Critical behavior of the three-dimensional Heisenberg antiferromagnetRbMnF3

Abstract

The magnetic critical scattering of the near-ideal three-dimensional Heisenberg antiferromagnet (AF) ${\mathrm{RbMnF}}_{3}$ has been remeasured using neutron scattering. The critical dynamics has been studied in detail in the temperature range ${0.77T}_{N}<T<{1.11T}_{N},$ where ${T}_{N}$ is the N\'eel temperature. In agreement with previous measurements, at ${T}_{N}$ and for wave vectors away from the AF zone center, the scattering has a quasielastic component in addition to the inelastic response predicted by renormalization-group and mode-coupling theories. Both components scale with the dynamic exponent $z=1.43\ifmmode\pm\else\textpm\fi{}0.04$, in agreement with dynamic scaling. On cooling below ${T}_{N}$ the inelastic peaks transform into the transverse spin waves and a crossover has been observed in the dispersion from a power-law relation ${\ensuremath{\omega}}_{q}{=Aq}^{z}$ at ${T}_{N}$ to a linear behavior ${\ensuremath{\omega}}_{q}=cq$ in the hydrodynamic region below ${T}_{N}.$ The quasielastic component evolves below ${T}_{N}$ into the longitudinal susceptibility identified in an earlier polarized neutron experiment. The intensity and energy width of the longitudinal scattering decrease on cooling below ${T}_{N}.$ Down to the lowest temperatures where the longitudinal susceptibility could be measured the leading term in the scaling behavior of the energy width was ${\ensuremath{\gamma}}_{q}\ensuremath{\approx}{q}^{1.58\ifmmode\pm\else\textpm\fi{}0.03}$ (hydrodynamic theory predicts a ${q}^{2}$ law). Possible explanations for the observed behavior of the longitudinal susceptibility are discussed.