Antiferromagnetic order driven by the molecular orbital order of C$_{60}$ in $α'$--tetra--$kis$--(dimethylamino)--ethylene--C$_{60}$
We have studied the ground state of a fullerene--based magnet, the $α'$--phase tetra--$kis$--(dimethylamino)--ethylene--C$_{60}$ ($α'$--TDAE--C$_{60}$), by electron spin resonance (ESR) and magnetic torque measurements. Below T$_N=7$ K, non--paramagnetic 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 $α'$--phase ground state. Below T$_N$, non--quadratic field dependence of 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 antiferromagnetic ground state driven by the cooperative orientational ordering of C$_{60}$ in the $α'$--TDAE--C$_{60}$.