With the increasing complexity of layered magnetic nanostructures there is a growing need for experimental methods capable of imaging the depth dependence of their properties. Nuclear resonant scattering of synchrotron radiation from isotopic probe layers provides an excellent tool for this purpose. In a recent experiment [1], we have investigated the exchange coupling of a soft-magnetic Fe layer deposited on a hard-magnetic FePt film. At the interface between the two layers, the moments of the soft-magnetic film are pinned to the unidirectional magnetisation MFePt of the hard-magnetic film. With increasing distance from the interface, the moments of the soft-magnetic film turn under the action of an external field Hext, resulting in a spiral magnetic structure, see Figure 8.


Fig. 8: Magnetic structure in the Fe layer of an Fe/FePt exchange spring as it forms in an external magnetic field of 160 mT. This image was obtained using an ultrathin probe layer embedded in the Fe film, shown on the right [1].


The structure of this spiral was imaged by employing a tilted ultrathin probe layer of 57Fe, as shown in Figure 8. Thus, the transverse displacement of the sample relative to the horizontally focussed beam enables one to select specific depths in the sample. The measurements benefited from the sample acting as an X-ray waveguide. Data acquisition times could be reduced drastically thanks to a strong coherent enhancement of the signal [2], so that systematic studies as a function of external parameters like temperature and external field became possible. One of these studies was aimed at the investigation of the exchange coupling between two ferromagnetic layers through a nonmagnetic spacer layer. Due to the confinement of spin-polarised electron waves in the spacer layer one expects an oscillatory behaviour of the coupling strength. This manifests itself in a periodic ferromagnetic/antiferromagnetic coupling between the two films as a function of the spacer layer thickness.

The sample investigated here consists of an Fe layer on a hard-magnetic FePt layer (L10 phase) with unidirectional anisotropy, separated by a wedge-shaped Cu layer, as sketched in Figure 9a. To determine the coupling angle between the films, a 57Fe probe layer is embedded in the centre of the Fe. Surprisingly, no variation of the coupling angle can be seen in remanence after saturation in an external field of 2.4 T. This situation changes drastically after application of a small external field in the reverse direction. Remarkably, one observes a modification of the remanent coupling angles, as illustrated in Figure 9. Below fields of 45 mT the strongest reorientation of the Fe moments occurs for thin spacer layer thicknesses, followed by a monotonous decrease with increasing Cu thickness. Above reverse fields of 50 mT an oscillatory behaviour of the coupling angle sets in. However, the coupling angles are below 40° and no evidence for an antiparallel component of the Fe magnetisation was found.

This interesting behaviour can be explained by the superposition of two coupling mechanisms in this layer system, i.e. dipolar coupling and interlayer exchange coupling. After saturation of the layer system the remanent state is determined by a dipolar coupling induced by interface roughness, resulting in a parallel alignment of both ferromagnetic layers. With increasing magnitude of the external field, this dipolar coupling is influenced by the external field, which leads to largest coupling angles at low spacer layer thicknesses. With further increase of the external field, another mechanism comes into play: small (e.g., soft-magnetic) regions of the FePt film turn into the direction of the reverse field. The presence of these antiparallel domains leads to a frustration of the moments in the Fe film. In this case an alignment of the Fe film towards the perpendicular direction (biquadratic coupling) becomes energetically more favourable. Since this interaction is mediated by the interlayer exchange coupling, it carries an oscillating signature[3].


Fig. 9: Lateral remanent magnetic structure of the Fe layer in an exchange-coupled Fe/Cu/FePt trilayer. (a) After saturation in an external field of 2.4 T and application of a reverse field of 30 mT and (b) a reverse field of 60 mT. (c) Colour coded map of the coupling angle as function of the spacer layer thickness and the reverse external field.


These studies show that remanent coupling angles between soft magnetic and hard magnetic thin films can be tuned by the application of external fields. It was found that such effects are even more pronounced if the external field is applied in a direction perpendicular to the unidirectional anisotropy of the FePt. This opens interesting perspectives for the manipulation of magnetic nanostructures with potential applications in magnetic data storage.

[1] R. Röhlsberger, H. Thomas, K. Schlage, E. Burkel, O. Leupold, and R. Rüffer, Phys. Rev. Lett. 89, 237201 (2002).
[2] R. Röhlsberger, T. Klein, K. Schlage, O. Leupold, and R. Rüffer, Phys. Rev. B 69, 235412 (2004).
[3] V. K. Vlasko-Vlasov et al., Phys. Rev. Lett. 86, 4386 (2001).

T. Klein (a), R. Röhlsberger (b), K. Schlage (a), H. Thomas (a), O. Leupold (b), and E. Burkel (a), submitted to Phys. Rev. Lett. (2004).
(a) Institut für Physik, Universität Rostock (Germany)
(b) HASYLAB@DESY, Hamburg (Germany)