“The energy spectrum of quadrupole impurity centers of different types in antiferromagnets

We study the energy spectrum and some properties of various quadrupole centers (magnetic impurities or magnetic impurity complexes that are symmetric with respect to the magnetic sublattices of the antiferromagnet). We allow for the effect of spin-phonon coupling on the quadrupole splitting parameter and show that such coupling can lead to a considerable decrease in the value of this parameter and even change its sign. We investigate the behavior of quadrupole centers with an orbitally degenerate ground state and of quadrupole impurity complexes formed by mixed-valence ions. We demonstrate that such centers may greatly affect the resonant, magnetic, and thermodynamic properties of antiferromagnets. Finally, we analyze the existing experimental data and show that several new effects can be observed in systems with such centers (in particular, a magnetic analog of the Jahn-Teller effect and a strong magnetoelectric effect).