Dissertation: Ultrafast Magnetization Dynamics: Element-selective studies of magnetic alloys using ultra short XUV pulses

  • Date:
  • Location: Ångströmlaboratoriet, Lägerhyddsvägen 1 Polhemsalen
  • Doctoral student: Rameez Saeed Malik
  • Contact person: Rameez Saeed Malik
  • Disputation

In this thesis, I investigate the ultrafast magnetization dynamics in 3d ferromagnets and their alloys with ultrashort laser pulses. The high harmonics generation (HHG) setup provides extreme-ultraviolet photons with energies 35-72 eV, which is the energy range where 3d metals have their M2,3 absorption edges. By employing HHG with the transverse magneto-optical Kerr effect, the magnetization of multiple elements in a magnetic system is probed and their dynamics are resolved separately on femtoseconds time scales.

The magneto-optical response of elemental Fe and Ni during demagnetization is investigated. This magneto-optical response is measured as an asymmetry in the intensity of reflected light for two opposite sample magnetization directions. Experiment and density functional theory calculations show that for Fe, the asymmetry is strongly dependent on the particular type of magnetic excitation. However, for Ni, it is relatively insensitive to the magnetic excitation. Next, the element-specific magnetization dynamics of FeNi alloys are investigated. A time delay in the Ni demagnetization relative to Fe is observed for all alloy compositions. This Ni-delay depends on the alloy composition and is related to changes in the exchange interactions.

Co2FeAl (CFA) Heusler alloys are unique due to their peculiar electronic structure and because they can exhibit very low damping. Experimentally, CFA films show a decreased damping with an increase in structural ordering. The demagnetization times of Fe and Co in CFA samples with different amount of ordering are similar for all samples. However, the remagnetization times exhibit a dependence on the structural ordering. Both the theoretical and experimental damping parameters correlate well with the remagnetization times. In FeCo alloys, the damping can be changed by doping with heavy metals. Here, the magnetization dynamics of Fe65Co35 films as a function of Re doping are investigated. We find no observable change in the demagnetization times for samples with increased damping. However, when increasing the Re doping and the damping, the remagnetization time becomes faster. Also, a fast increase of the asymmetry signal is observed at the Ru-edge during the demagnetization of FeCo. This effect is attributed to a super-diffusive spin current going from the FeCo layer to the Ru capping layer.

Last, the magnetization dynamics of a ferrimagnetic insulator is studied. The NiFe2O4 asymmetry shows oscillatory dynamics after an ultrashort laser pulse excitation. With 1.55 eV pump, these oscillations are strong. For 3.1 eV pump, demagnetization becomes dominant and the oscillations diminish.