Electron Spin Resonance (ESR) measurements have been performed on axial triangular antiferromagnets CsNiBr3 and CsMnI3 in their lowest three-dimensional (3 D) ordered phase. The frequency vs. magnetic field diagrams are obtained in the frequency range from 9 to 380 GHz and the magnetic field up to 160 kOe. Several significant disagreements between the experimental data and the theory based on the conventional model are found. It is shown that the disagreements can be removed by the diagonal interchain exchange coupling J2. The ESR theory is developed taking the J2 coupling into account. Fitting the present theory to experimental data, the interaction parameters of CsNiBr3 and CsMnI3 are determined.
- Diagonal interchain interaction
- Easy-axis anisotropy
- Electron spin resonance
- Triangular antiferromagnet