Order from disorder in the molecular-based spin ladder Cu(Qnx)Br$_{2}$

Copper quinoxaline dibromide is a molecular-based antiferromagnetic spin ladder in which Cu$_{2}$Br$_{4}$ dimers are linked into ladders by the quinoxaline molecules. The rung exchange occurs through the bridging bromide ions while the rail exchange occurs through the organic molecules [1]. It is possible to introduce randomness into this ladder by replacing bromide ions by chlorides, by replacing the quinoxaline molecules by the structurally equivalent 2,3-dimethylpyrazine, or by substituting diamagnetic cadmium ions into the copper sites. In all cases, order occurs at low temperatures ( 4 to 6 K) as evidenced by FC/ZFC studies, hysteresis and remnant magnetizations, as well as sharp anomalies in the susceptibility. [1] C. P. Landee \textit{et al}, \textit{Polyhedron }\textbf{22}, 2325-2329 (2003).