Antiferromagnetic Ordering Driven by the Molecular Orbital Order of ${\mathrm{C}}_{60}$ in ${\alpha }^{\prime }$-Tetra-Kis-(Dimethylamino)-Ethylene-${\mathrm{C}}_{60}$

We have studied the ground state of a fullerene-based magnet, the ${\alpha }^{\prime }$-phase tetra-kis-(dimethylamino)-ethylene-${\mathrm{C}}_{60}$ (${\alpha }^{\prime }$-TDAE-${\mathrm{C}}_{60}$), by electron spin resonance and magnetic torque measurements. Below $T_N=7\mathrm{K}$, 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 ${\alpha }^{\prime }$-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 antiferromagnetic ground state driven by the cooperative orientational ordering of ${\mathrm{C}}_{60}$ in the ${\alpha }^{\prime }$-TDAE-${\mathrm{C}}_{60}$.

Figure 1

(a) Angular dependence of magnetic torque signals at $T=1.5\mathrm{K}$. The inset shows the torque at 7 K (paramagnetic) and 1.5 K for $\theta =-45°$. The torque curve at 7 K is proportional to the square of magnetic field strength. (b) Angular dependence of peak in the torque signal as a function of $\theta$. The inset shows the angles between the magnetic field and the crystal axis.

Figure 2

(a) Temperature dependence of ESR spectra for $H\parallel a$ at 9 GHz region. Three peaks ($P$, $A$, and $F$) are observed, which correspond to the EPR, AFMR, and (extrinsic) FMR signals, respectively. (b) Temperature dependence of EPR and AFMR intensity for three crystallographic axes. (c) Angular dependence of AFMR signals within the $ab$ plane at 2.86 GHz. Each spectrum was taken every 10°. (d) Frequency ($\omega /|\gamma |$)-field relation at $T=1.5\mathrm{K}$. Solid line is the calculation with the easy-plane anisotropy along the $a$ axis.

Figure 3

(a) Low-temperature structure and (b) possible antiferromagnetic (AF) structure with molecular orbital order. Balls represent one of the ${\mathrm{C}}_{60}$ $t_{1u}$ orbitals. Blue and orange balls correspond to opposite spins.