MAGNETIC STUDIES OF THE 2-DIMENSIONAL, S=1 2 HEISENBERG ANTIFERROMAGNETS (5CAP)(2)CUCL4 AND (5MAP)(2)CUCL4/

The magnetic materials (5CAP)(2)CuCl4 and (5MAP)(2)CuCl4 form effectiv e square planar lattices of CuCl42- anions. The Cu2+ ions have weak an tiferromagnetic interactions in the plane through Cl-Cl contacts. Thes e planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)(2)CuC l4 and J=0.38 K for (5MAP)(2)CuCl4. chi(T,H=0) for these materials has a rounded maximum at T-peak=1.1 K for (5CAP)(2)CuCl4 and T-peak = 0.6 2 K for (5MAP)(2)CuCl4. A kink in chi(T,H=0) for the (5MAP)(2)CuCl4 su ggests a phase transition to three-dimensional long range order at T-C =0.44 K. Measurements of chi(T,H) show that the saturation field H-C f or the transition to a field induced saturated state is 3.78 T for (5C AP)(2)CuCl4 and 2.17 T for (5MAP)(2)CuCl4. Thus these materials are ex cellent model systems in which to explore the full H-T phase diagram o f the two-dimensional S=1/2 Heisenberg antiferromagnet. (C) 1997 Ameri can Institute of Physics.