Neutron-Scattering Evidence for the Spin State of a Molecule-Based Magnet with Interpenetrating Sublattices*

The molecule-based magnet [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] contains two interpenetrating cubic sublattices. Each sublattice is magnetically frustrated by the easy-plane anisotropy of the spin-3/2 diruthenium (II/III) paddlewheel complexes, which lie at the middle of each cube edge and are antiferromagnetically coupled by the exchange interaction J$_c \sim $ 1.7 meV to two spin-3/2 Cr(III) ions at the cube corners. Symmetry considerations suggest that each cubic sublattice has a non-collinear spin state with net moment along one of the cubic diagonals. The moments of the two interpenetrating sublattices are antiferromagnetically coupled at small magnetic fields and become aligned above a critical field of about 1000 Oe $\sim $ K$_c$/$\mu_B$, where K$_c \sim $ 2 x 10$^{-3}$ meV is the weak dipolar coupling between sublattices. Powder neutron-diffraction measurements on a deuterated sample confirm that the sublattice moments lie along the cubic diagonals and provide indications for substantial quantum corrections to the spin state of each sublattice.