Ultrafast spin dynamics are driven by interactions between different spin systems and their coupling to electronic and lattice excitations. In this project, we intend to study the microscopic processes behind these interactions in 3d transition metals (TM) and 4f rare earth (RE) metals. With state-selective and spin-sensitive time and angle-resolved photoemission techniques we determine the transient electronic structure, spin polarization, and magnetization. We study electron scattering, spin transport, ultrafast demagnetization, and magnetic switching initiated by tunable optical femtosecond (fs) laser pulses.

Work package 1 investigates spin-dependent carrier lifetimes and the transient spin polarization in TM thin films using fs laser pulses for spin-, time-, and angle-resolved photoemission spectroscopy. We plan to establish back-pump/front-probe experiments to study spin-dependent electron transport in layered systems. Furthermore, we will employ surface states (Dy, Ni, Fe) as sensors for ultrafast magnetization dynamics and investigate their exchange splitting and spin polarization (Stoner vs. spin-mixing behavior).

Work package 2 addresses the transient bulk band-structure of TM, RE and RE-TM double layers using tunable vacuum ultraviolet pulses from our higher-order harmonic generation (HHG) source. After optical excitation we will follow the exchange splitting and magnetic linear dichroism in Dy, map parts of the Fermi surface of Ni and Fe, and study ultrafast magnetization dynamics in FeTb and FeGd double-layer systems.

With these experiments we aim for a microscopic understanding and improved fundamental knowledge of spin dynamics. Our long-term goals are the control of magnetization dynamics and efficient all-optical magnetic switching. From the experimental side, we want to establish spin-, time-, and angle-resolved photoemission with HHG radiation to map the Fermi surface and its spin polarization after excitation with tunable nearinfrared laser pulses.

Publications (1^st Funding Period 2018 - 2021)

• The role of spin-lattice coupling for ultrafast changes of the magnetic order in rare earth metals 
  B. Andres, S. E. Lee, and M. Weinelt 
  Appl. Phys. Lett. 119, 182404 (2021)  - DOI: 10.1063/5.0067397

• Signature of magnon polarons in electron relaxation on terbium
  B. Liu, H. Xiao, G. Siemann, J. Weber, B. Andres, W. Bronsch, P. M. Oppeneer, and M. Weinelt
  Phys. Rev. B 104, 024434 (2021)  - DOI: 10.1103/PhysRevB.104.024434

• Effect of Rashba splitting on ultrafast carrier dynamics in BiTeI 
  A. S. Ketterl, B. Andres, M. Polverigiani, V. Voroshnin, C. Gahl, K. A. Kokh, O. E. Tereshchenko, E. V. Chulkov, A. Shikin, and M. Weinelt 
  Phys. Rev. B 103, 085406 (2021)  - DOI: 10.1103/PhysRevB.103.085406

• The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals 
  B. Frietsch, A. Donges, R. Carley, M. Teichmann, J. Bowlan, K. Döbrich, K. Carva, D. Legut, P. M. Oppeneer, U. Nowak, and M. Weinelt 
  Sci. Adv. 6 : eabb1601 (2020) - DOI: 10.1126/sciadv.abb1601

• Spin-resolved electronic structure of 3d transition metals during ultrafast demagnetization
  B. Andres and M. Weinelt
  J. Magn. Magn. Mater. 501, 166475 (2020)  - DOI: 10.1016/j.jmmm.2020.166475

• Origin of spin-polarized photocurrents in the topological surface states of Bi₂Se₃
  A. S. Ketterl, S. Otto, M. Bastian, B. Andres, C. Gahl, J. Minár, H. Ebert, J. Braun, O. E. Tereshchenko, K. A. Kokh, Th. Fauster, and M. Weinelt
  Phys. Rev. B 98, 155406 (2018)  - DOI: 10.1103/PhysRevB.98.155406