Frequency dependence of THz induced ultrafast demagnetization dynamics in amorphous ferromagnetic films

The interaction between magnetism and light is receiving considerable interest in recent years, after the ground-breaking experiments that showed that ultrashort (~100 fs) infrared light pulses can be used to demagnetize [1] or even switch [2] the magnetization of thin film ferromagnets. However, the fundamental physical processes governing the ultrafast magnetization have proven to be challenging to understand. Two main mechanisms have been put forward as possible ways of absorbing spin angular momentum: dissipation of spins through the electronic system, via the creation of super diffusive spin-currents [3], or via the phonon bath, through spin- orbit scattering of Elliot-Yafet type [4]. Although experimental evidence for both mechanisms has been reported, their relative contributions to ultrafast demagnetization remain debated with the accurate modelling of the infrared fs laser-induced highly non-equilibrium state remaining a key obstacle.
Here, we represent our recent work on thin amorphous ferromagnetic films using unique multicycle, strong THz fields from the High-field THz user facility TELBE [5] to drive ultrafast magnetization dynamics. With the THz pulse duration of the order of the electronic and spin scattering events, it was possible to assess the influence of elementary scattering processes on the sample magnetization while a non-equilibrium current is flowing in the magnetic material. Preliminary result shows that ultrafast magnetization depends both on the lattice ordering and the frequency of THz pulses used. This effect was quadratic in the THz field strength and could be separated from the magnetization precession around the THz magnetic field. Complementary THz conductivity measurements allow us to relate these observations to defect-induced spin-lattice scattering processes of Elliot-Yafet type [6].

References:

[1] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996).
[2] C. D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007).
[3] M. Battiato et al., Phys. Rev. Lett. 105, 027203 (2010).
[4] B. Koopmans et al., Phys. Rev. Lett. 95, 267207 (2005).
[5] B. Green et. al., Sci. Rep. 5, 22256 (2016).
[6] B. Koopmans, et al. Nature Mat. 9, 259 (2010).