Magnetic properties of the S = 1/2 Heisenberg spin-chain material (6MAP)CuCl₃

We report on the magnetic properties of the S = 1/2 Heisenberg spin-chain material (6MAP)CuCl₃. The magnetic susceptibility exhibits a pronounced maximum at T ≈ 70 K, which is a signature of low-dimensional magnetic interactions in (6MAP)CuCl₃, following by a pronounced Curie-like behavior at low temperatures. In order to understand the nature of low-temperature magnetic interactions, the excitation spectrum of (6MAP)CuCl₃ is probed using the electron spin resonance (ESR) technique. The angular dependence of the g-factor clearly indicates the existence of two magnetically non-equivalent sites of Cu-ions in neighboring chains. The ESR signal exhibits a significant linebroadening with decreasing temperature (starting at approximately 30 K), accompanied by a pronounced shift of the g-factor. The observed features can be explained by enhanced short-range-order spin correlations when approaching the long-range ordering at low temperature. Our suggestion is confirmed by the observation of a lowtemperature (T ~ 2.3 K) anomaly in the specific heat. The anomaly measured in field up to 14 T does not exhibit any significant dependence on the applied magnetic field.