During nanoindentation of MgO, a pop-in phenomenon has been observed, that is, an
abrupt increase in penetration depth at a critical load. Atomic force microscopy investigations
have been performed to characterise the dislocation structure in the vicinity of the indented
area. The pop-in has been found to be the first stage of plastic deformation during
nanoindentation. This phenomenon is ascribed to the nucleation and further propagation of
interstitial loops of dislocations beneath the indenter. A three-dimensional model for a flat
ended indenter is developed.