Time-resolved XMCD-PEEM

Many devices used in magnetic storage, like Magnetic Tunnel Junctions (MTJ) and Spin Valves (SV) rely on the Giant Magnetoresistance effect [1,2], the dependence of the resistance of a multilayered magnetic system on the relative orientation of the magnetization directions in different magnetic layers separated by non-magnetic spacer layers. The interplay between electron currents and local magnetization through the electron spins can also lead to the inverse effect, the switching of magnetization by a spin-polarized current, as has been shown recently [3,4]. In both cases, it is important to get microscopic information on the switching of each magnetic layer separately, and on the interaction between the magnetic layers through the non-magnetic spacer layers. This information can be obtained by combing the element selectivity of time-resolved X-ray Magnetic Circular Dichroism (XMCD) with the spatial resolution of Photoemission Electron Microscopy (PEEM). The first time-resolved XMCD-PEEM results were published in 2003 [5,6], and the technique has since then been used mainly for studying magnetization dynamics in soft magnetic Permalloy microstructures [7-10] and in FeNi/X/Co (with X = Cu, Al2O3) trilayer systems [11-16]. It has allowed obtaining detailed information on the dynamics of magnetic domains, domain walls and vortices in flux-closure magnetic patterns [7-10], on domain wall propagation speeds in continuous films [14] and on the influence of magnetic anisotropy [12], domain wall interactions [13] and layer topography [15] on the switching of the soft magnetic layer in trilayer systems.

All of the above mentioned experiments were performed in a pump-probe or stroboscopic mode, in which the system was excited using magnetic pulses that were synchronized with synchrotron x-ray pulses [5]. The temporal resolution of this technique is determined by the length of the x-ray bunches, which is typically in the order of 50-100 picoseconds. Magnetic pulses are applied to the sample at the repetition rate of the synchrotron x-ray bunches (several MHz) (see Figure). Details of the experimental setup used for time-resolved XMCD-PEEM by our group, in collaboration with the group of Prof. W. Kuch at the FU-Berlin, can be found in Refs. [5] and [16].

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[6] A. Krasyuk et al., Appl. Phys. A - Mater. Sci. & Proc. 76, 863 (2003).

[7] C.M. Schneider et al., Appl. Phys. Lett. 85, 2562 (2004).

[8] S.-B. Choe et al., Science 304, 420 (2004).

[9] J. Raabe et al., Phys. Rev. Lett. 94, 217204 (2005).

[10] A. Krasyuk et al., Phys. Rev. Lett. 95, 207201 (2005).

[11] W. Kuch et al., Appl. Phys. Lett. 85, 440 (2004)./br>
[12] J. Vogel et al., Phys. Rev. B 71, 060404(R) (2005).

[13] J. Vogel et al., Phys. Rev. B 72, 220402(R) (2005).

[14] K. Fukumoto et al., Phys. Rev. Lett. 96, 097204 (2006).

[15] F. Romanens et al., Phys. Rev. B 74, 184419 (2006).

[16] J. Vogel et al., J. Appl. Phys. 95, 6533 (2004).

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