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Experiments on THz magnetization dynamics provide insights into a yet unexplored time scale of transient magnetics and spintronics [1-3]. Coherent responses of the magnetization to THz pulses have been demonstrated [1,2]. Further, THz spin currents can be generated by non-equilibrium hot electrons [3]. These THz transients provide new ways to ultrafast spin control and its technical applications. Here, an analytical model is presented that describes transient magnetization dynamics up to the THz regime [4]. The model is used to determine the magnetization response to ultrafast multi and single-cycle THz pulses for a variety of parameters like carrier frequency, width, phase, frequency chirp, and polarization. The THz pulse shape and polarization provide a vectorial control of the magnetization on the sub-picosecond time scale. It is shown that an optimum timing for coherent magnetization control can be achieved.
Dynamics of the magnetization induce a spin current in an adjacent non-magnetic material. This effect s known as spin pumping [5]. Here, calculations of THz transient spin current generation by spin pumping are presented. An effective spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small at THz frequencies. At THz frequencies, the coherent reaction of the magnetization also causes a coherent spin current. In contrast to the dc spin current which scales with the susceptibility of the magnetization, the ac spin current does not vanish above the ferromagnetic resonance. Instead the THz ac spin current reaches a value that is comparable to the dc spin current at resonance. The behavior far above resonance can be used to efficiently generate ultrafast spin currents without the need for magnetic systems with very high resonance frequencies. Spin currents on picosecond time scales can be achieved by THz magnetization dynamics.
References:
[1] C. Vicario et al., Nat. Photonics 7, 720 (2013),
[2] T. Kampfrath et al., Nat. Photonics 5, 31 (2011)
[3] T. Kampfrath et al., Nat. Nanotechnl. 8, 256 (2013)
[4] L. Bocklage, Sci. Rep. 6, 22767 (2016) & J. Magn. Magn. Mater 429, 324 (2017)
[5] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys Rev. Lett. 88, 117601 (2002)
[6] D. Wei et al., Nat. Comm. 5, 3768 (2014)
[7] L. Bocklage, (submitted)
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