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

The magnetization process of an S = 1/2 antiferromagnet on the Kagomé lattice, [Cu₃(titmb)₂(OCOCH ₃)₆] · H₂O{titmb = 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6 trimethylbenzene}, has been measured at very low temperatures 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 experiments. A theoretical analysis using exact diagonalization yields J₁ = -19 ± 2 K and J₂ = 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.