SPECTROSCOPIC STUDIES OF THE 2-DIMENSIONAL MAGNETIC INSULATORS CHROMIUM TRICHLORIDE AND CHROMIUM TRIBROMIDE-II

The splitting, spectroscopic g-factors, polarized relative intensities and temperature dependence of the 2T1, 2T2 and 4T2 states of CrBr3 and the 4T2 state of CrCl3 are examined in detail. The 2T1 state is split into well-resolved 2A and 2E sublevels by the trigonal field, with no apparent complication from vibronic coupling. A hot-magnon-assisted transition to the 2A sublevel (hωmag = 75cm-1) is identified and its intensity vs temperature related to the nearest-neighbor two-spin correlation function, which is also used to estimate an excited-state exchange field of 1200 kG from the shift in energy of the 2E sublevel with temperature. A single-ion intensity mechanism is also found to be important, and the resulting strong dependence of the 2E sublevel intensity on exchange field direction and temperature (below Tc) is analyzed in terms of 2T1-4T2 and 2 T1-4T1 spin-orbit mixing. Closely similar results are found for the 2T2 state, including a strong dependence of the 2E sublevel intensity on exchange field direction and temperature (below Tc) and a hot-magnon-assisted transition to the 2A sublevel (hωmag = 48cm-1). Quantitative analysis is very difficult, however, because of a complicated sideband structure, the probable existence of a Jahn-Teller effect and uncertainty as to the rigid-lattice trigonal splitting. A partial interpretation of the structure, based on Jahn-Teller interaction with one or more eg, unit-cell vibrations, is proposed. A strong tetragonal Jahn-Teller effect is found in the 4T2 states in CrCl3 and CrBr3, as evidenced by a long progression in the 250 cm-1 and the 150 cm-1 eg, unit-cell vibration, respectively. For CrCl3, an analysis of the zero-phonon levels shows that EJT = 500 ±80 cm-1 and indicates that the excited-state exchange field is antiferromagnetic, Isotropie and slightly smaller in magnitude than the ferromagnetic ground-state exchange field. Vibrationally-excited levels of the 4T2 state are examined qualitatively, aided by the analysis of the zero-phonon levels. The closely-similar CrBr3 4T2 vibronic structure is also analyzed through a perturbation calculation, and a good description of the splitting, relative intensities and polarizations of the zero-phonon levels is obtained, yielding a value of EJT ≈ 400 cm-1. Spurious 4T2 zero-phonon structure is found in CrCl3 containing stacking faults but similar complications are not found in CrBr3. © 1979.