# Investigating Coherent Magnetization Control with Ultrashort THz Pulses  
[PERSON]  
1 Laboratoire de Chimie Physique-Matieere et Rayonnement (LPCMR), Sorbonne Universite,  
Centre National de la Recherche Scientifique (CNRS), 75252 Paris, France; [PERSON]ADDRESS] (X.L. ); [PERSON]AIL_ADDRESS] (E.J. ); [PERSON] (R.D. ); [PERSON]] (R.J.); [PERSON]S.C.); [PERSON]RESS] (J.E. );  
Institut Jean Lamour (UMR 7198), Universite de Lorraine, Centre National de la Recherche Scientifique (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (D.E. ); Deutsches Elektronen-Synchrotron, 22607 Hamburg, Germany; torsten.golz@class5 photonics.com (T.G. ); ekaterina.zpolowicz@class5 photonics.com (E.Z. ); rui.pan@desyde (R.P. ); [EMAIL_ADDRESS] (N.S.) Institut Jean Lamour (UMR 7198), Universite de Lorraine, Centre National de la Recherche Scientifique (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (D.E. );  
Institut Jean Lamour (UMR 7198), Universite de Lorraine, Centre National de la Recherche Scientifique (CNRS), 54011 Nancy, France; [EMAIL_ADDRESS] (CNRS), 54011 Nancy, France; greorgymalinowski@univ-lorraine. nitude) than that of a near IR photon which is generally used as pump in ultrafast magnetization experiments (0.004 eV versus 1.6 eV). Therefore, absorption of a THz photon can only occur by electrons very close to the Fermi level and only into lower energy levels than it is the case of an IR pump pulse (Figure 1). The THz pulse induced dynamics is then dominated by the interaction between the sample and the strong magnetic field of the THz pulse rather than by absorption and heating. An interesting question arises therefore, at which wavelength, or over which wavelengths range, we can observe the transition between the two behaviors. This knowledge will be fundamental to tailor ultrafast magnetic devices. Our goal is thus to study the transition from such a coherent coupling to the non-coherently excited ultrafast demagnetization phenomenon. Recently, the generation of THz pulses with much higher field amplitudes has become possible, which permitted a conceptually new approach for investigating low energy degrees of freedom in matter [4]. Additionally, advancements of accelerator technology achieved, for example at BL3 beamline of FLASH (DESY, Hamburg, Germany), made it possible to produce carrier-envelope phase stable THz pulses with high fields and adjustable high repetition rates [5]. Appropriate parameters settings of the THz undulator allows for the generation of narrow-band THz frequency and intense pulses, which can be used to trigger magnetization dynamics coherently. In the present work, we use synchronized THz-pump to an X-ray Free Electrons Laser (XFEL) source to investigate the THz induced magnetization dynamics in a Permalloy (Py) film. The tunable THz undulator allows us to select the pulse duration and the wavelength. A filter is used to remove higher orders generated by the undulator and to obtain a single frequency THz pulse. The extreme ultraviolet (XUV) radiation delivered by the XFEL, a powerful tool for probing magnetic phenomena at the nanometer and femtosecond scales [6; 7; 8; 9], is used to study the resonant magneto-optic response at the Ni \(M_{2,3}\) absorption edges (around 66.7 eV) [10]. Unfortunately, we did not succeed to achieve any ultrafast coherent magnetization control by the THz pump with a fundamental frequency of 100 \(\upmu\)m because the THz intensity was too low. However, after the removal of the filter from the THz undulator beam path, an ultrafast demagnetization of the Py film can be evidenced. The pump radiation is now composed of the fundamental frequency but also higher harmonics, that span from THz to IR wavelength range, which are responsible for this non coherent dynamics.