Vortices in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_{1.8}$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ thin films were directly observed by Lorentz microscopy as the sample temperature T was raised from 4.5 K under fixed magnetic fields H. Each time T was raised, vortices moved, eventually to settle in a new static configuration. Vortices began to form a regular lattice at a temperature depending on H. The vortex-lattice image remained visible until it gradually faded away around an irreversibility temperature ${\mathit{T}}^{\mathrm{*}}$(H). Careful examination of the disappearance temperature indicates that the vortex lattice may persist up to some temperature above ${\mathit{T}}^{\mathrm{*}}$(H).

• Received 30 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3371