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

Electron-spin-resonance measurements at high frequencies have been mode on the antiferromagnet FeI2 with strong uniaxial anisotropy. We have found sig nals whose g value is 16.5 together with the one with g = 7.0 reported befo re [A. R. Fert et al., Solid State Commun. 26, 693 (1978)]. We interpret th ese electron-spin resonance signals as originating from single-ion four mag non and two magnon bound states, respectively. The energies for creating th e magnon bound states are calculated using an Ising approximation. One of t he four magnon bound states is assigned to the one due to the exchange inte raction 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(B) (or J (0)/k(B)) = - 0.45 K.