Polarised neutron reflection studies in continuous films: Fe magnetic moment at a GaAs semiconductor interface
The current interest in magnetic/semiconductor interfaces stems from the potential applications that can ensue from the integration of magnetic materials with semiconductors (magnetoelectronics). However, sharp interfaces are a necessary requirement for these applications, as interdiffusion and magnetic dead layers are detrimental to optimal device performance. The Fe/GaAs(001) system is a good candidate for these applications, since high quality epitaxial Fe single crystals can be grown directly on the semiconductor substrate; recent X-ray circular dichroism (XMCD) measurements have suggested that no magnetic layers are present at the Fe-GaAs interface. It is therefore of great interest to determine the exact value of the magnetic moment of thin Fe layers on GaAs and also to study the structural properties of this interface, such as roughness. Polarised neutron reflection (PNR) is a technique particularly suited to tackle this kind of problem, due to its high sensitivity to both magnetic and structural parameters, such as magnetic moments, layer thickness and interface roughness. Despite the strong interest in the Fe/GaAs and Fe/InAs system, no precise determination of the interface moment is currently available.
The epitaxial Fe films were grown in ultrahigh vacuum by e-beam evaporation onto GaAs(001) and InAs(001) substrates, at room temperature. The thickness of the Fe films varied from 10 to 100 Å and the Fe films were capped with a protective Au layer. The samples were magnetically characterised by in situ MOKE magnetometry, and exhibit a dominant uniaxial magnetic anisotropy for small thicknesses, which is supplanted by the (bulk) cubic anisotropy term at larger thicknesses. The in-plane uniaxial anisotropy easy axis changes by 90o, from the Fe films grown on GaAs(001) (lattice mismatch of 1.3%) to the Fe films grown on InAs(001) (lattice mismatch of -5.6%), but this change in the anisotropy may be related to the interface (or in-plane lattice misfit) and not to this out of plane biaxial strain. The PNR measurements were performed at room temperature, with the magnetisation saturated by a large magnetic field parallel (or antiparallel) to the neutron spin. In Fig. 1 we present the PNR spectrum of a 100 Å Fe film deposited on InAs(001) and capped with a 30 Å Au film. The reflectivity for the spin down neutrons is much smaller than that for the spin up neutrons, and this is also reflected in the poorer statistics for the spin down reflectivity. Also shown in the figure are the fits to the data, which give a magnetic moment for the Fe film which is essentially bulk like, μFe = 2.26 μB. The value for the interface roughness obtained from the fit is of the order of 5-10 Å for the Fe/semiconductor interface, but larger for the top interface (~20 Å). Similar measurements for a 10 Å Fe/InAs(001) sample give a slightly reduced magnetic moment, indicating that some intermixing may occur at the Fe/InAs interface.
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