  The concept of charge-transfer (CT) transitions in ferrites is based on the
cluster approach and takes into account the relevant interactions as the
low-symmetry crystal field, spin-orbital, Zeeman, exchange and
exchange-relativistic interactions. For all its simplicity, this concept yield
a reliable qualitative and quantitative microscopic explanation of spectral,
concentration, temperature, and field dependences of optic and magneto-optic
properties ranging from the isotropic absorption as well as the optic
anisotropy to the circular magneto-optics. In this review paper, starting with
a critical analysis of the fundamental shortcomings of the "first-principles"
DFT-based band theory we present the main ideas and techniques of the cluster
theory of the CT transitions to be main contributors to circular magneto-optics
of ferrites.
