Zubov E. Ground state and collective phenomena in а narrow-band metal oxide systems with rare-earth ions

In work the magnetic structure and thermodynamic behaviour of a wide class of narrow-band metal oxides with rare-earth ions has been investigated. The perturbation theory is based on a scattering matrix formalism in which the effective self-consistent field is extracted. It has allowed to consider correctly the kinematic contributions in electron systems with a mixed valence. The results of the theory are used to prove the stability of the ferromagnetic ground state and to analyse the quantum canting for competing double and indirect exchanges with strong Hund's coupling of d-electrons. For manganites the strong influence of the quantum fluctuations on Curie temperature was found. The abrupt growth of the specific resistance near the metal-dielectric phase transition was predicted. The magnetic properties of low-symmetric double tungstates with Kramers rare-earth ions and competing magnetic-dipole and exchange interactions have been studied. The Luttinger-Tisza method was evaluated to estimate the internal magnetic energy of different magnetic structures in the Bravais lattice . This made possible to predict the magnetic structure of ground state of double tungstate with disprosium and erbium ions. On the basis of experimental data on specific heat the conclusion about low-dimensional character of magnetic interactions of rare-earth ions was made.