Paramagnetic spin waves in the one-dimensional antiferromagnet CsMnBr3

CsMnBr3 is a quasi-one-dimensional hexagonal antiferromagnet. We report on inelastic neutron scattering measurements that show well-defined spin-wave modes at a temperature of 15 K, which is almost twice the Néel temperature for three-dimensional ordering. The spin waves at large wave vectors have similar energies to those found in the three-dimensional ordered state at 4 K. At small wave vectors a mode is observed with energy 1.8 meV at zero wave vector, which is not seen in the three-dimensional state. The presence of dipolar interactions explains this effect qualitatively but not quantitatively. Measurements are also presented showing the effects of raising the temperature on the spin waves. At small wave vectors there is an apparent renormalization upwards of the spin wave peaks with increasing temperature, as has been predicted theoretically.