Single-ion magnon bound states in an antiferromagnet with strong uniaxial anisotropy

Abstract

Electron-spin-resonance measurements at high frequencies have been made on the antiferromagnet ${\mathrm{FeI}}_{2}$ with strong uniaxial anisotropy. We have found signals whose g value is 16.5 together with the one with $g=7.0$ reported before [A. R. Fert et al., Solid State Commun. 26, 693 (1978)]. We interpret these electron-spin resonance signals as originating from single-ion four magnon and two magnon bound states, respectively. The energies for creating the magnon bound states are calculated using an Ising approximation. One of the four magnon bound states is assigned to the one due to the exchange interaction either the third nearest-neighbor one in the c plane ${(J}_{3})$ or the between planes one ${(J}_{0})$ and its value is determined to be ${J}_{3}{/k}_{\mathrm{B}}$ (or ${J}_{0}{/k}_{\mathrm{B}})=\ensuremath{-}0.45$ K.