Magnetic properties of crystals of the molecular complex between fullerene C60 and an organic donor $9,9'$-trans-bis(telluraxanthenyl)

The EPR and static magnetic susceptibility of the crystalline molecular complex between fullerene C60 and an organic donor $9,9'$-trans-bis(telluraxanthenyl) (BTX) have been measured as functions of temperature. At temperatures $T$ above $130$ K, the samples exhibit anomalously high magnetic susceptibility exceeding the values calculated under the assumption that each molecule bears one paramagnetic spin $1/2$. A very intense magnetic resonance signal is also observed in the samples in the region of high $g$ factors ($g>4.5$). This allows the suggestion that the samples under study possess ferromagnetism (or superparamagnetism). The EPR signal and magnetic susceptibility sharply decay almost to zero as the temperature decreases below $100$–$120$ K. It is supposed that electron transfer from donor molecules BTX to C60 molecules takes place at temperatures above $110$ K. This electron transfer generates electron spins in the system, whereas the anomalously high magnetism is due to ferromagnetic correlations in the system of these spins.