Quantum Spin State and Magnetization Plateaus in anS=1KagoméHeisenberg Antiferromagnet

The organic Heisenberg antiferromagnet m -MPYNN·BF 4 has a two-dimensional Kagomé lattice of S = 1 dimers with a small trigonal distortion. This antiferromagnet indicates a nonmagnetic ground state with a gap of 0.2 K in zero field. To investigate the ground state of this quantum magnet, we measured the AC susceptibility and the magnetization as a function of the magnetic field. Magnetization plateaus in applied fields were sensitively observed from steep decrements in the AC susceptibility at low temperatures. They indicate characteristic behaviors to be expanded at finite temperatures. The magnetization curve indicates that the fractional plateaus are at 1/2 and 3/4 of the saturation magnetization. The observed plateaus are not commensurate with the unit cell of the Kagomé lattice, suggesting a contribution of long-distance interactions.