We have investigated by means of neutron diffraction and magnetic measurements the particle size effects on the structure and low-temperature spin arrangement in so-called half doped manganites. The study shows that the Mn3+/Mn4+charge ordering and CE-type antiferromagnetic structure characteristic for bulk are completely suppressed when particle size is decreased down to 20 nm, and a ferromagnetic state is stabilized instead. The reason is not in a lower energy of the latter state, but in the hindering of displacive processes through which the charge ordering develops. Our work is of a general importance since the room temperature crystal structures in present manganite particles are found not to deviate from bulk, disproving thus former speculations about the important role of surface stress.
