The growth of single crystals of FeCl₃, through sublimation and from the melt, is presented alongside a thorough investigation of their magnetostructural properties through a combination of DC magnetization and AC magnetic susceptibility measurements, single crystal X-ray diffraction (SCXRD), neutron powder diffraction (NPD) and small-angle neutron scattering (SANS). A new chiral polymorph of FeCl₃ is identified, crystallizing in the non-centrosymmetric space group P3₁. NPD and SANS reveal that a weakly first-order magnetic phase transition occurs from a paramagnetic phase with significant short-range correlations to an antiferromagnetic phase at T_N = 8.6 K, best described by the magnetic propagation vector k = (1/2, 0, 1/3) which differs from the previously reported magnetic structure of the well-known centrosymmetric polymorph (space group R3̄). We show that disordered crystallographic models including a large number of stacking faults are required to accurately reproduce the scattering observed in NPD patterns, preventing full determination of the magnetic structure. The magnetic field and temperature-dependent behavior of the intensities of the k = (1/2, 0, 2/3) and (1/2, 0, 5/3) magnetic Bragg peaks measured by SANS suggest that a field-induced spin reorientation occurs at H = 40 kOe when H‖c-axis and at a significantly lower field of H ≈ 25 kOe when H⊥c-axis. Above these magnetic fields in both cases the spins lie predominantly in the basal plane. The long-range magnetic ordering and the field-induced transitions observed in the neutron scattering experiments coincide with anomalies observed in the magnetisation versus both temperature and applied field along the principal crystal directions.

We present a comprehensive investigation of the magnetic properties of stage-1 graphite intercalated FeCl3 using a combination of DC and AC magnetic susceptibility, thermoremanent magnetization, and field-dependent magnetization measurements. This van der Waals system, with a centrosymmetric honeycomb lattice, combines frustration and disorder, due to intercalation, and may be hosting topologically nontrivial magnetic phases. Our study identifies two magnetic phase transitions at Tf1≈4.2 K and at Tf2≈2.7 K. We find that the paramagnetic state, for T>Tf1, is dominated by short-range ferromagnetic correlations. These build up well above Tf1 and lead to a significant change in magnetic entropy, which reaches ΔSMPk=-5.52 J kg-1K-1 at 7 T. Between Tf1 and Tf2, we observe slow spin dynamics characteristic of a cluster glasslike state, whereas for T<Tf2, our results indicate the onset of a low-temperature long-range ordered state. The analysis of the experimental results leads to a complex phase diagram, which may serve as a reference for future investigations searching for topological nontrivial phases in this system.