Neutron topographic investigation of the Morin transition in flux-grown crystals of $\alpha$-Fe< sub> 2 O< sub> 3

The Morin transition is investigated by neutron diffraction topography on high crystalline quality flux-grown single crystals of cr-Fe,O,. They display a widely spread transition, which is complete for some of the crystals, partial for others and even "reentrant" in one case. Topographs indicate that the transition takes place through a succession of abrupt transitions of regions of the sample, and give images of these regions. We discuss the possible relationship between these regions and the crystal perfection.