Diamagnetism is defined as the generation of a spontaneous magnetization of a material which directly opposes the direction of an applied field. However, the fundamental origins of the diamagnetism in superconductors and normal materials are very different. In superconductors the diamagnetism arises from the persistent screening currents which flow to oppose the applied field; in normal materials diamagnetism arises as a direct result of an orbital rotation of electrons about the nuclei of an atom induced electromagnetically by the application of an applied field. Very recently, it has been shown theoretically that the Meissner effect may exhibit paramagnetism in some layered superconductors but so far this paramagnetic intrinsic Meissner effect has not been experimentally observed. Mario Rabinowitz and his colleagues showed that a virtual violation of the Meissner effect is possible.
Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field B=0 within them. This means that their magnetic susceptibility, = −1.
They detected this effect only indirectly; because the magnetic flux is conserved by a superconductor, when the interior field decreased the exterior field increased. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.
Observation of the Meissner effect is difficult, because the applied fields have to be relatively small (the measurements need to be made far from the phase boundary). This is because the penetration depth is temperature-dependent and tends to be infinitely close to the phase boundary




