Magnetic phases ofCsCuCl3: Anomalous critical behavior

We report high-resolution measurements of the specific heat of the hexagonal antiferromagnet ${\mathrm{CsCuCl}}_3$ with ferromagnetically coupled ${\mathrm{Cu}}^{2+}$ chains in magnetic fields parallel to the hexagonal axis. The zero-field data can be described assuming a second-order phase transition with a chiral XY critical exponent α near 0.35, except very close to the Néel temperature where the transition seems to be of weakly first order. This might be an indication for the failure of XY chiral universality. The previously observed first-order transition from an ‘‘umbrella’’ to a coplanar spin structure at high fields merges the transition line to the paramagnet in a multicritical point (${\mathit{T}}_{\mathit{c}}$=10.59 K, ${\mathit{B}}_{\mathit{m}}$=5.5 T). The transition to the new coplanar structure from the saturated paramagnetic phase above ${\mathit{B}}_{\mathit{m}}$ seems, rather unexpectedly, also to be describable as a second-order transition with α≊0.23. © 1996 The American Physical Society.