Low-temperature magnetic structure and electron paramagnetic resonance properties of the quasi-one-dimensional S=12 Heisenberg helimagnet CuCl2·2NC5H5

We present results of electron paramagnetic resonance (EPR) and neutron-diffraction studies of ${\mathrm{CuCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{NC}}_{5}{\mathrm{H}}_{5}$, which is regarded as one of the best known uniform spin-1/2 Heisenberg antiferromagnet chain systems. We reveal that at ${T}_{N}=1.12$ K, ${\mathrm{CuCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{NC}}_{5}{\mathrm{H}}_{5}$ undergoes the transition into the magnetically ordered spiral state with $Q=(0.5,0.4,0.5)$ r.l.u. It was shown that the zigzag interchain interactions result in a noticeable geometrical frustration, also affecting the ordered moment per ${\mathrm{Cu}}^{2+}$ and EPR properties, including the angular dependence of the linewidth. The temperature dependencies of the EPR parameters are discussed.