Damage caused to interlayer coupling of magnetic multilayers by residual gases

Abstract

The oscillatory interlayer indirect exchange coupling in Co/Cu multilayers is known to be highly sensitive to structural defects. In this paper the dependence of the antiferromagnetic exchange coupling on the background pressure in the vacuum chamber is investigated. Co/Cu multilayers were grown by dc magnetron sputtering in a system equipped with a leak valve to allow the introduction of low levels of air or ${\mathrm{O}}_{2}.$ Below a relatively narrow band of pressure the samples exhibit excellent antiferromagnetic coupling and consequently a high giant magnetoresistance, $\ensuremath{\sim}40%$ for {Co(8 \AA{})/Cu(8 \AA{})}$\ifmmode\times\else\texttimes\fi{}25$ samples. Above this transitional band of pressure no antiferromagnetic coupling, and hence no giant magnetoresistance, is observed. X-ray diffraction measurements reveal no change of any significance in any of the layer thicknesses or roughnesses. Whilst the high-field magnetic behavior is found to be isotropic in the sample plane, the reversal of the remanent moment around zero field is found to show a varying degree of uniaxial anisotropy. Lower remanent moments are found to be associated with a more isotropic reversal mechanism. A thorough characterization of the physical and magnetic microstructure by means of various modes of transmission electron microscopy is presented. Cross-sectional images reveal subtle changes in the crystallinity and layer quality of the samples as the background pressure is increased. Plan-view Lorentz microscopy reveals that the isotropic reversal mechanism of the low remanence samples involves a complete domain structure. The reversal mechanism of multilayers with a significant remanent moment varies markedly with field direction and can be dominated by rotation or comparatively simple domain processes. Samples with a significant giant magnetoresistance $\ensuremath{\sim}20%$ and remanent fractions $\ensuremath{\sim}0.7$ are found to still show highly anisotropic reversal mechanisms around zero field. Indeed when demagnetized along the easy axis the samples are in a single domain state at remanence. This is compelling evidence for substantial noncollinear ordering of the moments through biquadratic coupling.