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

In the proposed project we intend to study the ultrafast magnetization dynamics in magnetically ordered films of lanthanide metals. To this end we use femtosecond pulses of extreme-ultraviolet radiation from a tunable higher-order harmonics beamline to map transients of the exchange-split (5d6s)^3 valence band structure and follow the magnetic linear dichroism (MLD) of the 4f core-levels in angle-resolved photoemission. Femtosecond IR pulses will be used as pump pulses to initiate the ultrafast magnetization dynamics.Our previous photoemission work shows a significant difference in the response of the 4f spin system of pure Gadolinium and Terbium metal, which we attribute to their different orbital momentum (L=0 vs. L=3). For Gd, the dynamics of 5d and 4f spin systems occurs on largely disparate timescales (~0.8 vs. ~14ps), while for Tb both spin systems react synchronously and faster (~0.4ps). These results are not only in contrast to X-ray magnetic circular dichroism (XMCD) measurements but generally challenge the microscopic description of ultrafast magnetic switching in 3d4f alloys.We now plan for three types of specifically designed experiments which allow us to further develop the microscopic understanding of the 4f coupling and magnetization dynamics in the rare-earths. (i) The influence of the orbital momentum on the magnetization dynamics will be studied for GdTb compounds. Varying the GdTb composition we will map transient exchange splitting and Gd and Tb 4f MLD and address whether they respond differently in the alloy. (ii) We want to answer whether there is a different magnetization dynamics at the Gd surface as compared to its bulk. To separate surface from bulk contributions we will study a Gd film covered with one monolayer of europium (4f^7 (5d6s)^2). (iii) As a third approach, we will investigate the intermetallic compound TbFe, where the magnetization is determined by the interplay between the exchange couplings of the different sublattices. Depending on the concentration, TbFe films can show very low coercivity and we expect a significant weaker 4f spin - lattice coupling as in pure Tb films. This should result in slower 4f spin dynamics.

• DFG Programme: Research Grants