High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene $\left(\mathrm{TDAE}\right)-{\mathrm{C}}_{60}$ 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 ${\mathrm{TDAE}}^{+}$ 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.

• Received 1 July 2002

DOI:https://doi.org/10.1103/PhysRevB.68.144430