Structural and Magnetic Properties of [(CH₃)₃NH] CuCl₃2H₂O

The crystal structure at room temperature and the low-temperature heat capacity and magnetic susceptibilities of single crystals of [(CH3)3 NH]CuCl3 2H2O are reported. The monoclinic crystals belong to the space group P21c with a=7.479(10), b=7.864(11), c=16.730(23), =91.98(3)°, and Z=4. The structure consists of chains of edge-sharing [CuCl4(OH2)2] octahedra running along the a axis. Each copper atom is coordinated in a square plane by two water molecules and two chlorine atoms, with two more chlorine atoms at a longer distance. The susceptibility data can be fitted adequately between 1.5 and 20°K by a CurieWeiss law [ga=2.080.01, gb=2.020.01; gc=2.110.01; a=(0.380.03)°K, b=(0.360.03)°K, c=(0.410.03)°K], but the susceptibility parallel to the chain can be better fitted as an Ising linear chain. Measurements perpendicular to the chain are not as well fitted by the Ising model. The heat capacity, which consists primarily of a lattice contribution above 3°K, begins to rise as the temperature falls below 3°K, but long-range order does not set in above 1°K, the lowest temperature attained in this work. Both Ising and Heisenberg linear-chain models fit the low-temperature data with |Jk| \u3c1°K. The results are compared with those reported for the compounds CuCl2 2H2O and CuCl2 2NC5H5. © 1972 The American Physical Society

Order to disorder transition in the XY-like quantum magnet Cs2CoCl4 induced by noncommuting applied fields

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the 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 (b,c) 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 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.Comment: revtex, 14 figures, 2 tables, to appear in Phys. Rev.