Synthesis, structure, and magnetic properties of the two-dimensional quantum antiferromagnets (CuBr)A2B3O10 (A=Ca, Sr, Ba, Pb; B=Nb, Ta) with the 1/3 magnetization plateau

A 1/3 magnetization plateau was recently observed in the layered perovskite (CuBr)Sr2Nb3O10 with S=1/2 CuBr layers. In order to control the cell size and thus exchange parameters, we have prepared an isostructural series of (CuBr)A(2)B(3)O(10) (A=Ca, Sr, Ba, Pb; B=Nb, Ta) and investigated the magnetic properties. We found that the stability of the 1/3 magnetization plateau is systematically tunable by the lattice parameter a. A substantial role played by the superexchange pathway via BO6-BO6 in the magnetic properties is suggested in addition to that via Br. Specific heat and muon spin relaxation measurements show that the appearance of the plateau phase is accompanied by successive phase transitions in low fields-the first one is possibly a structural transition and the second one is a magnetic transition. Frustration may induce the structural distortion and stabilize the 1/3 plateau phase.