Author: Roman Adam

Affiliation(s): Research Centre Julich

Ultrafast dynamic processes in magnetic systems are a topic of strong interest due to their high relevance in both basic science and technology. Further progress in our understanding requires a detailed analysis of fundamental processes in complex magnetic materials employing a spatial resolution on the nanometer scale, a temporal resolution in the femtosecond range, as well as element and chemical selectivity. Pump-probe measurements of magnetization dynamics based on ultrafast lasers have already demonstrated an unmatched temporal resolution in the femtosecond range. On the other hand, earlier synchrotron-radiation experiments using relatively long pulse durations in the picosecond range have shown that the magnetization can be probed element-selectively at the M absorption edges of Fe, Co, and Ni, at photon energies ranging from 55eV to 68eV. Recently, newly developed ultrafast light sources based on the high-harmonic up-conversion (HHG) of femto-second laser pulses have been successfully employed to access the M absorption edges in a table-top experimental setup. Introducing these new ultrafast and coherent soft X-ray sources for probing magnetic materials promises to combine element-selectivity with a temporal resolution in the femto- to attosecond range and a spatial resolution on the nanometer scale for exploring the fundamental limits of magnetism. Our experiment was set up in a transverse magneto-optic Kerr effect geometry (T-MOKE) in which the reflectivity of the material changes depending on its state of magnetization. The strength of the T-MOKE signal is commonly specified by the asymmetry A=(I+-I-)/(I++I-) where I+ and I- denote the reflected intensities recorded for two opposite magnetization directions along the axis perpendicular to the plane of incidence. Reflecting laser-generated X-ray pulses off a magnetized Ni80Fe20 grating, large changes in the asymmetry of up to 6% at the M absorption edges of Fe and Ni were observed upon magnetization reversal. This strong magnetic contrast enables us for the first time to measure demagnetization dynamics of a compound material element-selectively using a tabletop X-ray pump-probe measurement technique. In this experiment, short laser pump pulses are first directed at the sample to destroy the magnetic order. Inherently synchronized X-ray probe pulses are then reflected from the targeted spot to measure the ultrafast element-specific demagnetization with a time-resolution of 55fs. Our results point towards a strong coupling of the magnetic moments of Ni and Fe forming the NiFe film. Our experiments demonstrate that the use of higher harmonics for probing magnetic materials can combine element-selectivity and femto-second time resolution, thereby opening a new pathway to address ultrafast spin dynamics in heterogeneous magnetic systems.

Consider for Oral Presentation?: Yes