Magnetization measurements on a series of Fe films grown by molecular beam epitaxy on GaAs (001) substrates and capped with a thin Au layer reveal interesting exchange bias (EB) properties at low temperatures. The observed exchange bias decreases rapidly with increasing temperature, and completely disappears above 30 K. While the Fe samples were not grown with an intentionally deposited antiferromagnetic (AFM) layer, X-ray reflectometry, X-ray absorption near-edge spectroscopy carried out near the L-edge of Fe, and comparison with similar Fe/GaAs samples capped with Al, which do not show exchange bias, suggest that the exchange bias in the GaAs/Fe/Au multilayers is caused by an AFM Fe oxide at the Fe/Au interface formed by penetration of oxygen through the Au capping layer. The observed exchange bias is accompanied by a strikingly asymmetric magnetization reversal of the Fe films occurring when the magnetic field is applied at angles away from the easy axis of the film. The observed asymmetry can be interpreted in terms of a competition between cubic, uniaxial, and unidirectional magnetic anisotropy characteristic of the exchange-biased Fe film.
We carried out a systematic study of magnetic order and magnetic interlayer coupling in Fe/(Ga,Mn)As bilayers using superconducting quantum interference device magnetometry, polarized neutron reflectometry, element-specific x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray specular reflectivity. Our results clearly show that Fe/(Ga,Mn)As bilayers are strongly exchange coupled at the interface. However, contrary to recent reports in the literature, we observe a ferromagnetic rather than antiferromagnetic coupling between the magnetic moments of the Mn ions and the Fe layer. It is interesting in this context that the surface region of the (Ga,Mn)As layer that is in direct contact with the Fe film displays a nearly identical coercivity to that of Fe (indicating perfect ferromagnetic coupling of that region), while the bulk of the (Ga,Mn)As layer, which is more weakly ferromagnetically coupled with Fe, shows a significantly smaller coercive field.
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