Optical spectroscopy of antiferromagnetics with different dimensionalities of structures (a review)

The results of investigations of spectral properties of antiferromagnetic compounds of transition metal ions with a high structural dimensionality are analyzed briefly. The problems connected with the evidence for the collective nature of optical excitations in magnetically ordered crystals are discussed. The conditions for observing the magnetic resonance splitting in two-sublattice antiferromagnets, which are a direct proof of the exciton nature of optical excitations, are considered. Optical transitions in antiferromagnetic crystals involving the participation of the spin-wave subsystem (exciton-magnon transitions) are described, a well as the principles of the unified mathematical approach for describing exciton-magnon processes in absorption and luminescence. The peculiarities of the formation of the fine structure of optical spectra of antiferromagnetic crystals with decreasing structural dimensionality are analyzed. One of the most important characteristics which predetermines the basic spectral properties of antiferromagnets in the visible region is the spectral distribution of the density of magnon states, which depends on the dimensionality of the magnetic structure of the crystals. The shape of the exciton-magnon absorption bands and the peculiarities of exciton scattering by spin waves in low-dimensional magnets are discussed. A number of new phenomena associated with the optical reorientation of ion spins in antiferromagnetic crystals, i.e., with the deviation of the spin-quantization axes of photoelectrically excited ions from the direction of magnetization of the corresponding sublattices, are described. It is shown that the small number of nearest neighbors surrounding a photoelectrically excited ion in a low-dimensional antiferromagnetic crystal creates conditions for the emergence of a basically new state, viz., the inhomogeneous magnetization state, which is a sort of spin coat consisting of nearest unexcited magnetic ions whose spins are not coaxial with the direction of magnetization of the corresponding sublattices.

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