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.

Magnetic systems having several different kinds of magnetic ions and/or competition among spin interactions frequently show canted spin order, frustration, or spin structure more complex than simple ferromagnets and antiferromagnets. It is difficult to understand its physical properties with simple theory or by the macroscopic measurement such as magnetization. Ferric chloride (FeCl3) is one such complex magnetic system having helical spin order with an uncommon period of 15. Another interesting fact about FeCl3 is that it was reported to undergo quantum phase transitions in magnetic field. FeCl3 has a hexagonal layered crystal structure  that is isomorphic with BiI3 and CrBr3. Iron ions located at the center of the chlorine octahedron form honeycomb layers that are stacked up along the c-axis with a slight shift in the ab-plane direction so that every third layer ends up on top of the bottom one. FeCl3 undergoes phase transition from paramagnetic to antiferromagnetic phase with decrease of temperature to around 9 K. The antiferromagnetic phase is not a simple collinear type but a helical structure with its rotation axis along the  direction.

There was an interesting report that the spin structure of FeCl3 changes in a magnetic field. It was claimed that the spins have a double cone heliconical order instead of a 15 period helix in the ground state when an external magnetic field with its direction along the c-axis increases to over 1.5 T. The spins are canted from the c-axis by about 54°, and the projection of one spin to the axis is parallel while the other is antiparallel to the field. When the external field increases to over 4 T, the spin state changes once more to the so-called spin flop state, in which the spin direction is perpendicular to the magnetic field. Supporting evidence for these results was found in the susceptibility measurement, which shows some changes around 1.5 T and 4 T. A change of the susceptibility near 1.5 T is observed, but even near 20 K, which is well above the transition temperature. The change of susceptibility around 4 T showed some characteristics of a first-order phase transition, but experimental evidence for the phase transition is not strong because the change was observed in a wide range of magnetic fields. The physics underlying these changes seems quite unclear.