Single Vortex Resolution Imaging of the Flux Front in a YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystal

We have imaged the vortex state in an optimally doped, detwinned YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single crystal, using magnetic force microscopy (MFM). The structure of the flux front, the boundary between vortices and anti-vortices, is studied with single vortex resolution, as it evolves with applied magnetic field. We find the front to be corrugated on a scale of several microns. On a smaller scale, the front is composed of alternating ``fingers'' of vortices and anti-vortices. We also observe stable vortex-antivortex pairs.