Magnetization of bilayer two-dimensional electron systems

We have measured the magnetization of strongly coupled bilayer two-dimensional electron systems. When sweeping the magnetic field steps appear in the magnetization whenever a transition between discrete energy levels takes place. At magnetic fields for which the Landau-level splitting dominates, the steps occurring at total filling factors nu(T) = 4j are related to transitions between adjacent Landau levels j - 1 and j; such steps can also be observed in single layers. Additional magnetization steps showing up in bilayers at half Landau-level filling nu(T) = 4j + 2 are associated with transitions from a symmetric to an antisymmetric state inside the same Landau level j. The observed size of the Landau-level steps in bilayers is considerably lower than expected theoretically. These findings are explained using a model with a large background density of states.