It is demonstrated that magnetic resonance in a magnetically ordered state is a sensitive indirect method for the investigation of the orbital ground state. This idea is illustrated for two perovskite titanates: LaTiO 3 and YTiO 3 . In contrast to the spin-wave energy spectra, antiferromagnetic resonance spectra in an external magnetic field reveal clear dependence on the orbital state and it can distinguish the state with strong orbital fluctuations from the static orbital order. Our theoretical analysis is based on the model, which explicitly takes into consideration the strong correlation among lattice, orbital, and magnetic degrees of freedom.

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The magnetic structure of LaMnO3 and the antiferromagnetic resonance frequencies in it are investigated as functions of magnetic field H and temperature T. A previously developed model that includes the orbitally dependent superexchange and single-ion anisotropy is used. The temperature dependences described in the framework of the mean field approximation are in good agreement with experiment. The H–T phase diagrams are constructed for magnetic field directed along the principal crystallographic directions (a,b,c). The problem of the origin of the weak noncollinearity of the Mn3+ spins is solved. An optimal method of obtaining the expression for the magnetic free energy is proposed.

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The present work is devoted to the theoretical interpretation of nuclear magnetic resonance (NMR) spectra on nonmagnetic lanthanum ion in charge-ordered La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 compounds. It is shown that the anisotropic hyperfine interaction, which arises due to the polarization of lanthanum’s external p-shells, plays a crucial role in spectrum forming. Our model allows to explain experimental spectra. The work is partially supported by CRDF REC-005.