Lineare optische anisotropie von antiferromagnetischem NiO im äusseren magnetfeld

Single crystals of NiO exhibit an optical anisotropy below their Néel temperature T_N = 523.7 K even without external magnetic fields. Linear dichroism and birefringence increase in proportion to the square of the antiferromagnetic long range order parameter. The spectra of both effects show detailed structures in the visible range: the dichroism exhibits a large peak at 2.12 eV and some smaller ones below and above; the birefringence increases steadily towards the fundamental absorption edge (3.75 eV); and, some structures are superposed in the visible range of the small 3d⁸-absorption bands. In external magnetic fields a large rise of the Néel temperature is detected by optical anisotropy. It is proportional to the square of the field strength: T_N(H) = T_N(O) + 0.585 K/Tesla². Below T_N optical anisotropy shows a slight magnetic field dependence. Our results are discussed by means of a microscopic model of the magneto-optical anisotropy in NiO.