The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The eA'ect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-Aip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.
%e investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to = 20 A. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.
The recently discovercd oscillating exchange effcct in iron-chromium rnultilayers can bcst be studied on an epitaxial iron sandwich with a wcdge shaped chromium interlayer. Domain patterns arc analyzed as wcll as magnetization curves on such samplcs using magnctooptical techniques. Unusual domains are found in zones around thc transitions between ferromagnetic and antifcrromagnetic exchange. Also thc rnagnctij.ation curves for largcr chromium thickness show some peculiar fcaturcs. The interprctation of these patterns invokcs coupling bctween the iron layers which favours a right angle betwccn the magnetization directions in both layers. Such a non-collinear interaction can be derived phenomcnologically from a biquadratic coupling as introduccd earlicr in thc analysis of spin structures in oxidcs. A biquadratic coupling can be understood either as a second order Heisenberg or a sccond order Dzyaloshinskii interaction. In the course of these investigations domain observations proved to be a uscful and scnsitivc method to detcct locally the nature of interactions between fcrromagnetic layers.Die kurzlich cntdecktc oszillicrendc Wechselwirkung in Eisen Chrom-Vielfachschichtcn kann am bcsten an einer cpitaxialcn Schichtanordnung mit ciner kcilformigcn Chromzwischenschicht studiert wcrden. Die auf solchen Proben beobachtbarcn Domanenstrukturen sow.ie dic Magnctisierungskurven werden mit magnctooptischen Vcrfahrcn untcrsucht. Im Bercich der b e r g a n g c zwischen fcrromagnetischer und antifcrromagnetischer Kopplung werden Zonen ungewijhnlicher Muster cntdeckt. Auch die Magnetisierungskurven fur groflerc Chrom-Schichtdickcn zeigtcn cigentumlichc Zuge. Die Beobachtungen lassen sich erkliren, wcnn einc zusltzliche Kopplung der Schichtcn angenommen wird, die cincn rcchten Winkel zwischcn den Magnetisierungsrichtunecn in bciden Schichtcn begunstigt. Phiinomenologisch ergibt sich eine solche nicht-kollineare Kopplung aus dcr Annahme eincr biquadratischcn Wechselwirkung, wic sie fruher in dcr Analyse von Spinstrukturen rnagnetischer Oxide eingcfuhrt wurde. Einc biquadratische Kopplung lant sich entwcder als Heisenberg-oder DryaloshinskiiWechselwirkung zweiter Ordnung interpretiercn. Dornanenbeobachtungen erwiesen sich im Laufe dieser Untersuchung als empfindlichc Methodc zur IJntcrsuchung des Charaktcrs der Kopplung in Vielfachschichten.
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