Antiferromagnetic ordering driven by the molecular orbital order of C-60 in alpha '-tetra-kis-(dimethylamino)-ethylene-C-60

We have studied the ground state of a fullerene-based magnet, the alpha'-phase tetra-kis-(dimethylamino)ethylene-C-60 (alpha'-TDAE-C-60), by electron spin resonance and magnetic torque measurements. Below T-N = 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 alpha'-phase ground state. Below T-N, 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 T-N indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferro-magnetic ground state driven by the cooperative orientational ordering of C-60 in the alpha'-TDAE-C-60.