Speaker
Description
To push writing of magnetic information to ultrafast time scales, it is essential to understand the response of a magnetically ordered solid to an instantaneous perturbation. A model experiment in the case of a ferromagnet (such as Fe) is an excitation by a femtosecond laser pulse. The resulting magnetization quenching is known to proceed on a surprisingly fast time scale of 100 fs, and this observation is still a topic of extensive research [1,2]. One open question is how the transfer of spin angular momentum to the crystal lattice depends on the nature of the photoexcited electron distribution, that is, thermal (Fermi-function-like) vs nonthermal. In case of antiferromagnets, recent advances demonstrate possibilities to coherently control antiferromagnetic order through electric currents by means of the so-called Néel spin-orbit torque (NSOT) [3]. Switching of CuMnAs was achieved using free-space terahertz pulses [4], but just in before/after manner. Thus, the intrinsic timescales of this process are still unknown.
In a first experiment, we tune the electron distribution directly after optical excitation by variation of the photon energy. Low photon energies (in particular smaller than the thermal energy of 25 meV at 300 K) are expected to induce a transient distribution that enables a much smaller phase space for electron scattering than a distribution induced with optical photon energies (~1 eV). We perform a direct comparison of the ultrafast demagnetization (UDM) dynamics in Fe thin film following excitation with ultrashort optical (3 eV) and terahertz (4 meV) pump pulses. By using deconvolution procedure, we are able to push the time resolution down to 130 fs and demonstrate identical demagnetization dynamics for both types of excitation [5]. Supported by a simple spin-flip scattering model, this result shows that the UDM depends only on the amount of energy put into the electron system. Therefore, UDM-related effects, such as toggle switching of GdFeCo, can be achieved using pulsed laser source of any wavelength.
In a second experiment, we employ a THz-pump optical-probe setup to investigate ultrafast dynamics of antiferromagnetic order induced by NSOT in Mn2Au. The direction of the Néel vector was prealigned via spin-flop transition in a high magnetic field (60 T). We observe a signal that depends linearly on the driving THz field and is consistent with NSOT-driven spin dynamics both in frequency and symmetry. The recorded spin motion corresponds to a strongly damped magnon mode with frequency of 0.6 THz. Based on our results, we can estimate important material-specific parameters and calculate THz pulse field strengths that one needs to switch the antiferromagnetic order of Mn2Au on picosecond timescale.
References
[1] E. Beaurepaire, J. C. Merle, A. Daunois, and J. Y. Bigot, Ultrafast Spin Dynamics in Ferromagnetic Nickel, Phys. Rev. Lett. 76, 4250 (1996).
[2] B. Koopmans et al., Explaining the Paradoxical Diversity of Ultrafast Laser-Induced Demagnetization, Nat. Mater. 9, 259 (2010).
[3] P. Wadley et al., Electrical switching of an antiferromagnet, Science 351, 6273 (2016).
[4] K. Olejnik et al., Terahertz electrical writing speed in an antiferromagnetic memory, Sci. Adv. 4, 3 (2018).
[5] A. L. Chekhov, Y. Behovits, J. J. F. Heitz, et al., Ultrafast demagnetization of iron induced by optical vs terahertz pulses, PRX (accepted for publication).
