This is caused by persistent (*) currents which form on the surface of the material as it is cooled below the critical temperature, which completely cancel out the flux inside it, resulting in perfect diamagnetism.
It was first observed in 1933 by its eponymous discoverers in tin and lead specimens. Rather than a sharp boundary at the edge of the sample, there is a gradual decay of the magnetic field across the penetration depth of the material. This is caused by persistent (*) currents which form on the surface of the material as it is cooled below the critical temperature, which completely cancel out the flux inside it, resulting in perfect diamagnetism. It allows the diamagnetic levitation of small items, including live frogs. FTP, name this uniquely defining property of superconductors, a phenomenon characterized by the complete expulsion of a magnetic field.
It was first observed in 1933 by its eponymous discoverers in tin and lead specimens.
{Meissner}-Ochsenfeld {effect}