Martin Weinelt

Time-resolved absorption spectroscopy and magnetic circular dichroism with circularly polarized soft x-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. By employing these methods, groundbreaking results have been obtained, for instance, for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free-electron laser facility FLASH, key capabilities for ultrafast XAS and XMCD experiments have recently improved. In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft x-ray pulses with a duration of a few tens of femtoseconds covering the L3,2-edges of the important 3d transition metal elements with pulse energies of several μJ. Here, we present first experimental results with such ultrashort x-ray pulses from the FL23 beamline employing XMCD at the L3,2-edges of the 3d metals, Co, Fe, and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump-laser setup, FLASH can offer important and efficient experimental instrumentation for ultrafast demagnetization studies and other investigations of ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.

The use of advanced x-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in x-ray free-electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient x-ray absorption spectroscopy and x-ray magnetic circular dichroism, respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft x-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co L3-edge absorption. Despite employing a weaker electronic excitation level, we find a comparable demagnetization for the Co 3d-states in CoPt compared to previous measurements on CoPd. This is attributed to the distinctly different spin–orbit coupling between 3d and 4d vs 3d and 5d elements in the corresponding alloys and multilayers.

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Strongly differing static dipole moments of the trans and cis isomers of photochromic azobenzene allow for optical switching of the work function of azobenzene-functionalized self-assembled monolayers (SAMs). We apply these properties in a fundamental experiment to manipulate the area size of the switched SAM. Azobenzene molecules were excited by ultraviolet laser illumination, and the transient isomerization profile of the SAM was spatially resolved recording photoemission electron microscopy images. Thereby, we demonstrate the spatial tuning of the SAM's work function and discuss the role of the laser spot profile in generating sharp edges or gradual changes of the work function.

A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4 f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4 f -electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5 d -4 f -electron scattering, altering the 4 f -orbital state and consequently the MCA with important implications for magnetization dynamics in 4 f -metals and more general for the excitation of localized electronic states in correlated materials.

Strongly differing static dipole moments of the trans and cis isomers of photochromic azobenzene allow for optical switching the work function of azobenzene-functionalized self-assembled monolayers (SAMs). We apply these properties in a fundamental experiment to manipulate the area size of the switched SAM. Azobenzene molecules were excited by ultraviolet laser illumination and the transient isomerization profile of the SAM was spatially resolved recording photoemission electron microscopy (PEEM) images. Thereby we demonstrate the spatial tuning of the SAM work-function and discuss the role of the laser spot-profile in generating sharp edges or gradual changes of the work function.

Spin-based electronics (spintronics) aims at extending electronic functionalities, which rely on the electron charge as information carrier, by the spin of the electron. To make spintronics competitive and compatible with other information carriers like photons and electrons, their speed needs to be pushed to femtosecond time scales and, thus, terahertz frequencies. In ultrafast and terahertz spintronics, femtosecond optical and terahertz electromagnetic pulses are used to induce spin torque and spin transport and to monitor the subsequent time evolution. The two approaches, sometimes referred to as femto-magnetism and terahertz magnetism, have provided new, surprising, and relevant insight as well as applications for spintronics. Examples include the ultrafast optical switching of magnetic order and the generation of broadband terahertz electromagnetic fields. This APL Special Topic Collection is dedicated to provide a platform for the newest developments and future trends in the very active, dynamic, and exciting research field of ultrafast and terahertz spintronics.

Laser-induced spin transport is a key ingredient in ultrafast spin dynamics. However, it remains debated to what extent ultrafast magnetization dynamics generates spin currents and vice versa. We use time- and spin-resolved photoemission spectroscopy to study an antiferromagnetically coupled Gd/Fe bilayer, a prototype system for all-optical switching. Spin transport leads to an ultrafast drop of the spin polarization at the Gd surface, demonstrating angular-momentum transfer over several nanometers. Thereby, Fe acts as spin filter, absorbing spin majority but reflecting spin minority electrons. Spin transport from Gd to Fe was corroborated by an ultrafast increase of the Fe spin polarization in a reversed Fe/Gd bilayer. In contrast, for a pure Gd film, spin transport into the tungsten substrate can be neglected, as spin polarization stays constant. Our results suggest that ultrafast spin transport drives the magnetization dynamics in Gd/Fe and reveal microscopic insights into ultrafast spin dynamics.