MAGNETOOPTIC RESPONSE IN ULTRATHIN NI-FE BILAYERS

The Kerr signal from a sample of 10 ML (monolayers) of 'bcc' Ni epitaxially grown on 6.6 ML of bcc Fe on the (001) surface of Ag, published previously, exhibited the superposition of two symmetries. One of these was the expected reversal of the signal with reversal of the field. The other was symmetric with respect to the absolute value of the field. We were unable to explain this result under the assumption that the magnetization was constrained to lie in the plane of the ultrathin layers with the consequence that we were measuring the longitudinal Kerr effect. We have since found that the symmetric signal is a polar Kerr effect. It is most likely a manifestation of the magneto-optic magnetic gradient effect recently discovered by Schafer and Hubert. The magnetic gradients come from a profusion of walls generated on magnetization reversal by the defect structure of the Ni layer. The possibility that it may be a real polar Kerr effect resulting from rotation of magnetization out of the plane because of Dzialoshinsky-Moriya interactions is also considered.