Critical-current density of melt-grown single-grain Y–Ba–Cu–O disks determined by ac susceptibility measurements

The field amplitude and frequency dependent complex ac susceptibility χ( H m , f ) of three Y–Ba–Cu–O disks made by a top-seeded melt growth technique has been measured at 77 K with the ac field applied along the c -axis of the samples (parallel to their thickness). A procedure based on the Bean model has been developed to calculate the critical-current density J c near the surface of the sample from the measured χ( H m ) for the case where the maximum imaginary component χ '' is not reached.