Spin dynamics in the singlet-singlet ground state magnet LiTbxY1−xF4; a neutron scattering study

A neutron inelastic scattering study is presented of the spin dynamics at and above Tc in the singlet-singlet ground state ferromagnet LiTbxY1-xF4 for x=1.0 and x=0.6. Above Tc the dynamics are found to be very well described by an overdamped relaxation-coupled oscillator (RCO) lineshape with the frequencies of magnetic excitons given by the mean-field dispersion relation. The damping parameter is found to be much greater in the pure material than in the dilute material. At Tc the RCO lineshape is found not to fit the data for small q, rather a Lorentzian function has been used to extract a characteristic frequency. In the pure system the value for the dynamical critical exponent z is found to be 0.79+or-0.26. In the dilute system an extra component to the scattering is found to obscure the asymptotic critical behaviour for small q. However for larger q a power-law dependence of the characteristic frequency is found with exponent 0.8+or-0.1.