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resistance oscillations, and aperiodic magnetoresistance oscillations. These effects are shown to arise from the internal structure of overlapping Landau bands and are well accounted for by perturbation calculations.

The theory of 2D electrons in non-uniform magnetic fields has an extensive research literature in which a range of new semiclassical and quantum effects are predicted [1-3]. Until recently there have been very few experimental studies however. A two-dimensional electron gas (2DEG) subject to a magnetic field which varies spatially on a length scale which is longer than the electron mean free path can be considered to have a spatially varying conductivity tensor [4]. This can lead, for example, to large magnetoresistances due the admixture of longitudinal and Hall resistances [4]. The more interesting case, which we focus on in this paper, is that of large amplitude magnetic fields that vary on length scales that are small compared to the electron mean free path. In this case the Lorentz force can strongly modify the electron trajectories leading to a wide range of new semiclassical effects. The nature of the electron states of the 2DEG will also be dependent on the form of the magnetic field leading to the possibility of new quantum mechanical effects. In this paper we will be concerned with the situation in which the Zeeman energies are negligibly small. It is then only the component of the magnetic field perpendicular to the 2DEG that affects the electron states. One approach to the realisation of large amplitude spatially varying magnetic fields is the use

Exchange anisotropy occurs at the interface between an anti ferromagnetic (AF) layer and a ferromagnetic layer, and results in a ferromagnet hysteresis loop displaced along the field axis. We have performed off-specular neutron reflectometry in order to characterize the domain structure in Co layers that are exchange biased by FeMn. This allows us to determine the domain direction distributions and lateral magnetic correlation lengths for the Co layers as a function of field with the exchange bias in two different thermally prepared states: high field cooled and zero field cooled. We find that a reversing exchange-biased layer is characterized by a very short (submicron) magnetic lengthscale, indicating domains much smaller than those in the ferro-magnet after ac demagnetization at temperatures above the blocking temperature. This indicates that the bias is not spatially uniform across the entire interface, underlining the complexity of the AF spin structure.