Magnetic properties of TDAE-${\mathrm{C}}_{60}$ and TDAE-${\mathrm{C}}_{70}$, where TDAE is tetrakis(dimethylamino)ethylene

Magnetic properties of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ complexes with tetrakis(dimethylamino)ethylene (TDAE) have been studied based on the measurements of the Faraday-balance magnetization and the electron-spin resonance (ESR). It has been found that both TDAE-${\mathrm{C}}_{60}$ and TDAE-${\mathrm{C}}_{70}$ possess about one radical spin per fullerene molecule. TDAE-${\mathrm{C}}_{60}$ has been confirmed to show a ferromagnetic-type transition at ${\mathit{T}}_{\mathit{c}}$=16.7–17.5 K. Appearance of a broad and intense ESR signal below ${\mathit{T}}_{\mathit{c}}$ for TDAE-${\mathrm{C}}_{60}$ suggests that the radical spins in the magnetically ordered state are rather localized on each ${\mathrm{C}}_{60}$ molecule and are ferromagnetically correlated. On the other hand, TDAE-${\mathrm{C}}_{70}$ does not show ferromagnetism at temperatures above 4.5 K.