Observation of a transient magnetization plateau in a quantum antiferromagnet on the Kagome lattice

The magnetization process of an S = 1/2 antiferromagnet on the Kagome lattice, [Cu-3(titmb)(2)(OCOCH3)(6)] . H2O{titmb = 1, 3, 5-tris(imidazol-1-ylmethyl)-2.4.6 trimethyl-benzene}, has been measured at very low temperature's in both pulsed and steady fields. We have found a new dynamical behavior in the magnetization process. A plateau at one third of the saturation magnetization M-s appears in the pulsed-field experiments for intermediate sweep rates of the magnetic field and disappears in the steady-field experiment's. A theoretical analysis using exact diagonalization yields J(1) = 19+/-2 K and J(2) = 6+/-2 K. for the nearest-neighbor and second-nearest-neighbor interactions. respectively. This set of exchange parameters explains the very low saturation field and the absence of the plateau in the thermodynamic equilibrium as well as the two-peak feature in the magnetic heat. capacity observed by Honda et. al. (Z. Honda et al., J. Phys. Condens. Matter. 14 (2002) L625). Supported by numerical results we argue that a dynamical order by disorder phenomenon could explain the transient appearance of the M-s/3 plateau in pulsed-field experiments.