The low temperature magnetic properties of MnP nanocrystals (15-40 nm), both in GaP:MnP epilayers and MnP films, are significantly different compared to bulk and cannot apparently be explained by differences in the structure. A simple model of localized spins is used to describe the magnetic screw structure confined to nanocrystals. The results indicate that the observed magnetic behaviour is related to the nanometric size and to changes in the coupling constants most probably localized at an external grain shell. The nucleation of helical regions at the surface of the ferromagnetic grains is proposed as a possible mechanism for the reversal of the magnetization. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3658393]