Inelastic scattering of unpolarized neutrons on dimer clusters

On the basis of a microscopic calculation of the inelastic neutron scattering spectra for scattering by exchange dimer clusters, we have determined the conditions for applicability of the semiphenomenological approach to determination of the scattering cross sections based on the spin model. The scattering cross sections obtained in the spin model are distinguished from the exact values by the fact that modified "form factors of the S→S′ transitions" are used in the exact calculation instead of the magnetic form factors of the individual atoms. In the case of weak overlap of the one-electron wave functions for different centers, the exact scattering cross sections coincide with the cross sections calculated on the basis of the spin model. We have investigated the inelastic neutron spectrum for scattering by a dimer d²-d¹ of mixed valency. Tunneling transfer of the "extra" electron leads to splitting of the exchange line corresponding to the transition with δS=1 in the system with localized electrons and the appearance of new lines. The spectrum consists of a series of "forbidden" transitions (with conservation of parity and δS=1) and allowed transitions (with a change in parity and δS=0, 1). The results obtained allow us to determine the parameters of Heisenberg and double exchange in mixed-valency dimers using the inelastic neutron scattering spectra.