Inelastic neutron scattering in the weakly coupled triangular spin tube candidate CsCrF4

We performed inelastic neutron scattering (INS) experiments to measure spin dynamics in a polycrystalline sample of the spin tube candidate CsCrF4. The compound exhibits a successive phase transition from a paramagnetic phase through an intermediate-temperature (IT) phase of a 120 degrees structure to a low-temperature (LT) phase of another 120 degrees structure. An elaborate comparison of observed and calculated neutron spectra in the LT phase reveals that the spin Hamiltonian is identified as antiferromagnetic spin tubes including perturbative terms of intertube interaction, Dzyaloshinskii-Moriya interaction, and single-ion anisotropy. A phase diagram for the ground state is classically calculated. A set of parameters in the spin Hamiltonian obtained from the INS spectra measured in the LT phase is quite close to the boundary of the phase of the 120 degrees structure of the IT phase. The INS spectra measured in the IT phase are, surprisingly, the same as those in the LT phase on the level of powder-averaged spectra, even though the magnetic structures in the IT and LT phases are different. Identical dynamical structures compatible with two different static structures are observed. No difference in the observed spectra indicates no change in the spin Hamiltonian with the temperature, suggesting that the origin of the successive phase transition is a spin-entropy-driven mechanism in the spin system located near the phase boundary.