Short- and long-range correlations in the S=1/2 ferromagnetic chain system (${\mathrm{C}}_6$${\mathrm{D}}_{11}$${\mathrm{ND}}_3$)${\mathrm{CuBr}}_3$

The short- and long-range spin correlations in the quasi-one-dimensional S=(1/2) system (${\mathrm{C}}_6$ ${\mathrm{D}}_{11}$ ${\mathrm{ND}}_3$)${\mathrm{CuBr}}_3$ (CHAB-D14) have been investigated by (quasi) elastic neutron scattering experiments. The temperature dependence of the sublattice magnetization below $T_N$=1.56 K reflects the presence of ferromagnetic chains in this compound. The intrachain spin correlations in the paramagnetic region show a crossover from isotropic Heisenberg behavior at high temperatures to XY behavior below ∼3 K, and are surprisingly well described by a classical spin model.