Ba 3 Mn 2 O 8 is a spin-dimer compound based on pairs of S =1, 3d 2 , Mn 5+ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material associated with closing the spin gap to the excited triplet states. Specific heat, magnetocaloric effect, and torque magnetometry measurements were performed in magnetic fields up to 32 T and temperatures down to 20 mK. For fields above H c1 ϳ 8.7 T, these measurements reveal a single magnetic phase for H ʈ c but two distinct phases ͑approximately symmetric about the center of the phase diagram͒ for H Ќ c. Analysis of the simplest possible spin Hamiltonian describing this system yields candidates for these ordered states corresponding to a simple spiral structure for H ʈ c and to two distinct modulated phases for H Ќ c. Both single-ion anisotropy and geometric frustration play crucial roles in defining the phase diagram.

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi2O6 in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below ∼100 K. This lattice modulation is characterized by a resolution limited wave vector qIC =(0,∼0.13,0) and its 2 nd and 3 rd harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.

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