Exchange coupling through spin-density waves in Cr(001) structures: Fe-whisker/Cr/Fe(001) studies

Abstract

Exchange coupling through a spin-density wave in Fe-whisker/Cr/Fe(001) structures has been studied using Brillouin light scattering (BLS) and magneto-optical Kerr effect (MOKE). The Fe-whisker(001) substrates provide nearly ideal templates: they are characterized by atomic terraces having dimensions in excess of several micrometers. Such templates are essential for the study of short-wavelength exchange coupling which is mediated by the intrinsic spin-density wave in Cr(001). Atomically smooth Cr(001) layers similar to those of the Fe-whisker surfaces can be grown at raised substrate temperatures. Angular resolved auger electron spectroscopy measurements have shown that the Fe-whisker/Cr(001) interfaces are affected by an atom exchange placement mechanism (interface alloying). It will be shown that this interface alloying at the Fe-whisker/Cr interface profoundly affects the behavior of the short-wavelength oscillations. The phase of the short-wavelength oscillations is reversed compared to that expected for the spin-density wave in Cr(001). The strength of coupling is significantly decreased from that obtained from first-principles calculations, and the first crossover to antiferromagnetic coupling occurs at 4 ML. BLS and MOKE have shown unambiguously that the exchange coupling in Fe-whisker/Cr/Fe(001) structures can be described by bilinear and biquadratic terms. Experiments carried out using Cu and Ag atomic layers between the Cr(001) and Fe(001) films, i.e., heterogeneous interfaces, have shown that the exchange coupling in Cr(001) is strongly affected by electron multiple scattering. It will be argued that the exchange coupling through thick (>8 ML) and atomically smooth Cr(001) spacers can be described by localized interactions (Heisenberg type) and by electron multiple-scattering (quantum well state) contributions. This is in good accord with recent first-principle calculations by Mirbt and Johansson. However, interface alloying severely affects the behavior of the exchange coupling for Cr thicknesses less than 8 ML. In this thickness regime the overall coupling exhibits mostly a long-wavelength behavior with a small superimposed short-wavelength contribution. This initial Cr thickness regime is responsible for changes in the phase of the short-wavelength oscillations and for the reduced strength of the exchange coupling due both to the localized and to the multiple-scattering contributions. We have observed no significant dependence of the exchange-coupling strength on the Fe film thickness for samples having the structure $\mathrm{F}\mathrm{e}\ensuremath{-}\mathrm{w}\mathrm{h}\mathrm{i}\mathrm{s}\mathrm{k}\mathrm{e}\mathrm{r}/11\mathrm{Cr}/n\mathrm{F}\mathrm{e}/20\mathrm{Au}$ where n specifies an iron film thickness between 5 and 40 ML. However, preliminary data show that the exchange coupling is significantly increased in specimens for which both sides of the iron film are covered by Cr, i.e., for structures of the form $\mathrm{F}\mathrm{e}\ensuremath{-}\mathrm{w}\mathrm{h}\mathrm{i}\mathrm{s}\mathrm{k}\mathrm{e}\mathrm{r}/11\mathrm{Cr}/n\mathrm{F}\mathrm{e}/11\mathrm{C}\mathrm{r}/20\mathrm{Au}.$ It appears that electron resonant states in the iron film play no important role in the strength of the exchange coupling when the iron is bounded on one side by the gold, but that they do become important when the iron film is bounded by Cr on both sides. BLS and MOKE studies on Fe-whisker/Cr/Mn/Fe(001) samples revealed that the antiferromagnetic state of Mn is composed of compensated (001) atomic planes. The results of the above experimental studies will be compared to recent theories. Points of agreement and of disagreement between the experimental results and recent first-principles calculations will be explicitly pointed out.