Exchange Interaction between the Excited States of Magnetic Ions

Within the framework of the LDA + GTB multielectron approach to the electron structure of Mott–Hubbard insulators a scheme is developed for constructing the effective low-energy Hamiltonian that includes not only the ground state of the magnetic cations, but also the excited terms. The mathematical apparatus of the theory are Hubbard operators built on the many-electron states of the cation in the d n configuration. The occupation of the excited term under optical pumping can change the sign of the exchange interaction of the excited cation with the neighboring cation in the ground state. Another variant of the occupation of the excited states is connected with a spin crossover when the excited and the ground terms change over, for example, at high pressure. Examples are given for such crystals as FeBO 3 , Nd 0.5 Gd 0.5 Fe 3 (BO 3 ) 4 and NiO.