Ultrafast magnetization dynamics of lanthanide metals and alloys

In this study, the laser-induced magnetization dynamics of the lanthanide ferromagnets Gadolinium (Gd), Terbium (Tb) and their alloys is investigated using femtosecond (fs) time-resolved x-ray magnetic circular dichroism (XMCD), the magneto-optical Kerr effect (MOKE) and magnetic second harmonic generation (MSHG). The magnetization dynamics is analyzed from the time scale of a few fs up to several hundred picoseconds (ps). The contributions of electrons, phonons, spin fluctuations, as well as the temporal regimes corresponding to the spin-orbit and exchange interactions are disentangled. In addition to possible applications in magnetic storage devices, understanding magnetization dynamics in lanthanides is also important because of their different magnetic structure compared to well-studied itinerant ferromagnets. Lanthanides are model Heisenberg-ferromagnets with localized 4f magnetic moments and long range magnetic ordering through indirect exchange interaction. By optical excitation of the conduction electrons, which mediate the exchange interaction, and studying the induced dynamics of the localized 4f and delocalized 5d6s magnetic moments, one can obtain insight into the angular momentum transfer at ultrafast time scales. Moreover, lanthanides offer the possibility to tune spin-lattice coupling via the 4f shell occupation and the concomitant changes in the 4f spin and orbital moments due to Hund’s rules. Utilizing this fact, the importance of spin-lattice coupling in laser-induced demagnetization is also analyzed by comparing the magnetization dynamics in Gd and Tb. By investigating the magnetization dynamics of localized 4f moments of Gd and Tb using time-resolved XMCD, it is found that the demagnetization proceeds in both metals in two time scales, following fs laser excitation, which are classified as: (i) non-equilibrium (t ~ 1 ps) and (ii) quasi- equilibrium (t ≫ 1 ps), with respect to equilibration of electron and phonon temperatures. The characteristic demagnetization time in this non-equilibrium regime is similar for Gd and Tb, while in the quasi-equilibrium regime it differs following the strength of the spin-orbit coupling. To disentangle different microscopic mechanisms, conduction electron magnetization dynamics of Gd(0001) is investigated in further detail using time-resolved MOKE. By comparing the dynamics of the 4f moments with the delocalized 5d6s moments, an insight into the angular momentum transfer is obtained and the importance of the intra-atomic exchange interaction is analyzed. The critical spin fluctuations strongly affect the static magnetic properties near Curie temperature (TC). In this study, a real time observation of the critical fluctuations in laser-induced magnetization dynamics near the ferro- to paramagnetic phase transition is described. Moreover, it is concluded that the spin fluctuations contribute to the magnetization dynamics in the quasi- equilibrium regime as well as to the recovery of magnetization while the non- equilibrium dynamics is weakly affected by these fluctuations. The well known phonon distribution as a function of equilibrium temperatures (T0) allowed us to investigate the role of phonons in magnetization dynamics. From the observed temperature dependence of demagnetization in the quasi-equilibrium regime (t ≫ 1 ps), it is concluded that the phonons contribute to the amplitude of demagnetization while the demagnetization time is not affected by them. In order to disentangle different microscopic contributions in the non- equilibrium regime (t ∼ 1 ps), magnetization dynamics is investigated for different laser fluences and equilibrium temperatures by analyzing the MOKE rotation and ellipticity. A slowing down of magnetization is observed with increasing T0. Using input from theoretical modeling by the Landau-Lifshitz- Bloch equation, it is shown that both electrons as well as phonons contribute to demagnetization in non-equilibrium demagnetization. Analyzing the dynamics further in the non-equilibrium regime (t < 300 fs) directly after laser excitation, the observation of magnetic as well as non-magnetic contributions is reported. The comparison of the surface sensitive MSHG and the bulk sensitive MOKE signal gave us the opportunity to investigate the spin- dependent transport processes, which occur from the surface to the bulk of Gd. Finally, owing to the tunability of spin-orbit coupling in GdTb alloys, ultrafast magnetization dynamics of these alloys is investigated as a function of Tb concentration. The characteristic quasi-equilibrium demagnetization time increases six times by decreasing the Tb content from 70% to Gd metal, due to the known spin-orbit coupling of the system. The non-equilibrium demagnetization time, on the other hand, changes only weakly with concentration due to the fact that this time scale is faster than the spin- orbit coupling.

Author dc:creator

• Sultan, Muhammad

• Ultrafast dynamics
• magnetizatism
• lanthanides

Licence dc:rights.uri

• http://www.fu-berlin.de/sites/refubium/rechtliches/Nutzungsbedingungen

Language dc:language

eng

Identifier URI

http://dx.doi.org/10.17169/refubium-4606

OAI record oai:identifier

https://refubium.fu-berlin.de/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Arefubium.fu-berlin.de%3Afub188%2F402