Small and large angle precession in exchange biased bilayers

Small and large angle excitations in exchange bias systems have been investigated in real time by means of all-optical pump-probe experiments. Time resolved magneto-optics has been employed as a probe of the transient magnetic phenomena upon laser excitation. Due to an increased spin temperature upon photoexcitation, an unpinning of the interfacial exchange coupling takes place resulting in a collapse of the unidirectional anisotropy. An effective internal anisotropy pulse field with a rise time of the order of the pump laser pulse duration (τ =8.3 ps) is launched which governs the time evolution of the effective field acting on the magnetization of the ferromagnetic layer of the bilayer system. The time evolution of the exchange bias shift field and the zero-field susceptibility reveal a fingerprint of the internal field pulse and can be understood taking ultrafast thermal activation into account [1]. The excess energy of the spin system - upon a sudden increase of the interfacial spin temperature – can lead to the excitation of a high frequency precessional response of the magnetization of the ferromagnetic layer for both the easy and the hard axis geometry [2,3]. Even, the initial deflection direction of the magnetization of the ferromagnet away from the initial equilibrium orientation can be controlled on the picosecond timescale in the hard axis geometry. The magnitude of the internal pulse field, and thus the torque acting on the magnetization of the ferromagnetic layer, can be controlled by the absorbed photons [3]. Hence, the precessional motion depending on the magnitude of the anisotropy pulse field, i.e., the precession angle can
be investigated. Moreover, by comparing the measured real time precessional motion with solutions of the Landau-Lifshitz and Gilbert equation the dependence of the effective Gilbert parameter α on the precession angle can be studied. The extracted Gilbert parameter depends not or only rather weakly on the magnitude of the internal pulse field. There are no non-linear effects present for this material system. Both small and large angle precession of the ferromagnetic layer upon photoexcitation can be modeled with reasonable values of the Gilbert parameter within the Landau-Lifshitz and Gilbert framework.
Employing the known antiferromagnetic thickness dependence of the exchange bias field Heb, the exchange bias field dependence of the Gilbert parameter α was investigated. For this purpose a wedge shaped exchange bias bilayer with a fixed thickness of the ferromagnetic layer and a varying thickness of the antiferromagnetic layer along the sample was prepared and measured.
The extracted Gilbert parameter from the time-resolved Kerr traces and thus, the dissipation rate increases linearly with the exchange bias field magnitude. Local fluctuations of the interfacial exchange coupling, due to interface roughness, can increase the two-magnon relaxation probability, which in terms of an additional dissipation channel finally leads to an increased Gilbert damping parameter [4,5].
The work is supported by the Graduiertenkolleg 792 of the Deutsche Forschungsgemeinschaft and the European Communities Human Potential programs HPRN-CT-2002-00318
ULTRASWITCH and HPRN-CT-2002-00296 NEXBIAS.

References
[1] M.C. Weber, H. Nembach, and J. Fassbender, J. Appl. Phys. 95, 6613 (2004).
[2] G. Ju, A. V. Nurmikko, R.F.C. Farrow, R.F. Marks, M.J. Carey, and B.A. Gurney, Phys. Rev. Lett. 82, 3705 (1999).
[3] M.C. Weber, H. Nembach, B. Hillebrands, and J. Fassbender, Phys. Rev. B, submitted for publication.
[4] S.M. Rezende, A. Azevedo, M.A. Lucena, and F.M. de Aguiar, Phys. Rev. B 63, 214418 (2001).
[5] M.C. Weber, H. Nembach, B. Hillebrands, and J. Fassbender, J. Appl. Phys, in press.