Polymer phase of the tetrakis(dimethylamino)ethylene-C₆₀ organic ferromagnet

High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene (TDAE)-C₆₀ single crystals and stability of the polymeric phase was established in the P-T parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at a temperature of 520 K, revealing an unexpectedly high thermal stability of the polymer. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The TDAE⁺ donor-related unpaired electron spins, formerly ESR silent, become active above a temperature of 320 K, which demonstrates that the magnetic properties are profoundly defined by miniscule reorientation of TDAE molecules.