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We report comprehensive inelastic neutron scattering measurements of themagnetic excitations in the 2D spin-5/2 Heisenberg antiferromagnet Rb2MnF4 as afunction of temperature from deep in the Neel ordered phase up to paramagnetic,0.13 < kBT/4JS < 1.4. Well defined spin-waves are found for wave-vectors largerthan the inverse correlation length $\eta^{-1}$ for temperatures up to near theCurie-Weiss temperature, $\Theta_{CW}$. For wave-vectors smaller than$\eta^{-1}$, relaxational dynamics occurs. The observed renormalization ofspin-wave energies, and evolution of excitation line-shapes, with increasingtemperature are quantitatively compared with finite-temperature spin-wavetheory, and computer simulations for classical spins. Random phaseapproximation calculations provide a good description of the low-temperaturerenormalisation of spin-waves. In contrast, lifetime broadening calculatedusing the first Born approximation shows, at best, modest agreement around thezone boundary at low temperatures. Classical dynamics simulations using anappropriate quantum-classical correspondence were found to provide a gooddescription of the intermediate- and high-temperature regimes over allwave-vector and energy scales, and the crossover from quantum to classicaldynamics observed around $\Theta_{CW}/S$, where the spin S=5/2. Acharacterisation of the data over the whole wave-vector/energy/temperatureparameter space is given. In this, $T^2$ behaviour is found to dominate thewave-vector and temperature dependence of the line widths over a largeparameter range, and no evidence of hydrodynamic behaviour or dynamical scalingbehaviour found within the accuracy of the data sets.

journal of statistical mechanics

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