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Multilayer films made by alternate deposition of two materials play an important role in electronic and optical devices such as quantum-well lasers and x-ray mirrors.[1] In addition, novel phenomena like giant magnetoresistance and dimensional crossover in superconductors have emerged from studies of multilayers. While sophisticated x-ray techniques are widely used to study the morphology of multilayer films, progress in studying the electronic structure has been slower. The short mean-free path of low-energy electrons severely limits the usefulness of photoemission and related electron spectroscopies for multilayer studies.
Soft x-ray fluorescence (SXF) is a bulk-sensitive photon-in, photon-out method to study valence band electronic states.[2] Near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) measured with partial photon yield can give complementary bulk-sensitive information about unoccupied states.[3] Both these methods are element-specific since the incident x-ray photons excite electrons from core levels. By combining NEXAFS and SXF measurements on buried layers in multilayers and comparing these spectra to data on appropriate reference compounds, it is possible to obtain a detailed picture of the electronic structure.
The Fe/Si multilayer system well illustrates the power of combining the SXF and NEXAFS techniques. Fe/Si multilayers exhibit a large antiferromagnetic (AF) interlayer exchange coupling that is apparently similar to that previously observed in metal/metal multilayers like Fe/Cr.[4] The observation of strong antiferromagnetic coupling was initially surprising, since this coupling is believed to be a manifestation of spin-density oscillations in the non-magnetic metallic spacer layer of a multilayer.[5] The interpretation of the Fe/Si coupling data was hampered by lack of knowledge about the strongly intermixed iron silicide spacer layer, which was variously hypothesized to be a metallic compound in the B2 CsCl structure[4] or a Kondo insulator in the more complex B20 structure.[6] If the spacer layer is not metallic, then the usual theories of interlayer exchange coupling do not apply[5] and the coupling must involve a novel mechanism. Using transmission electron microscopy (TEM), the spacer layer has been identified as a metastable cubic iron silicide closely lattice-matched to bulk Fe.[7] However, since the exact stoichiometry of the silicide was not determinable by diffraction means, the question of whether the spacer layer is a metal or not has remained unanswerable. SXF and NEXAFS are ideal techniques to resolve exactly this type of issue.
SXF and NEXAFS measurements were performed on five different Fe/Si multilayer films at the Advanced Light Source on beamline 8.0, which is described in detail elsewhere.[8] SXF data has previously been used to study buried layers of BN[9] and Si.[10] Data taken at the Fe L-edge closely resembles bulk Fe for all Fe/Si multilayers. NEXAFS spectra were acquired by measuring the total Si L-emission yield with the same detector used for fluorescence. The resulting data are expected to be comparable to those acquired by electron counting.[3] The films used in this study were grown using ion-beam sputtering (IBS) in a chamber with a base pressure of 2×10-8 torr. The deposition conditions were the same as those used in previous studies.[8] All multilayers were characterized using x-ray diffraction and magnetometry. Reference spectra were obtained from a crystalline silicon (c-Si) substrate piece, an amorphous silicon (a-Si) film made by IBS, and a fragment of an FeSi2 sputter target.
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