Magnetization plateaus in two-dimensional spin systems

We study the magnetization process in two-dimensional spin systems and disc uss the appearance of a plateau structure. The following three systems are considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange m odel on the triangular lattice, (2) Shastry-Sutherland-type lattice [which is a possible model for SrCu2(BO3)(2)], and (3) 1/5-depleted lattice (for C aV4O9). We find in these systems that magnetization plateaus appear owing t o insulator-superfluid transitions of magnetic excitations, where the plate au state has insulating CDW order. (C) 2000 Elsevier Science B.V. All right s reserved.