Coexistence of exchange bias and magnetization pinning in the MnOx/GaMnAs system

Coexistence of exchange bias (HE) and magnetization (M) shift was observed in as-grown and field-annealed MnOx/Ga0.95Mn0.05As bilayers. It was found that HE initially decreases with the annealing time ta and then increases when ta > 30 min, while the M shift remains almost unchanged with ta. X-ray photoelectron spectroscopy (XPS) analysis reveals that MnOx is composed of MnO and Mn3O4, and the volume amount ratio of Mn3O4 to MnO increases with increasing ta. A simple model based on a uniform MnO-Mn3O4 interface with constant 'pinned' uncompensated interfacial spins is proposed to account for the observed exchange-biased phenomena in the bilayers.