Magnetization dynamics in lanthanide metals probed by photoelectron spectroscopy with higher-order harmonic radiation

• Applicant: Professor Dr. Martin Weinelt
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2010 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 169010978

• Final Report Year: 2020

Final Report Abstract

In summary, we performed the proposed experiments and obtained valuable insights into the nonequilibrium behavior of the lanthanides electronic structure. We have shown that the magnetic systems of single-crystalline Gd and Tb films exhibit very different behavior after laser excitation. Especially Gd is driven far from equilibrium, demonstrating that the intra-atomic exchange interaction, which is generally assumed to be quasi-instantaneous, is broken on a ten picosecond timescale. The distinct minority and majority spin dynamics in the valence band, reflecting the magnetization of the itinerant 5d6s electrons, give insights into the ultrafast processes comprising fast exchange scattering which drives the minority band response as well as slower EY-like spin-flips which determine the majority band dynamics. The difference in the 4f response between Gd and Tb are attributed to ordinary spin-lattice coupling and can adequately be described by Gilbert damping using a ratio between the damping parameters of both metals which is also found in between their magnetocrystalline anisotropy energies. The strong spin-lattice coupling in Tb lead, thereby, to a fast response which keeps the magnetic system – contrary to Gd – close to equilibrium. Besides these main results we examined in Gd the pump laser fluence and temperature dependence of the valence band dynamics and found comparable results to those obtained earlier by MOKE. With our method we contributed to this extremely active field by the additional insight into the origin of the dependency. The temperature dependence is driven by the fast minority bulk band, which we explain in terms of electron-electron and electron-magnon scattering. The fluence dependence on the other hand is driven by the slower majority bulk band, which can be explained by EY-scattering. In addition to these physical insights, our studies raised questions about the comparability of different magnetic techniques in the ultrafast regime. In particular, the discrepancy between our MLD and published XMCD dynamics needs further investigation. In conclusion, we have gained significant microscopic insights into the role of inter- and intra-atomic exchange coupling during laser driven demagnetization. The former, which is predominately influenced by Elliot-Yafet and exchange scattering is reflected by the dynamics of the itinerant electrons. The latter is, surprisingly, not sufficient to establish magnetic equilibrium on the ultrafast timescales and the dynamics of the localized spins are basically determined by spin-lattice interaction alone. This discrepancy, which shows that intra-atomic nonequilibrium conditions persists over several picoseconds, was observed for the first time and was not expected from existing theoretical understanding. With these key insights we have contributed significantly to the field of laser-driven magnetization dynamics, emphasizing that the role of lanthanide alloyants in all optical magnetic switching experiments of RE-TM ferrimangets could be ultimately determined by the interplay of their itinerant with the localized electrons.

Publications

• “Femtosecond Laser Excitation Drives Ferromagnetic Gadolinium out of Magnetic Equilibrium”, Physical Review Letters 109, 057401 (2012)
  R. Carley, K. Döbrich, B. Frietsch, C. Gahl, M. Teichmann, O. Schwarzkopf, P. Wernet and M. Weinelt
  
• “A high-order harmonic generation apparatus for time- and angle-resolved photoelectron spectroscopy”, Review of Scientific Instruments 84, 075106 (2013)
  B. Frietsch, R. Carley, K. Döbrich, C. Gahl, M. Teichmann, O. Schwarzkopf, P. Wernet and M. Weinelt
  
• “Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal”, Nature Communications 6, 8262 (2015)
  B. Frietsch, J. Bowlan, R. Carley, M. Teichmann, S. Wienholdt, D. Hinzke, U. Nowak, K. Carva, P. M. Oppeneer and M. Weinelt
  
• “Magnetization Dynamics of Itinerant and Localized Electrons in Lanthanide Metals”, PhD Thesis, Freie Universität Berlin (2015)
  B. Frietsch
  
• “The valence band structure of Gadolinium studied with time-resolved photoemission”, in Ultrafast Magnetism I (J.-Y. Bigot, W. Hübner, T. Rasing and R. Chantrell, eds.), Springer Proceedings in Physics 159, 274–277 (2015)
  B. Frietsch, J. Bowlan, R. Carley, M. Teichmann, J. Wolter, and M. Weinelt
  
• “Transient band structures in the ultrafast demagnetization of ferromagnetic gadolinium and terbium”, Physical Review B 91, 014425 (2015)
  M. Teichmann, B. Frietsch, K. Döbrich, R. Carley, and M. Weinelt
  
• “Fluence-dependent dynamics of the 5d6s exchange splitting in Gd metal after femtosecond laser excitation”, Japanese Journal of Applied Physics 55, 07MD02 (2016)
  B. Frietsch, R. Carley, M. Gleich, M. Teichmann, J. Bowlan and M. Weinelt