Magnetic studies of the two-dimensional, S=1/2 Heisenberg antiferromagnets (5CAP)2CuCl4 and (5MAP)2CuCl4

Abstract
The magnetic materials (5CAP)2CuCl4 and (5MAP)2CuCl4 form effective square planar lattices of CuCl42− anions. The Cu2+ ions have weak antiferromagnetic interactions in the plane through Cl–Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)2CuCl4 and J=0.38 K for (5MAP)2CuCl4. χ(T,H=0) for these materials has a rounded maximum at Tpeak=1.1 K for (5CAP)2CuCl4 and Tpeak=0.62 K for (5MAP)2CuCl4. A kink in χ(T,H=0) for the (5MAP)2CuCl4 suggests a phase transition to three-dimensional long range order at TC=0.44 K. Measurements of χ(T,H) show that the saturation field HC for the transition to a fieldinduced saturated state is 3.78 T for (5CAP)2CuCl4 and 2.17 T for (5MAP)2CuCl4. Thus these materials are excellent model systems in which to explore the full H–T phase diagram of the two-dimensional S=1/2 Heisenberg antiferromagnet.