Magnetization plateaus in weakly coupled dimer spin system

I study a spin system consisting of strongly coupled dimers which are, in t urn, weakly coupled in a plane by zigzag interactions. The model can be vie wed as the strong-coupling limit of a two-dimensional zigzag chain structur e typical, e.g., for the (ac) planes of KCuCl3. It is shown that the magnet ization curve in this model has plateaus at 1/3 and 2/3 of the saturation m agnetization, and an additional plateau at 1/2; the critical fields are cal culated perturbatively. It is argued that for the three-dimensional lattice structure of the KCuCl3 family the plateaus at 1/4 and 3/4 of the saturati on can be favored in a similar way, which might be relevant to the recent e xperiments on NH4 CuCl3 by Shiramura er at, J. Phys. Sec. Jpn. 67, 1548 (19 98). [S0163-1829(99)07705-X].