Observation of vortex structure in MgB$_2$ single crystals by Bitter   decoration technique
  We report the observation of superconducting vortices in pure and lightly Al doped MgB$_2$ single crystals. Low field experiments allow for the estimation of the London penetration depth, lambda ~ 1900 $\AA$ for T$\sim$6 K. Experiments in higher fields (e.g. 200 Oe) clearly show a triangular vortex lattice in both real space (13 $\mu$m by 13 $\mu$m Bitter decoration image of over 1000 vortices) and reciprocal space. 
L. Ya. Vinnikov, J. Karpinski, S. M. Kazakov, J. Jun, J. Anderegg, S.   L. Bud'ko, P. C. Canfield