Neutron and Optical Studies of Domains in NiO

Antiferromagnetic domains in NiO have been studied by neutron diffraction and optical observation of single crystals. There are two kinds of antiferromagnetic domain walls, T (twin) and S (spin-rotation). A T wall is produced by rotation of the antiferromagnetic pattern of magnetic moments and an S wall by rotation of the direction of the magnetic moments. The T walls can be observed with polarized light, and both T- and S-type domains can be studied by neutron diffraction. Crystals with simple T-wall structures are produced by annealing at high temperature. The crystallography of the T walls can be understood in terms of a small rhombohedral deformation which accompanies the antiferromagnetic ordering. The T walls are easily displaced by the application of small mechanical stresses or magnetic fields. A crystal without T walls may still contain S walls, and these S walls also are displaced or rotated by magnetic fields.