Control of electron, magnetic, and lattice states by optical excitations in magnetically ordered materials has attracted considerable attention due to their potential applications in electronic and magnetic recording media functioning on an ultrafast time scale below nanosecond (ns, 10⁻⁹ second, GHz range). To capture their non-equilibrium dynamics, ultrafast time-resolved experiments have been carried out using a bunched synchrotron light source, and x-ray free electron lasers [1]. FePt thin films have drawn intense research interest owing to their potential for high density recording applications by using their magnetism. Recently, Lambert et al. have showed that circularly polarized laser pulses can induce a small helicity-dependent magnetization in FePtAgC granular film [2]. This finding suggests the possibility of all-optical switching of the magnetism in FePt having very short recording-time windows on the ps time scale.

Fig. 1. (a) Overview of the setup for Tr-XAS and Tr-RSXS measurements at BL07LSU of SPring-8. The Ti-sapphire laser with a ~1 kHz repetition rate, which is synchronized with the bunches of the synchrotron, is introduced into the experimental chamber. (b) Electrical circuits for the detectors.

Fig. 2. (a) Time-evolutions of XMCD taken in the PEY mode at(a) L₃ (hν=707.0 eV) and (b) L₂ (hν=720.2 eV) edges for the FePt thin film at room temperature. (c) Laser-fluence dependence of Tr-XMCD intensities (μ+ and μ−) at t=30 ps on the L₃ edge (hν=707.0 eV). The dashed lines denote results of the linear fittings having a threshold.

Here, we present a setting for time-resolved (Tr-) x-ray absorption spectroscopy (XAS) and Tr- x-ray magneto circular dichroism (XMCD) in the partial electron yield (PEY) and total fluorescence yield (TFY) modes to measure non-transmissive as-grown samples at nearly normal incidence [3]. The results of Tr-XMCD experiments for as-grown FePt films with non-transmissive substrates are reported. A photo-induced ultrafast demagnetization at the Fe sites of the FePt thin film are observed within the experimental time resolution of 50 ps for Tr-XMCD at SPring-8.

Figure 1 shows an overview of the experimental setup for Tr-XAS, Tr-XMCD, and Tr- resonant soft x-ray scattering (RSXS) measurements in the soft x-ray region at BL07LSU of SPring-8. Tr-RSXS is performed at the θ < 90º side of the experimental chamber. The scattering is detected by the micro-channel plate (MCP) or avalanche photodiode (APD) installed on the 2θ motion of the diffractometer. On the other hand, XAS and XMCD in the PEY or TFY modes are measured at the θ > 90º side of the chamber. Emitted photoelectrons or x-ray fluorescence are caught by another MCP topped on the linear motion. The femtosecond Ti:sapphire laser with a wavelength of 800 nm is introduced into the XAS and RSXS chamber. The laser irradiates samples and photo-induced dynamics of the electronic and structural evolutions are examined by means of a pump-probed technique. The laser pulses with 1 kHz repetition rate are synchronized with selected bunches of the synchrotron and delayed electronically. The signals of the MCP or APD are amplified and gated on the oscilloscope or Boxcar integrator which is triggered by the laser pulses (Fig. 1 (b)).

The time evolutions of the FePt thin film with 16 mJ/cm² laser irradiations are given in Fig. 2 (a) at hν=707.0 eV (L₃ edge) and (b) at 720.2 eV (L₂ edge), respectively. As can be seen, almost similar time-evolutions are observed for the Fe L_2,3 edges. XMCD exhibits a reduction in its intensity by ~90% of the original value ~30 ps after the pump pulse. Then the XMCD exhibits a slow recovery of the magnetization with a time constant of ~150 ps. In addition, the magnetization is not recovered to the original value even at t=1500 ps, which is completed before the next bunch arrives after ~342 ns. Figure 2 (c) gives the Tr-XMCD intensities at t = 30 ps as a function of the laser fluence. Threshold-like behavior is observed at 6.5 mJ/cm² for t = 30 ps, which is estimated by the linear fittings for the fluence dependences. These results indicate that the photoinduced effect will not be a simple thermal effect.

The less-distorted PEY method presented in this study will provide an opportunity for applying a sum rule analysis to XMCD spectra in the ultrafast pump-probed spectroscopy for various magnetic materials, which would be very useful to explore the all-optical switching in spintronics devices such as FePt, Co/Pt multi-layers and so forth. On the other hand, the bulk sensitive TFY method will allow us to measure the dynamics of bulk samples showing various quantum phenomena.

References

• [1] A. Kirilyuk, A. V. Kimel, and T. Rasing, Rev. Mod. Phys. 82, 2731 (2010) and reference there in.
• [2] C.-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman, M. Aeschlimann, and E. E. Fullerton, Science 345, 1337 (2014).
• [3] K. Takubo, K. Yamamoto, Y. Hirata, Y. Yokoyama, Y. Kubota, S. Yamamoto, S. Yamamoto, I, Matsuda, S. Shin, T. Seki, K. Takanashi, and H. Wadati, Appl. Phys. Lett. 110, 162401 (2017).

Authors

• K. Takubo, K. Yamamoto, Y. Hirata, Y. Yokoyama, Y. Kubota, S. Yamamoto, S. Yamamoto, I. Matsuda, S. Shin, T. Seki^a, K. Takanashi^a, and H. Wadati
• ^aInstitute for Materials Research, Tohoku University