Antiferromagnetic ordering driven by the molecular orbital order of C60 in αâ€-Tetra-Kis-(Dimethylamino)-Ethylene-C60

We have studied the ground state of a fullerene-based magnet, the αâ€-phase tetra-kis-(dimethylamino)-ethylene-C60 (αâ€-TDAE-C60), by electron spin resonance and magnetic torque measurements. Below TN=7  K, nonparamagnetic field dependent resonances with a finite excitation gap (1.7 GHz) are observed along the a axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the αâ€- phase ground state. Below TN, nonquadratic field dependence of the magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle-dependent torque signals below TN indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferromagnetic ground state driven by the cooperative orientational ordering of C60 in the αâ€-TDAE-C60.