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SXF data have also been taken on an Fe/Si multilayer with tSi = 14Å but which was held at a reduced temperature of 120 K during growth (data not shown). The valence band spectra of the film grown at reduced temperature with tSi = 14Å look virtually identical to data on the film grown at 60°C but with tSi = 20Å. The most likely explanation for this similarity is that both films have amorphous iron silicide spacer layers. The amorphous state of the spacer layer in these films must be due to the reduced Fe content compared with films which have thinner Si layers or are deposited at higher temperature. Multilayers with amorphous spacer layers do not display antiferromagnetic interlayer coupling.[4,7]
A comparison of the data of Figs. 3 and 4 show that the peaks in the spectrum of the epitaxial AF-coupled multilayer are narrower than those in the spectrum of the polycrystalline AF-coupled multilayer. This suggests that a higher degree of local order occurs in epitaxial films. The nature of this order and the exact structure of the silicide spacer layer phase are not yet known. TEM studies have shown that the spacer layer in AF-coupled multilayers is a crystalline cubic iron silicide in the B2 CsCl phase or fcc DO3 phase.[7] The TEM diffraction patterns are not consistent with the B20 structure, whose SXF data most closely resembles that of the AF-coupled multilayers. Jia et al. do report SXF data on the DO3-structure Fe3Si phase but the spectrum of this compound has a much more prominent and narrow non-bonding s feature.[11] The presence of an Fe3Si spacer can be ruled out on other grounds since this compound is ferromagnetic, inconsistent with the presence of antiferromagnetic interlayer coupling. The possibility remains, however, that the spacer layer is in the DO3 structure but at a different stoichiometry. No SXF data on the metastable B2 silicide phase have been reported although photoemission measurements show that it is metallic.[13] The magnetic properties of the B2 phase and hypothetical off-stoichiometry phases are not known. The observation of large biquadratic coupling in Fe/Si multilayers[14,15] suggests that an antiferromagnetic or ferrimagnetic order may be present in the spacer layer.
When examined together, the SXF and NEXAFS data show that Fe/Si multilayers with crystalline metallic silicide spacer layers have antiferromagnetic interlayer coupling, while similar multilayers with amorphous silicide spacer layers show no interlayer coupling. Whether the amorphous silicide layers are metallic or semiconducting is a topic for further study. Theoretical calculations will be necessary to get a better estimate of the stoichiometry and magnetic properties of the silicide spacer in the AF-coupled multilayers. The present data should lay to rest any speculation that the interlayer exchange coupling in Fe/Si multilayers involves a novel mechanism. The clarity of these results on thin buried silicide layers illustrates the power of photon-counting spectroscopies with their intrinsic bulk sensitivity for the study of multilayer films.
We would like to thank P.E.A. Turchi, P.A. Sterne and J. van Ek for helpful discussions. This work was supported by the Division of Materials Science, Office of Basic Energy Sciences, and performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48, by National Science Foundation Grant No. DMR-9017996 and DMR-9017997, by a Science Alliance Center for Excellence Grant from the University of Tennessee, by the U.S. Department of Energy (DOE) Contract No. DE- AC05-84OR21400 with Oak Ridge National Laboratory and by the Louisiana Educational Quality Support Fund and DOE-EPSCOR Grant LEQSF (93-95)-03 at Tulane University. This work was performed at the Advanced Light Source, which is also supported by the Office of Basic Energy Sciences, U.S Department of Energy, under contract No. DE-AC03-76SF00098.
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