# Investigating Coherent Magnetization Control with Ultrashort THz Pulses
## 2 Materials and Methods  
THz radiation is produced by the THz undulator of the BL3 beamline at FLASH, a purpose built planar electromagnetic undulator with 9 full periods of 40 cm [12]. It can generate radiation in a very broad spectral range from the mid infrared to the far infrared spectral regions corresponding to wavelengths between 1 \(\upmu\)m and 300 \(\upmu\)m (respectively 300 THz and 1 THz). The pulse duration and intensity depends on the wavelength chosen: the THz pulse has about ten optical cycles (corresponding to a pulse duration of about 33.3 fs and 10 ps respectively) and the maximum pulse energy is 150 \(\upmu\)] [5]. The THz undulator is implemented in series, and can be operated simultaneously to the XUV undulator of FLASH for the realization of THz pump-XUV probe experiments (see Figure 2). To do so, an electron bunch is accelerated in the tunnel, passes into the XUV undulator and generates an XUV pulse. The electrons bunch then passes through the THz undulator and generates a THz pulse. Those two pulses are separated by a holey mirror and then recombine onto the sample thanks to a series of mirrors. Since both THz and XUV pulses are emitted from the same electron bunch, they are inherently synchronized [5,12]. In more detail, the experimental setup consists of two vacuum chambers. The incoming XUV beam (red in Figure 2) passes through the first chamber and is reflected back and focused by a spherical mirror in the second chamber (not shown in the Figure 2). The tilts of this back mirror are motorized in order to be able to change the position of the beam on the sample. This allow us to find the spatial overlap of the probe beam with the THz beam. The reflected and focused XUV beam comes back to  
Figure 2: Layout of the experimental setup. XUV probe pulse (red) and THz pump pulse (blue) are produced by the XUV undulator and the THz undulator respectively. The zoom on the main chamber shows the THz beam focused onto the sample by a parabola and the probe beam going through the chamber before coming back focused onto the sample by a spherical mirror (not shown). Two APDs are used to measure the intensity of the incoming probe beam (measuring the partial reflection on a Si\({}_{3}\)N\({}_{4}\) window) and the signal reflected by the sample. through a center aperture in the THz off-axis parabola (orange in the zoom of Figure 2) and eventually arrives on the sample. In the meantime, the THz beam enters the main chamber perpendicularly to the XUV beam and is focused onto the sample by the off-axis parabola. The exact spots sizes on the sample could not be measure in-situ but the characteristics of the beams and the focusing mirrors should yield a focal spot diameter of lower than 100 \(\upmu\)m for XUV and larger than 400 \(\upmu\)m for THz ensuring a uniform excitation of the probed area. To probe the magnetization dynamics, we used the time-resolved Transverse Magneto-Optical Kerr Effect (T-MOKE). The basic principles of this technique have been presented in many reports [13; 14; 15]. The geometry is represented in Figure 3. The incident angle on the sample is chosen to be close to 45\({}^{\circ}\) to maximize the T-MOKE signal. Two avalanche photodiodes (APD) measure the XUV beam incoming intensity, \(I_{0}\), and the reflected signal, \(I_{\mathrm{r}}\), from the sample. An electromagnet is used to magnetize the sample in plane in two opposite directions, \(M_{+}\) and \(M_{-}\), giving rise to two different signal intensities, \(I_{+}\) and \(I_{-}\), measured as \(\frac{I_{\mathrm{r}}}{I_{0}}\) for positive and negative magnetizations respectively. The sample studied consisted of a 10 nm thin Py film sputtered on a silicon substrate with a 3 nm thin Ta buffer and capped with a 3 nm thin Al layer to prevent oxidation. To measure the magnetization dynamics of this sample, we tuned the XFEL to a photon energy of 66.6 eV, in resonance with the \(M_{2,3}\) absorption edges of Ni, and recorded \(I_{+}\) and \(I_{-}\) for delays between the THz pump and the XUV probe ranging from \(-\)5 ps to 15 ps.