Magnetization plateaux in a generalized ladder model

A spin-12 antiferromagnetic generalized ladder model is constructed which consists of four-spin plaquettes, coupled through weaker exchange interactions, to two-spin rungs. In an extended parameter regime, the exact ground state of the ladder is determined. In this state, the four-spin plaquettes and the rungs are in their ground-state spin configurations. In the presence of an external magnetic field, the magnetization/site has a plateau structure as a function of the magnetic field.