ABSTRACT
The isotopic sensitivity of nuclear resonant scattering enables one to probe selected parts of a layer system by depositing ultrathin layers of a Mössbauer isotope. Enriching an element with its Mössbauer isotope (e.g., Fe by 57Fe) does not disturb the chemical integrity of the layer system. Precise control over the isotopic probe layer thickness thus enables one to reach atomic resolution in the determination of magnetic properties. This is particularly interesting for the study of magnetic heterostructures where the magnetic properties critically depend on the interfacial structure. Prominent examples are nanocomposites that consist of exchange-coupled soft-and hard-magnetic phases. Such materials are interesting candidates for new permanent magnets with giant magnetic energy products [18]. Thin bilayers consisting of a hard-and a soft-magnetic material are ideal model systems to investigate the fundamental properties of this coupling mechanism. As a characteristic property of such systems, the magnetization of the soft-magnetic film at the interface is pinned to the hard-magnetic film as a result of the exchange interaction. With increasing distance from the interface, the exchange coupling becomes weaker and the magnetization may rotate under the action of an external field. If, for example, the external field is applied orthogonal to the magnetization direction of the hard layer, the magnetic moments in the soft layer arrange in a spiral structure along the normal, as shown schematically in Fig. 8.5. Because of the reversible nature of this rotation, this is called the exchange-spring effect [19]. While a number of micromagnetical models have been developed to describe this behavior [20,21] direct measurements of the in-depth spin structure are scarce. Here, we show how to apply nuclear resonant scattering from ultrathin probe layers of 57Fe to directly measure the in-depth magnetic structure of exchange-spring bilayers [22]. The sample was a bilayer consisting of 11 nm soft-magnetic Fe on 30 nm hardmagnetic FePt. To minimize oxidation of the Fe layer, it was coated with a 3 nm thick Ag layer. All the layers were prepared by RFmagnetron sputtering in a high-vacuum system with a base pressure
of 2◊10-7 mbar. The FePt layer was produced by cosputtering of Fe and Pt to obtain a composition close to Fe55Pt45. Subsequent annealing of this layer resulted in the formation of the hardmagnetic LI0 phase with a coercivity of 0.95 T at room temperature. An ultrathin inclined probe layer of 57Fe was embedded within the Fe layer, as shown in Fig. 8.5. The sample was mounted in a cryomagnet system and exposed to an external field of about 3.5 T to saturate the FePt and to introduce a remanent uniaxial magnetization along the direction of the incident wave vector k0. During the measurements, the sample was cooled to 4.2 K and subjected to variable external fields H perpendicular to the beam direction. To measure the depth dependence of the spin rotation, the sample was displaced transversely to the beam. Time spectra taken at various positions Dx are shown in Fig. 8.6 for an external field of H = 160 mT.
