Collapse of Magnetic Order of the Quasi One-Dimensional Ising-Like Antiferromagnet BaCo2V2O8 in Transverse Fields

We have performed magnetization measurements of the quasi one-dimensional (1D) $S = 1/2$ Ising-like anti ferromagnet BaCo 2 V 2 O 8 in transverse fields. Different from the expectation by the 1D $S = 1/2$ $XXZ$ model, a steep suppression of the antiferromagnetic order is observed for $H\parallel[100]$, whereas the experimental results for $H\parallel[110]$ agree with the $XXZ$ model. We show that these anisotropic behaviors are caused by a characteristic screw chain structure in BaCo 2 V 2 O 8 , which gives rise to a peculiar angular dependence of effective fields upon application of uniform external magnetic fields.