  The magnetic susceptibility and magnetization of two new layered S=1/2
Heisenberg antiferromagnets with moderate exchange are reported. The two
isostructural compounds, (2-amino-5-chloropyridinium)2CuBr4 ((5CAP)2CuBr4) and
(2-amino-5-methylpyridinium)2CuBr4 ((5MAP)2CuBr4), contain S=1/2, Cu(II) ions
related by C-centering, yielding four equivalent nearest neighbors. The crystal
structure of newly synthesized compound, (5CAP)2CuBr4, 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 be 8.5(1) K, and 6.5(1) K and ordering temperatures of
5.1(2) K and 3.8(2) K for (5CAP)2CuBr4 and (5MAP)2CuBr4 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)2CuBr4 at 18.8 T and in (5CAP)2CuBr4 at 24.1 T. The magnetization curves
are consistent with recent theoretical predictions. Single crystal
magnetization measured at 2.0 K indicate a spin flop transition at 0.38 T and
0.63 T for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively.
