First experiments with ultrashort, circularly polarized soft x-ray pulses at FLASH2

Area of Science:

• Condensed matter physics
• Materials science
• Ultrafast science

Background:

• Time-resolved X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) are crucial for studying magnetic material dynamics.
• Previous studies have significantly advanced ultrafast magnetization dynamics, particularly in magnetic alloys.

Purpose of the Study:

• To present the first experimental results using ultrashort, circularly polarized soft X-ray pulses at the FLASH free-electron laser.
• To evaluate the capabilities of the upgraded FLASH2 beamline for ultrafast XMCD experiments.

Main Methods:

• Utilized time-resolved absorption spectroscopy and magnetic circular dichroism (XMCD) with circularly polarized soft X-rays.
• Employed ultrashort X-ray pulses (tens of femtoseconds) from the upgraded APPLE-III undulator at the FLASH2 beamline.
• Conducted XMCD measurements at the L-edges of Cobalt (Co), Iron (Fe), and Nickel (Ni).

Main Results:

• Achieved significant dichroic difference signals for Co, Fe, and Ni.
• Demonstrated a degree of circular polarization close to 100% for the generated soft X-ray pulses.
• Confirmed the effectiveness of the pulse-length preserving monochromator and improved pump-laser setup.

Conclusions:

• The upgraded FLASH facility provides powerful instrumentation for ultrafast XMCD studies.
• These capabilities are vital for investigating ultrafast demagnetization and spin dynamics in 3d transition metals, multilayers, and alloys.