Magnetic state of FeCl3 investigated by NMR

The spin state of FeCl3 was measured by Nuclear Magnetic Resonance (NMR) as a function of temperature and magnetic field. The sublattice magnetization obtained from the 57Fe NMR spectrum fits well with the theoretical prediction for an antiferromagnet with a magnetic anisotropy field of less than 70 mT in the ab-plane. The field dependence of the 57Fe NMR spectrum shows that a spin rotation plane of helical order starts to align perpendicularly to the external field direction as the field increases from 0 and ends around 4 T with no phase transition. From the spin tilting angle analysis, we obtained the quantitative relation among the exchange coupling constants.