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Currently there is a strong interest in the physics of magnetic films and multilayers that are coupled by the exchange-spring effect [1]. This effect is observed at a boundary between a hard- and a soft-magnetic phase : Directly at the interface, the magnetization of the soft magnetic film is pinned to the magnetization of the hard magnetic film. With increasing distance form the interface, the magnetization of the soft magnetic film may rotate under the influence of an external field. Such magnets have important applications in the design of new permanent magnets with an energy product that is significantly above that of existing magnets [1]. In a bulk permanent magnet based on nanocrystalline structure, the hard-magnetic grains provide the high anisotropy and coercive fields while the soft-magnetic grains enhance the saturation magnetization. For a microscopic understanding of the magnetic behaviour of such novel magnets it is essential to investigate suitable model systems like exchange-spring coupled thin films and multilayers. In this experiment we have studied the spin structure in a thin Fe film on a hard-magnetic FePt layer in an external magnetic field.
A 30 nm thick FePt layer was deposited on a superpolished Si wafer by radiofrequency sputtering from an Fe target covered with Pt chips. Annealing for 30 min at 500°led to the formation of the tetragonal FePt phase. Magnetooptical Kerr effect measurements were performed to determine the coercive force of these layers to be in the range of 1 Tesla. An external magnetic field of 300 mT was applied during the annealing process to induce a uniaxial magnetic anisotropy. Afterwards a 33 nm thick Fe layer, enriched to 95% in 57Fe, was deposited on the hard-magnetic FePt layer.
We have studied the magnetic structure of the Fe layer subjected to an external magnetic field by nuclear resonant scattering (NRS) of synchrotron radiation. This method is ideally suited for this purpose because it is sensitive to the magnitude and the direction of the magnetic fields at the nuclei. Nuclear probes as 57Fe can be employed to to reveal the spin structure of magnetic materials with very high spatial resolution, reaching even sub-monolayer sensitivity [2, 3]. Due to the isotopic sensitivity of NRS, the measured signal yields the properties of the Fe in a magnetic environment which can be an important advantage compared to other methods like x-ray magnetic dichroism and the magnetooptical Kerr effect.
The experiment was performed at beamline BW4 of HASYLAB. A permanent magnet provided an
external magnetic field of 300 mT that could be oriented in the plane of the sample. The sample
was illuminated at 3.8 mrad, the critical angle for electronic charge scattering from Fe at 14.4
keV. In this geometry we obtained an average countrate of about 10 Hz that allowed to obtain
datasets with reasonable statistical quality within 1 h. Time spectra for various orientations of the
external magnetic field are shown in fig. 1a. The solid lines are simulations by using the program
package CONUSS. We found that the spin direction of the Fe film aligned between the external
field and the axis of the uniaxial anisotropy of the Fe film. With increasing azimuthal angle
the
direction of the Fe spins lags more and more behind the external field direction, as depicted in
fig. 1b. This behaviour, schematically sketched in fig. 1c, is in contrast to the model of an exchange
spring magnet based on Fe/SmCo that is shown in fig. 1d. In this case the magnetization of the soft
magnetic layer is strongly pinned to the uniaxial magnetization of the hard magnetic layer. Thus,
in an external field the moments in the soft-magnetic film follow a helical rotation as a function
of depth. We assume that in the system investigated here the coupling between the Fe and FePt is
much weaker which is the observed behaviour. Further studies using thin 57Fe probe layers within
the soft-magnetic layer should provide more information about the coupling mechanism.
 Figure 1: a) Time spectra of NRS from the Fe/FePt layer system for various azimuthal angles of
the external magnetic field. b) Azimuthal angle 
of the Fe layer. c),d) Spin structures of Fe/FePt and Fe/SmCo exchange-coupled layer systems.
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References
[1] E. E. Fullerton, J. S. Jiang, M. Grimsditch, C. H. Sowers, and S. D. Bader, Phys. Rev. B 58, 12193 (1998).
[2] L. Niesen, M. F. Rosu, A. Mugarza, R. Coehoorn, R. M. Jungblut, F. Rooseboom, A. Q. R. Baron, A. I. Chumakov, and R. Rüffer, Phys. Rev. B 58, 8590 (1998).
[3] R. Röhlsberger, J. Bansmann, V. Senz, K. L. Jonas, A. Bettac, O. Leupold, R. R¨uffer, E. Burkel, and K.H. Meiwes-Broer, Phys. Rev. Lett. 86, 5597 (2001).
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