OBSERVATION OF DYNAMIC INTER-ACTION OF VORTICES WITH ION-IRRADIATED DEFECTS BY LORENTZ MICROSCOPY

The dynamic interaction of vortices (quantized magnetic flux lines) wi th pinning centers was observed directly by Lorentz microscopy using a 300 kV field emission electron microscope. To clarify the pinning eff ect on the vortex configuration, we produced a defect array in a singl e-crystalline thin film of Nb by irradiating it with an ion beam. The vortices could not form a single-crystalline lattice structure but bec ame divided into domain structures due to the existence of the defect array. When the magnetic field was changed, all the domains resisted b eing moved for a while, and then some domain boundaries suddenly began to move in an avalanche manner and to break down into pieces that flo wed like a ''river'', that flowed along the domain boundaries located near the defect. At higher temperatures, the ''river'' became wider an d finally the vortex lattice flowed while keeping its lattice form.