Two-dimensional S=1/2 Heisenberg antiferromagnets: Synthesis, structure, and magnetic properties

The magnetic susceptibility and magnetization of two layered S=½ Heisenberg antiferromagnets with moderate exchange are reported. The two isostructural compounds, (2-amino-5-chloropyridinium) 2 CuBr 4 [(5CAP) 2 CuBr 4 ] and (2-amino-5-methylpyridinium) 2 CuBr 4 [(5MAP) 2 CuBr 4 ], contain S=½, Cu(II) ions related by C centering, yielding four equivalent nearest neighbors The crystal structure of the synthesized compound, (5CAP) 2 CuBr 4 , shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to he 8.5(2) K and 6.5(2) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively. High-field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP) 2 CuBr 4 at 18.8 T and in (5CAP) 2 CuBr 4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single-crystal magnetization measured at 2.0 K indicates a spin-flop transition at 0.38 T and 0.63 T for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively.