Designing spin-spin interactions with one and two dimensional ion crystals in planar micro traps J. Welzel, A. Bautista-Salvador, C. Abarbanel, V. Wineman-Fisher, C. Wunderlich, R. Folman and F. Schmidt-Kaler Eur. Phys. J. D, 65 1-2 (2011) 285-297 Published online: 27 July 2011 DOI: 10.1140/epjd/e2011-20098-y