ANTIFERROMAGNETIC COUPLING BETWEEN LAYERS IN CO/CR MULTILAYER (INVITED)

It has been shown by several groups that Fe layers in epitaxial Fe/Cr sandwiches or multilayers (MLFs) are coupled antiferromagnetically (AF) for very small Cr thicknesses. One way this is manifest in the MLF is that for a fixed Fe thickness (30 Å) the hysteresis curves are progressively flatter as the Cr thickness decreases. This is the opposite behavior to that expected if the Fe layers were uncoupled or ferromagnetically coupled. We have observed the same type of behavior in a series of nominally 21-Å Co/X Cr MLFs where X varies from 14 to 57 Å. These MLFs have been well characterized with both x-ray scattering and cross-sectional high-resolution electron microscopy. They are highly polycrystalline, nonideal MLFs with appreciable mixed interfaces. We have determined that Co/Cr MLF have an interface or exchange anisotropy with an average value of ∼7 kOe in a direction opposite to that of the in-plane magnetization. Such an AF interaction is expected from Cr layers having interface thicknesses comparable to that of the pure Cr regions. It provides a natural mechanism for the Cr layers to align the Co layers either AF or FM depending on the local structural details. Both types of alignment are likely to be present in a MLF and the regions having AF alignment will cause the observed flattening of the hysteresis loops. The magnetoresistance of these MLFs has a behavior similar to, but much smaller than, the "giant" magnetoresistance seen in the epitaxial Fe/Cr MLF and single-crystal Fe whiskers. The magnetoresistance in pure Fe has been attributed to the domain structure and it is likely responsible for this effect in the Co/Cr MLF as well.