Field-induced magnetic order of Cs₃Cr₂Br₉ and Cs₃Cr₂Cl₉

The magnetic dimer compounds Cs₃Cr₂Br₉ and Cs₃Cr₂Cl₉ are well characterized magnetic dimer compounds. In zero field the spin 3/2 moments are bound in an hexagonal lattice of magnetic singlets,¹⁾ but at finite magnetic fields there are transitions to field-induced magnetic phases. In the Bromide compound this transition occurs at sufficiently low fields, ∼ 1.5T, that neutron diffraction results can give details of the static and dynamic magnetic structure even in the field induced phase. High magnetic field measurements have been used to determine the equation of state up to saturation. In this paper we discuss different approaches to the calculation of the magnetization and transverse magnetic order in the two compounds. Compared to spin 1/2 dimer materials, such as TlCuCl₃, the higher spin allows for more important effects of anisotropy, and the frustration, leads to possibily incommensurate order. Two main theoretical methods will be discussed: extension of the standard theory of Tachiki and Yamada for spin 1/2, to include anisotropy and mixing of higher levels, and an approach based on Bose-Einstein Condensation for the onset of magnetic order. This description must include frustration that gives a continuous degeneracy in momentum space to the condensing bosons.