Antiferromagnetic order driven by the molecular orbital order of C${}_{60}$ in ${\alpha }^{\prime }$--tetra--$kis$--(dimethylamino)--ethylene--C${}_{60}$

Abstract

We have studied the ground state of a fullerene--based magnet, the ${\alpha }^{\prime }$--phase tetra--$kis$--(dimethylamino)--ethylene--C${}_{60}$ (${\alpha }^{\prime }$--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 ${\alpha }^{\prime }$--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 ${\alpha }^{\prime }$--TDAE--C${}_{60}$.

4 pages, 3 figures