Bls Studies of Exchange Coupling in The Iron Whisker/Cr/Fe System
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ABSTRACT Brillouin light scattering (BLS) and magneto-optic Kerr effect have been carried out on a series of specimens consisting of a Fe(001) whisker substrate upon which thin Cr(001) and Fe(001) layers have been deposited by means of molecular beam epitaxy in ultrahigh vacuum. The Fe film was 20 monolayers (ML) thick, and the Cr(001) films were grown having various thicknesses. It is demonstrated that BLS thin film frequencies measured in the saturated magnetic state with the thin film magnetization parallel with the applied magnetic field can be used to obtain the exchange coupling strength between the thin film and whisker magnetizations both for antiferromagnetic coupling and for ferromagnetic coupling, provided that the ferromagnetic coupling is not too strong. It is also shown that the coupling strength is extremely sensitive to the quality of the chromium growth: a small deterioration in the growth conditions has been found to reduce the exchange coupling by nearly a factor of two.
INTRODUCTION 2 In a beautiful series of experiments UngurisI et al, Pierce ,3 4 et al, and Stroscio et al have used scanning electron microscopy with polarization analysis (SEMPA) and scanning tunneling microscopy (STM) to investigate the growth and magnetic structures in thin Cr(001) and Fe(001) layers grown by means of molecular beam epitaxy (MBE) on very clean and perfect Fe(001) whisker structure was used to investigate the substrates. A trilayer exchange coupling between the iron whisker substrate and a thin separated by a Cr(001) iron film, 5-10 monolayers thick (ML), interlayer whose thickness varied from 0 to 40 ML over a distance of -0.5 mm. They showed that the coupling alternated between antiferromagnetic coupling (AF) and ferromagnetic coupling (FM) depending on the thickness of the chromium interlayer, and that the pattern of changes from AF to FM coupling depended on the substrate temperature at which the chromium growth took place. The smoothest growths, corresponding to a layer-by-layer growth mode, occurred when the substrate temperature was held at 350C; the resulting magnetization patterns could be described as the 2 superposition of two oscillatory terms - a short wavelength component having a period of 2.105±.005 ML and a long wavelength component having a period of 12±1 ML. Unfortunately, the SEMPA 131
Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
technique can not be used to provide a quantitative measure of exchange coupling strengths, and it can only be used in very small applied magnetic fields. One must therefore turn to other techniques such as Brillouin light scattering (BLS) or magnetooptic Kerr effect (MOKE) in order to obtain a quantitative measure of the exchange coupling between iron layers separated by a chromium interlayer. We are interested in the use of BLS to measure the strength of the exchange coupling between a thin Fe(001) film separated from a bulk whisker substrate by a Cr(001) interlayer. The principle of the method is simple. The magnetization in an isolate
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