Metamagnetic phase transition in a diruthenium compound with interpenetrating sublattices

The diruthenium compound Ru₂(O₂CMe)₄]₃Cr(CN)₆] may be the only known material that contains two weakly-coupled, magnetically-ordered sublattices occupying the same three-dimensional volume. Due to the strong easy-plane anisotropy on each Ru₂ complex, the moment of each sublattice is constrained to one of the eight cubic diagonals. At low fields, the two sublattices are antiferromagnetically aligned by weak dipolar and deformation energies. But above a metamagnetic critical field of about 1000 Oe, the sublattice moments become ferromagnetically aligned and the net magnetization increases dramatically. We have successfully modeled this metamagnetic transition by assuming that the individual sublattice spin configurations are only weakly distorted by the magnetic field. This model suggests that the ground state of each sublattice undergoes a phase transition at a pressure of about 7 kbar. The drop in the sublattice moment and the rise in the sublattice susceptibility above 7 kbar can be explained by a high- to low-spin transition (S = 3/2 to 1/2) on the mixed-valent diruthenium complexes.