Magnetization plateaus in a solvable 3-leg spin ladder

We present a solvable ladder model which displays magnetization plateaus at fractional values of the total magnetization. Plateau signatures are also shown to exist along special lines. The model has isotropic Heisenberg inte ractions with additional many-body terms. The phase diagram can be calculat ed exactly for all values of the rung coupling and the magnetic field. We a lso derive the anomalous behavior of the susceptibility near the plateau bo undaries. There is good agreement with the phase diagram obtained recently for the pure Heisenberg ladders by numerical and perturbative techniques.