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Results

Figure 1 shows hysteresis loops for three representative Fe/Si multilayers. The polycrystalline (Fe30Å/Si20Å)x50 multilayer grown on glass has a magnetization curve that shows no sign of interlayer exchange coupling. This multilayer has magnetic properties like those of bulk Fe. The epitaxial (Fe40Å/Si14Å)x40 multilayer grown on MgO has a low remanent magnetization and a high saturation field, which are the classic signs of antiferromagnetic interlayer coupling. Data on the polycrystalline (Fe30Å/Si14Å)x50 multilayer fall somewhere in-between these two extremes. Detailed characterization of these films has been published previously.[7]

For purposes of comparison to the Fe/Si multilayer SXF spectra, SXF reference spectra taken at the Si L-edge for the c-Si and a-Si samples are shown in Figure 2. The spectra resemble previously published Si data.[2,10] The peaks near 89 and 92 eV in the c-Si spectrum originate from non-bonding s states and sp-hybridized states, respectively.[10,11] These features are broadened by disorder in a-Si.

Figure 3 shows the Si L-edge valence band emission spectra of the FeSi₂ reference sample and the same two polycrystalline Fe/Si multilayers whose magnetization data are shown in Figure 1. The FeSi₂ data has two primary features, namely s-orbital features near 90 eV, and a shoulder which extends up to 99 eV and is comprised mostly of states with d symmetry. These features have been previously identified in semiconducting bulk FeSi₂ specimens.[11]

In Figure 3 the spectrum for the polycrystalline antiferromagnetically coupled multilayer with t_Si = 14Å\ looks similar to the FeSi₂ data, while the spectrum for the polycrystalline uncoupled multilayer with t_Si = 20Å is more like c-Si. Peaks in the AF-coupled multilayer spectrum are noticeably narrower than those in the FeSi₂ reference spectrum. Studies of bulk iron silicides have shown that peaks in the Si emission spectra narrow as the iron content increases and Si-Si coordination decreases.[11] Thus the data of Fig. 3 indicate that the Fe atomic fraction in the spacer layer of the AF-coupled multilayers is higher than 1/3. Overall the shape of the spectrum from the AF-coupled multilayer is more reminiscent of SXF data on bulk B20 FeSi than of data on bulk FeSi₂.[11] The uncoupled multilayer data in Fig. 3 have a sharp peak near 92 eV which coincides with a feature in the c-Si spectrum although the shape of the higher energy part of the valence band more closely resembles the FeSi₂ data. The narrowness of the 92 eV feature is evidence for a significant Fe content and low Si-Si coordination in the spacer layer of the uncoupled multilayer. These observations are consistent with the TEM determination that the spacer layer in the uncoupled multilayers is amorphous iron silicide.[7]

The presence of significant Fe in the silicide spacer layer of the Fe/Si multilayers strongly suggests that the silicide is metallic. Unambiguous confirmation of the metallic nature of the silicide is obtained by plotting together the SXF and NEXAFS spectra as in Fig. 4. For this data set the spectrometer energy calibration was accomplished through comparison with earlier work on c-Si L-emission[12] and through alignment of the elastically scattered photon peak to the incident photon energy. The overlap of the valence band features from the SXF and the conduction band features from NEXAFS is therefore convincing evidence that the silicide spacer layer of the multilayer is metallic. While the Si bands near the Fermi level clearly show the energy gap which is expected in a semiconductor, the slope of the silicide bands near E_F suggests that the Fermi level falls in the middle of an energy band. A more detailed interpretation of these spectral features will require electronic structure calculations.

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