Spin Dynamics in the One-Dimensional Antiferromagnet(CD3)4NMnCl3

Abstract

Recent studies of the instantaneous magnetic correlations in ${(\mathrm{C}{\mathrm{D}}_{3})}_{4}$NMn${\mathrm{Cl}}_{3}$ using quasielastic-neutron-scattering techniques have shown that the Mn${\mathrm{Cl}}_{3}$ chains in this compound exhibit purely one-dimensional paramagnetic behavior down to 1.1 \ifmmode^\circ\else\textdegree\fi{}K. The interactions between ${\mathrm{Mn}}^{2+}$ ions along the chain are such that a molecular field theory would predict an ordering at \ensuremath{\sim} 76 \ifmmode^\circ\else\textdegree\fi{}K. It was found that both the spatial and thermal variation of the instantaneous correlations could be quantitatively accounted for using Fisher's theory for the classical Heisenberg linear chain. In this paper we report a detailed study of the time-dependent magnetic correlations in ${(\mathrm{C}{\mathrm{D}}_{3})}_{4}$ NMn${\mathrm{Cl}}_{3}$ using inelastic---neutron-scattering techniques. It is bound that at low temperatures, for $q\ensuremath{\gg}\ensuremath{\kappa}$ and $\ensuremath{\omega}\ensuremath{\ne}0$, the Van Hove scattering function $\mathcal{S}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}, \ensuremath{\omega})$ may be accurately described by spin-wave theory with a dispersion relation $\ensuremath{\hbar}\ensuremath{\omega}=\frac{6.1}{sin\ensuremath{\pi}{q}_{{c}^{*}}^{z}}$ meV over the entire one-dimensional Brillouin zone, even though there is no long-range order. As the temperature is increased from 1.9 to 40 \ifmmode^\circ\else\textdegree\fi{}K these "spin waves" typically weaken in intensity and broaden asymmetrically, with the scattering increasing on the low-energy side. In no case were both well-defined spin waves and a central diffusive component observed simultaneously, although the latter, if weak, could have been masked by the large incoherent scattering.