ABSTRACT
Antiferromagnetic (AFM) multilayers can be used as model systems to study the behavior of finite magnetic spin systems. Here, we choose the archetypal Fe/Cr multilayer as an example. In such multilayers, the adjacent Fe layers are exchange coupled through the nonmagnetic Cr spacer in which standing waves of spin polarized conduction electrons mediate the coupling. The coupling is an oscillatory function of the Cr spacer layer thickness. At a Cr thickness of d = 1.3 nm, one observes the maximum of the AFM coupling strength. Due to the AF coupling of the Fe layers, the magnetic unit cell is twice as large as the chemical unit cell. Probing such a structure with a spin-sensitive scattering method leads to half-order Bragg reflections that are located between the Bragg peaks of the chemical structure [23]. Nuclear resonant scattering of synchrotron radiation exhibits a similar sensitivity to magnetic superstructures as neutron scattering does. The very high brilliance of modern synchrotron radiation sources, however, leads to particular advantages of the NRS technique such as high spatial resolution via microfocusing and short data acquisition times in the range of <1 hour for a reflectivity scan. The Fe/Cr multilayer in this example consisted of 20 bilayers of (1.3 nm Fe/1.0 nm Cr) on a Si wafer, produced by sputter deposition in an Argon atmosphere from a Cr target and a Fe target enriched to 95 percent in 57Fe. The angular dependence of the electronic and nuclear reflectivity is shown in Fig. 8.7. The chemical periodicity of the multilayer with a period of 2.3 nm results in the first-order electronic Bragg reflection at a grazing angle of 0.75°. The same peak is seen also in the nuclear reflectivity. However, in contrast to the electronic reflectivity, the nuclear reflectivity shows peaks also at the angular positions of the half-order reflections, corresponding to a spatial period of 4.6 nm. This is the period of a magnetic superstructure that results from the AFM coupling of the neighboring iron layers.
