Flux quantization in a superconducting microdisk

Magnetic flux distribution of a YBa 2 Cu 3 O 7− δ microdisk with 50 μm in diameter and 0.48 μm in thickness has been studied by scanning SQUID microscopy. All vortices observed in the microdisk with high- κ each carries flux quantum φ 0 and are loosely distributed in magnetic fields up to 30 μT. Number of the vortices increases one by one with increasing magnetic field. Average magnetic field for each increase in the number is greater than that expected from the size of the microdisk, which is probably due to expulsion of magnetic field from the rim of the microdisk. These results show that multi-vortex states are stable in the microdisk.