We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-$\frac{1}{2}$ $\mathrm{XY}$-like antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CoCl}}_4$ using single-crystal neutron diffraction. In zero-field, interchain couplings cause long-range order below $T_N=217\mathrm{}\left(5\right)\mathrm{}\hspace{0.5em}\mathrm{mK}$ with chains ordered antiferromagnetically along their length and moments confined to the $\mathrm{}(b,c)\mathrm{}$ plane. Magnetic fields applied at an angle to the $\mathrm{XY}$ planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the $\mathrm{}(b,c)\mathrm{}$ plane, proposed to be a spin-liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region $2.10<\mathrm{}{H}_{\mathrm{SL}}<2.52\mathrm{}\hspace{0.5em}\mathrm{T}\mathrm{}\Vert a.\mathrm{}$ The observed phase diagram is discussed in terms of known results on $\mathrm{XY}$-like chains in coexisting longitudinal and transverse fields.

• Received 18 October 2001

DOI:https://doi.org/10.1103/PhysRevB.65.144432