Anisotropic phase diagram of the frustrated spin dimer compound Ba ₃ Mn₂ O₈

Heat-capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba3 Mn2 O8 in the field range from 0 to 18 T. For fields oriented along the c axis, a single magnetically ordered phase is found in this field range whereas for fields oriented along the a axis, two distinct phases are observed. The present measurements reveal a surprising nonmonotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (Hc1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next-nearest Mn ions, the Zeeman energy, and single-ion anisotropy. This Hamiltonian also predicts a nonmonotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.