Magnetic structures and excitations ofCsMnI3: A one-dimensional Heisenberg antiferromagnet with easy-axis anisotropy

We have performed measurements on the easy-axis Heisenberg antiferromagnet ${\mathrm{CsMnI}}_3$ to determine the magnetic structure and excitations. The structural anisotropy of the hexagonal perovskite structure (space group P$6_3$/mmc) leads to quasi-one-dimensional behavior such that the ratio of the intra- to interchain exchange ${\mathit{J}}_1$/${\mathit{J}}_2$≊200. Transitions to three-dimensional ordered magnetic states occur at ${\mathit{T}}_{\mathit{N}1}$=11.41±0.01 K and ${\mathit{T}}_{\mathit{N}2}$=8.21±0.02 K. Superconducting quantum interference device (SQUID) magnetometer measurements were performed with magnetic fields of up to 5.2 T applied parallel to the crystal c axis. No other phase transitions were detected at temperatures down to 1.8 K. Neutron-diffraction measurements confirmed that the low-temperature phase has antiferromagnetic order within the chains and a canted spin structure similar to that of the low-temperature phase of ${\mathrm{CsNiCl}}_3$ with a canting angle of 51±1°. In the higher-temperature phase the moments were found to lie almost parallel to the c axis, and to have a very small ferrimagnetic component along the c axis in each a-b plane. Inelastic neutron scattering was used to measure the dispersion of magnetic excitations in both ordered magnetic phases. The form of the dispersion was very similar in both cases and was adequately fitted to linear spin-wave theory, giving exchange parameters ${\mathit{J}}_1$=-0.198±0.002 THz, ${\mathit{J}}_2$=-0.0010±0.0001 THz, and single-ion anisotropy D=-0.000 50±0.000 02 THz. There were, however, several deviations that suggest that linear spin-wave theory does not give a complete description of the magnetic excitations in this class of magnet.