Cs₃Cr₂Br₉ (Cr³⁺: s = 3/2) provides an interesting example of an interacting spin-dimer system. In zero field, the S = 0 singlet ground state is separated from the first excited triplet state by an energy gap of 0.2 meV. In a magnetic field, because of the Zeeman splitting of the S = 1 state, a phase transition occurs at a critical field H_c1 ~ 1.5 T, for T < 500 mK, when the lowest gap closes. Above Hc1, the field-induced magnetic ordering is due to the inter-dimer couplings and concerns the spin components perpendicular to the applied field.

The magnetic structure of Cs₃Cr₂Br₉ has been determined for H = 6 T applied in the hexagonal (a, b) plane (H // a-b).

The transverse ordering is a commensurate + + - - structure, described by the propagation wave vector k = (1/4, 1/4, 0), with the Cr³⁺ magnetic moments of 1.60(1) μB parallel to the dimers (along the c-axis). A small parallel magnetisation of 0.37(3) μB/Cr³⁺ also appears. The field dependence between H_c1 and 6 T of the magnetic superlattice peaks reveals large hysteresis and incommensurabilities, accounted for by frustration effects.

More details in: Grenier B., Inagaki Y., Regnault L.P., Wildes A., Asano T., Ajiro Y., Lhotel E., Paulsen C., Ziman T., Boucher J.P. - Ordering and excitations in the field-induced magnetic phase of Cs₃Cr₂Br₉ , Physical Review Letters 92, 177202-1-177202-4 (2004).