Magnetic properties of $FeCl_3$ investigated by ${}^{57}Fe$ NMR

$FeCl_3$ crystallizes in a hexagonal layered structure and has a spiral antiferromagnetic phase below $T_N=8.8 K$, where the spins are spirally ordered with $2\pi/15$ period along [140] direction. It is reported that the magnetic phase changes to an order with two sublattices above $1.5 T$ and there is a spin- flop transition above $4.0 T$. We observed ${}^{57}Fe$ ferromagnetic nuclear magnetic resonance(NMR) signal for $FeCl_3 $ for the first time at low temperature. The sublattice magnetization $M(T)$ obtained from the temperature dependence of the NMR frequency is well fit by the Bloch's $T^2$ law for antiferromagnets. The spectral change with external field provided a microscopic evidence for the spin-flop transition. The transition was not abrupt but progressive with increasing field.