Effect of roughness, frustration, and antiferromagnetic order on magnetic coupling of Fe/Cr multilayers

The interplay between interfacial disorder and the antiferromagnetic order in Cr leads to complex behavior in Fe/Cr multilayers. Measurements of interlayer coupling are discussed for samples with different amounts of disorder ranging from optimally fabricated trilayers of Fe/Cr/Fe on Fe(0 0 1) whiskers, to trilayers with increasing degrees of interfacial roughness, and finally to superlattices of Fe/Cr. The coupling of ferromagnets through noble-metal spacer layers can be described by a model that consists of bilinear coupling averaged over thickness fluctuations and extrinsic biquadratic coupling induced by the thickness fluctuations. This, the conventional model, also describes much of the behavior observed for Fe/Cr multilayers. However, in this case, the antiferromagnetism in Cr leads to results not explained by the conventional model. For nearly ideal interfaces, the Fe–Cr coupling can induce order in Cr, modifying the temperature dependence of the interlayer coupling. In addition, interfacial disorder can frustrate the antiferromagnetic order in the Cr, leading to a variety of ordered states which have been observed by neutron scattering. Each of these ordered states, in turn modifies the interlayer coupling in unexpected ways. The different ways in which the systems minimize the frustration can explain the experimental results.