MAGNETIZATION INSTABILITY IN A 2-DIMENSIONAL SYSTEM

The spin magnetization of a system of fermions confined to two dimensi ons, and subject to a strong magnetic field, has been predicted to exh ibit a first-order phase transition as a function of its Zeeman energy . We present strong experimental evidence that such a paramagnetic to ferromagnetic phase transition occurs at Landau level filling nu = 4 i n the two-dimensional hole gas. We demonstrate that our data are not e xplained by perturbative effects such as Landau level anticrossing, an d show that exchange coefficients measured at odd filling factors are consistent with the predicted size of the gap at nu = 4.