A study of the quantum classical crossover in the spin dynamics of the 2D S=5/2 antiferromagnet Rb2MnF4:: neutron scattering, computer simulations and analytic theories

We report comprehensive inelastic neutron scattering measurements of the magnetic excitations in the 2D spin-5/2 Heisenberg antiferromagnet Rb2MnF4 as a function of temperature from deep in the Neel ordered phase up to paramagnetic, 0.13 < k(B)T/4JS < 1.4. Well-defined spin waves are found for wavevectors larger than the inverse correlation length xi(-1) for temperatures up to near the Curie-Weiss temperature, Theta(CW). For wavevectors smaller than xi(-1), relaxational dynamics occurs. The observed renormalization of spin wave energies, and evolution of excitation lineshapes, with increasing temperature are quantitatively compared with finite-temperature spin wave theory and computer simulations for classical spins. Random phase approximation calculations provide a good description of the low temperature renormalization of spin waves. In contrast, lifetime broadening calculated using the first Born approximation shows, at best, modest agreement around the zone boundary at low temperatures. Classical dynamics simulations using an appropriate quantum classical correspondence were found to provide a good description of the intermediate and high temperature regimes over all wavevector and energy scales, and the crossover from quantum to classical dynamics observed around Theta(CW)/S, where the spin S = 5/2. A characterization of the data over the whole wavevector/energy/temperature parameter space is given. In this, T-2 behaviour is found to dominate the wavevector and temperature dependence of the linewidths over a large parameter range, and no evidence of hydrodynamic behaviour or dynamical scaling behaviour found within the accuracy of the datasets. An efficient and easily implemented classical dynamics methodology is presented that provides a practical method for modelling other semiclassical quantum magnets.