Spin transition polymer with a large hysteresis around room temperature: optical response and electron paramagnetic resonance

A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.