Importance of in-plane anisotropy in the quasi-two-dimensional antiferromagnet BaNi2V2O8

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi2V2O8 is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At mu H-0(*)similar or equal to 1.5 T, a crossover to a high-field state, where T-N(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T-N and H-* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.