Quantum theory of the magneto-optical effect induced by Ni2+ ions in barium ferrites

To reveal the physical origin of the giant magneto-optical enhancement of Ni2+ ions in barium ferrite, quantitative calculations of the contributions of both the intra-ionic electric dipole transition between the 3d(8) and 3d(7)4p configurations of the Ni2+ ions and the intra-ionic electric dipole d --> d transition induced by odd-parity crystal field terms are presented. It is deduced that the 3d(8) --> 3d(7)4p transition is important in the origin of the considered magneto-optical enhancement. The most important factor is the Ni-Fe superexchange interaction; since it is strong enough, the Faraday rotation produced by the Ni2+ ions is large though the energy difference between the 3d(8) and 3d(7)4p configurations is large. It is demonstrated that though the intra-ionic electric dipole d --> d transition does produce Faraday rotation peaks in the visible range, their magnitude is too small to explain the observed Faraday rotation. The effect of the spin-orbit interaction on the Faraday rotation is analysed. The spin-orbit interaction of the ground configuration plays a very important role in the occurrence of Faraday effects; but the Faraday rotation does not increase linearly with the strength of the spin-orbit coupling. On the contrary, the spin-orbit interaction of the excited configuration has almost no effect on the Faraday rotation. It is shown that the mixing of the different multiplets of the ground term induced by the crystal field has a great influence on the magneto-optical properties.