Two-dimensional $S=\frac{1}{2}$ Heisenberg antiferromagnets: Synthesis, structure, and magnetic properties

The magnetic susceptibility and magnetization of two layered $S=\frac{1}{2}$ Heisenberg antiferromagnets with moderate exchange are reported. The two isostructural compounds, $(2\mathrm{}-\mathrm{amino}-5-\mathrm{chloropyridinium}{)}_2{\mathrm{CuBr}}_4$ $\left[\right(5\mathrm{C}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4]$ and $(2\mathrm{}-\mathrm{amino}-5-\mathrm{methylpyridinium}{)}_2{\mathrm{CuBr}}_4$ $\left[\right(5\mathrm{M}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4],$ contain $S=\frac{1}{2},$ Cu(II) ions related by C centering, yielding four equivalent nearest neighbors. The crystal structure of the synthesized compound, $(5\mathrm{C}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4,$ shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the $\mathrm{ab}$ 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(2) K and 6.5(2) K and ordering temperatures of 5.1(2) K and 3.8(2) K for $(5\mathrm{C}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4$ and $(5\mathrm{M}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4,$ respectively. High-field magnetization experiments on both compounds show upward curvature of $M(H,T).$ Magnetization measurements made at $T=1.3\mathrm{}\hspace{0.5em}\mathrm{K}$ show saturation occurs in $(5\mathrm{M}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4$ at 18.8 T and in $(5\mathrm{C}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4$ 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 $(5\mathrm{C}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4$ and $(5\mathrm{M}\mathrm{A}\mathrm{P}{)}_2{\mathrm{CuBr}}_4,$ respectively.