  The propagation of domain walls in a ferromagnetic film is largely determined
by domain wall pinning at defects in the material. In this letter we map the
effective potential landscape for domain wall pinning in Permalloy films by
raster scanning a single ferromagnetic vortex and monitoring the hysteretic
vortex displacement vs. applied magnetic field. The measurement is carried out
using a differential magneto-optical microscopy technique which yields spatial
sensitivity $\sim 10$ nm. We present a simple algorithm for extracting an
effective pinning potential from the measurement of vortex displacement vs.
applied field. The resulting maps of the pinning potential reveal distinct
types of pinning sites, which we attribute to quasi-zero-, one-, and
two-dimensional defects in the Permalloy film.
