Importance of In-Plane Anisotropy in the Quasi Two-Dimensional   Antiferromagnet BaNi$_{2}$V$_{2}$O$_{8}$
  The phase diagram of the quasi two-dimensional antiferromagnet BaNi$_{2}$V$_{2}$O$_{8}$ is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to $\mathbf{c}$. At $\mu_0H^{*}\simeq1.5$ 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. 
W. Knafo, C. Meingast, K. Grube, S. Drobnik, P. Popovich, P. Schweiss,   P. Adelmann, Th. Wolf, and H. V. L\"ohneysen