Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide—I

Abstract

The general aspects of the crystal field spectra of the two-dimensional magnetic insulators CrCl 3 and CrBr 3 are considered in detail, with special emphasis on the broad, spin-allowed 4 T 2 and 4 T 1 bands. A fit of the point-charge model Hamiltonian to the spectra is presented, with attention to the magnitude of the trigonal field effects. The temperature dependence of the 4 T 2 and 4 T 1 dipole strengths in the range 4.2< T < 300°K is analyzed in terms of single-ion (odd-parity trigonal field and odd-parity vibronic) and exchange mechanisms. A moment analysis of the M.C.D. spectra is performed, which (together with dipole strength temperature dependence data) yields a reasonable interpretation of the spin-allowed transition mechanism. The odd-parity crystal field is found to be the most important parity-breaking mechanism in both the 4 T 2 and 4 T 1 bands. Both also receive some contribution from the vibronic mechanism. This process is more significant for the 4 T 2 state than for the 4 T 1, and may involve a slightly anharmonic low-energy vibration. Evidence for a small exchange contribution is found in the 4 T 2 band of both systems. Factors influencing the bandshapes are examined quantitatively. The antiresonance formalism of Sturge et al. is applied to the anomalous lineshapes of certain transitions in absorption and M.C.D. and one particularly complex bandshape (that for 4 A 2 → 4 T 2 - 2 E - 2 T 1 in CrCl 3) is analyzed through a computation.