The geometry of the Brillouin zone of antiferromagnetic NiO (C c 2/c) is studied. The corepresentations of the magnetic little group are deduced for the points and lines of symmetry. The magnon symmetries are investigated and the selection rules for two magnon electric dipole and magnetic dipole absorption are deduced. 494 A. P. CBACKNELL AND S. J. JOSHUA in the direction of the intersection of the sheet with the vertical plane through the z-axis and [110]. We shall then classify the eigenstates of the crystal both above and below T N by finding the irreducible corepresentations of the group of the wave vector for each of the points and hnes of symmetry of the Brillouin zone of the appropriate magnetic space group of NiO (7).

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