Excitation of Multiple-Magnon Bound States in CoCl2·2H2O

Abstract

The results of far-infrared transmission measurements on Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O at helium temperatures are reported and compared with theoretical predictions. Antiferromagnetic resonance, ferrimagnetic resonance, and ferromagnetic resonance have been observed in the respective metamagnetic phases of this material. Furthermore, these magnons appear to interact with an unexpected excitation which is believed to be an optical phonon. The most striking feature of the data, however, is the appearance of absorption lines in each phase which shift with magnetic field at a rate corresponding to $g$ values of about 14, 21, 28, and even 35, as compared to the $g$ value of ∼7 for the single magnons. Furthermore, the energy of each of these $n$-fold multiple excitations is markedly less than $n$ times the energy of a onefold one. These excitations are identified as clustered spin reversals or magnon bound states, and this is the first direct observation of such states. In the simple Ising-model approximation, such clusters of $n$ adjacent spin reversals are eigen-states, and the Ising-model energies qualitatively describe the observed energy spectra in all three phases. Using the theoretical results of the preceding paper, the small non-Ising terms are included in the theory, and excellent quantitative agreement is obtained. The reasons why bound states can be observed in Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O are also discussed.