THE ENERGY-SPECTRUM OF QUADRUPOLE IMPURITY CENTERS OF DIFFERENT TYPESIN 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 antiferromag net). We allow for the effect of spin-phonon coupling on the quadrupol e splitting parameter and show that such coupling can lead to a consid erable decrease in the value of this parameter and even change its sig n. We investigate the behavior of quadrupole centers with an orbitally degenerate ground state and of quadrupole impurity complexes formed b y mixed-valence ions. We demonstrate that such centers may greatly aff ect the resonant, magnetic, and thermodynamic properties of antiferrom agnets, Finally, we analyze the existing experimental data and show th at several new effects can be observed in systems with such centers (i n particular, a magnetic analog of the Jahn-Teller effect and a strong magnetoelectric effect). (C) (C) 1997 American Institute of Physics.