Low-temperature magnetic structure and EPR properties of the quasi-1D S = 1/2 Heisenberg helimagnet CuCl₂·2NC₅H₅

We present results of electron paramagnetic resonance (EPR) and neutron diffraction studies of CuCl₂·2NC₅H, which is regarded as one of the best known uniform spin-1/2 Heisenberg antiferro-magnet chain systems. We revealed that at T_N = 1.12 K CuCl₂·2NC₅H₅ 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 zig-zag interchain interactions result in a noticeable geometrical frustration, also affecting the ordered moment per Cu²⁺ and EPR properties, including the angular dependence of the linewidth. The temperature dependencies of the EPR parameters are discussed.