Observation of dynamic interaction of vortices with ion irradiated defects by Lorentz microscopy

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