Electron Spin resonance in triangular antiferromagnets

We give an overview of our studies on the electron spin resonance (ESR) in the hexagonal triangular antiferromagnets of ABX₃ type using millimeter and submillimeter waves and static and pulsed high magnetic fields. Novel ESR modes, which cannot be understood by the conventional theory of the antiferromagnetic resonance, were observed. Examples of this include that there exists the gapless excitation even for the easy-axis anisotropy. The ESR modes can be classified by the signs of the exchange interaction Jo along the c-axis and the anisotropy. The ESR modes were calculated within the framework of the mean-field approximation using three-sublattice and six-sublattice models for the ferromagnetic and antiferromagnetic Jo interaction, respectively. The experimental results were well described by the present calculations.