Epitaxial Fe/Fe 0.5 Si 0.5 /Si-wedge/Fe 0.5 Si 0.5 /Fe structures are prepared by thermal evaporation with Fe 0.5 Si 0.5 boundary layers grown by coevaporation at 200°C. Magnetic properties are examined with Brillouin light scattering and longitudinal magneto-optic Kerr effect hysteresis. The interlayer coupling is found to increase in excess of 8 mJ/m 2 by introducing a boundary layer at the bottom interface. The coupling maximum shifts from 7 to 3 Å nominal Si thickness. This effect is related to reduced interdiffusion with the formation of an epitaxial, pinhole-free spacer at smaller thickness. Together with the strong increase of the coupling for decreasing spacer thickness, this results in an enhancement of the coupling. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1499229͔ Fe/Si/Fe structures are attracting interest due to strong antiferromagnetic ͑AF͒ coupling which is of significance in applications using artificial antiferromagnets and ferrimagnets as, for instance, in magnetic sensors 1 or more recently in antiferromagnetically coupled ͑AFC͒ storage media for hard disk drives. 2 Fe/Si/Fe structures are complex objects for the study of magnetic and transport properties because of the interdiffusion at interfaces with the possible formation of iron silicides of different structure and composition. [3][4][5][6][7] As was shown earlier, Si grown on Fe tends to interdiffuse and to crystallize in epitaxially stabilized CsCl-type, metallic Fe 0.5 Si 0.5 ͑Ref. 3͒ and exhibits an exponential decay of coupling versus spacer thickness ͑Refs. 5 and 7͒. This unusual behavior was related to a new type of exchange coupling across metallic-type spacers 5 in contradiction with the standard quantum interference model ͑QIM͒ of interlayer coupling. 8 The QIM predicts an exponential decay of AF coupling only for nonconducting spacers but oscillatory coupling for metallic spacers.In order to distinguish metallic and insulating-type spacers, we have previously prepared epitaxial Fe/Fe 1Ϫx Si x /Fe trilayers by codeposition of Fe and Si instead of relying on interdiffusion. We achieved a spacer composition close to Fe 0.5 Si 0.5 ͑Ref. 9͒ and spacers with variable Si content x in the range 0.4ϽxϽ1 ͑Ref. 10͒. We deposited Fe 0.5 Si 0.5 spacer layers at an elevated temperature (200°C) in order to form metallic, epitaxial iron silicides and obtained weak oscillatory coupling ͑less than 1 mJ/m 2 ͒. 9 In these samples, the coupling strength increased with decreasing temperature in agreement with the QIM for metallic spacers. 8 Second, we showed that the interlayer exchange coupling strongly increases with the nominal Si content x in epitaxial Fe 1Ϫx Si x spacers exceeding 5 mJ/m 2 for nominally pure Si spacers. With the increase of nominal Si content x, the thickness of the strongest AF coupling (t max ) shifted to a smaller Si thickness. We concluded that the very strong coupling ͑more than 5 mJ/m 2 ͒ observed for nominally pure Si spacers is not due to metallic iron silicides, but due to highly resistive, Si-rich spacers...

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