Vortex configurations, matching, and domain structure in large arrays of   artificial pinning centers
  High-resolution scanning Hall probe microscopy has been used to image vortex configurations in very large periodic arrays of artificial pinning sites. Strong matching effects are seen at fields where either one or two vortices can sit at a site; with three vortices per site, however, no clear matching is observed. Matching effects have been also been observed at several fractional multiples of the matching field, including 1/5, 1/4, 1/3, 1/2, and 3/4. These fractional values are characterized by striking domain structure and grain boundaries. 
S. B. Field, S. S. James, J. Barentine, V. Metlushko, G. Crabtree,   H.Shtrikman, B. Ilic, and S. R. J. Brueck