  Spin-1/2 compounds A3Cu3AlO2(SO4)4 (A=K, Rb, and Cs) have one-dimensional
(1D) inequilateral diamond chains. We analyze the temperature dependence of the
magnetic susceptibility and determine the magnetic exchange interactions. In
contrast to the azurite, a dimer is formed on one of the sides of the diamond.
From numerical analyses of the proposed model, we find that the dimer together
with a nearly isolated 1D Heisenberg chain characterizes magnetic properties
including magnetization curve and magnetic excitations. This implies that a
dimer-monomer composite chain without frustration is a good starting point for
describing these compounds.
