Upward-renormalization of fluctuations in CsMnBr3 with increasing temperature

The temperature dependence of the spin-wave gap in the triangular antiferromagnet CsMnBr3 was studied above the three-dimensional ordering temperature TN = 8.3 K along the main symmetry directions using inelastic neutron scattering. We find at TN two gapped dispersive modes, whose energy increases with temperature. Moreover, the width of the spin-wave band along the [110] direction increases also. In a second session, polarization analysis was applied in order to extract explicitly the components with in-plane and out-of-plane character. The results show that both gapped modes (with and symmetry) renormalize upwards with rising temperature. We show that this behavior is not compatible with spin-wave theory. In addition, we find a new magnetic anomaly in the paramagnetic phase. ©2003 The American Physical Society