Vortex distributions near surface steps observed by scanning SQUID microscopy

We have used a scanning superconducting quantum interference device microscope to image individual vortices near lithographically patterned surface steps in weak-pinning superconducting thin films of amorphous MoGe. The field-cooled vortex distributions are strongly influenced by the surface steps, with an enhanced vortex density along the thin side of the steps and a wide vortex-free region along the thick side of the steps. The surface steps induce orientational order that persists for many intervortex spacings away from the steps. We study the effects of surface step pinning for different magnetic-field strengths and step heights by analyzing the intervortex spacing and computing vortex correlation functions. For certain sample configurations, we are able to apply a Lorentz force to the vortices and observe an asymmetric vortex response near the surface steps. We discuss the interaction between a vortex and a surface step and consider possible mechanisms for generating the vortex distributions which we observe upon field cooling.