Crystallography and Domain Walls in Antiferromagnetic NiO Crystals

Twin-domain walls in large crystals of antiferromagnetic NiO have been observed by specular reflection in optically polished and annealed crystals. These walls can be moved in annealed crystals by small mechanical stresses (106 d/cm2) or by moderate magnetic fields (25 000 oe) to yield completely untwinned crystals. The ease with which a twin wall moves appears to be limited by a spin-rotation energy loss. Crystallographic measurements of the rhombohedral distortion agree with the x-ray data, and verify the 111 antiferromagnetic contraction. The anisotropy in the magnetic susceptibility vs field has been measured for untwinned crystals, and is compared with earlier work.