the peak AC field amplitude shifts to higher values when the fluxoid motion changes from irreversible to reversible. This is quite different from the prediction of Bean´s critical state model in which the peak value and the peak AC magnetic field are predicted to be constant. Since the reversible phenomenon is expected to be remarkable in superconductors with small sizes, the third harmonic AC susceptibility measurement was carried out for a superconducting powder of alkali metal doped fullerene. Experimental results can be qualitatively explained by the theoretical results.<>