# Investigating Coherent Magnetization Control with Ultrashort THz Pulses
## 3 Results  
First, we tuned the THz undulator to obtain a fundamental wavelength of 100 \(\upmu\)m. To suppress higher harmonics, we used a 50 \(\upmu\)m long-pass filter. This should results in a pump pulse consisting of about 10 oscillations of a 3 THz electromagnetic field and a pulse duration of about 3.3 ps. The coupling between the magnetic field of this radiation and the magnetization of the sample should results in oscillations in the magnetic signal with a period of about 333 fs (the period of the THz radiation and hence of the associated magnetic field). From previous experiments [3], a THz fluence of about 10 mJ cm\({}^{-1}\) should be sufficient to trigger this effect (1 mJ cm\({}^{-1}\) were enough for a single cycle 2.1 THz pulse). Figure 3: Time resolved T-MOKE configuration showing the probe (red) and pump (blue) beams, their footprints on the sample (red and blue spots) which is situated in the air gap of an electromagnet. The two opposite directions of magnetization (\(M_{+}\) and \(M_{-}\)) of the sample are represented (violet and cyan arrows) as well as the corresponding pulse intensities \(I_{+}\) and \(I_{-}\) (violet and cyan envelopes). Figure 4a shows the sample reflectivity as function of time delay for the two opposite magnetization direction of the sample. Because of T-MOKE, the reflectivity for \(M_{+}\) is almost 3 times higher than the reflectivity for \(M_{-}\). However, there is no effect of the THz field on the magnetization of the sample. After investigating at different THz wavelengths we came to the conclusion that the magnetic field associated to the THz pump was too weak to induce a magnetic response. This was most probably due to a combination of two factors: the intensity in the fundamental was a little lower than expected and the focal spot was too large to compensate and obtain the right fluence. To increase the pump pulse intensity, we removed the long-pass filter. This should increase the pulse intensity by a factor of at least 4 [5]. By doing so, we were able to trigger a modification of the sample magnetization state with the THz pump (Figure 4b). To analyze this magnetization dynamics in more detail, we have plotted the so-called asymmetry, defined as \(A=\frac{I_{+}-I_{-}}{I_{+}+I_{-}}\), in Figure 5 for two different THz fundamental wavelength, 45 and 100 \(\upmu\)m. The curves appear very similar to the typical curves obtained by performing a demagnetization experiment with femtosecond IR pulses. The magnetization dynamics is nonetheless very different to the typical ultrafast demagnetization in transition metals thin films [16; 17]. First, as seen on Figure 5, the demagnetization occurs on a much longer timescale of a few picoseconds. Moreover, the timescale of the dynamics depends on the fundamental wavelength of the THz pump. Finally, the demagnetization curves appear to be the results of several different demagnetization processes triggered successively. All together, these point out to the fact that our measurements are dominated by the shape of the THz pulse. Figure 4: Normalized T-MOKE intensity for two opposite magnetization directions (**a**) for a single frequency THz pump (3 THz) and (**b**) for a pump consisting of the complete radiation content delivered by the THz undulator (fundamental frequency of 6.7 THz). In the first case, the magnetization does not change while in the latter case the pump triggers a modification of the magnetization on a timescale of a few ps.