Femtosecond x-ray absorption spectroscopy of spin and orbital angular momentum in photoexcited Ni films during ultrafast demagnetization

We follow for the first time the evolution of the spin and orbital angular momentum of a thin Ni film during ultrafast demagnetization, by means of x-ray magnetic circular dichroism. Both components decrease with a 130 +/- 40 fs time constant upon excitation with a femtosecond laser pulse. Additional x-ray absorption measurements reveal an increase in the spin-orbit interaction by 6 +/- 2 % during this process. This is the experimental demonstration quantifying the importance of spin-orbit mediated processes during the demagnetization

Temperature-induced reversal of magnetic interlayer exchange coupling

For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a non-magnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth $M_5$ thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness $d_Y$, a temperature-induced sign reversal of IEC was observed for constant $d_Y$, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.Comment: 4 pages, 4 figures; accepted version; minor changes and new Figs. 2 and 4 containing more dat

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet.

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field

Transient electronic and magnetic structures of nickel heated by ultrafast laser pulses

We investigate the evolution of the Ni electronic and magnetic structure on fs to ps time scales following fs-laser excitation. Within 200 fs after excitation the Ni 3d ferromagnetic moment is reduced as probed by x-ray magnetic circular dichroism. At the same time the Ni 3d electronic structure undergoes pronounced changes as demonstrated by x-ray absorption spectroscopy. We show that the latter persists also into thermal equilibrium which is reached on the ps time scale. Cluster calculations identify a reduction in 3d-4sp hybridization possibly associated with phonon-driven spin-flip excitations

Transient electronic and magnetic structures of nickel heated by ultrafast laser pulses

We investigate the evolution of the Ni electronic and magnetic structure on fs to ps time scales following fs-laser excitation. Within 200 fs after excitation the Ni 3d ferromagnetic moment is reduced as probed by x-ray magnetic circular dichroism. At the same time the Ni 3d electronic structure undergoes pronounced changes as demonstrated by x-ray absorption spectroscopy. We show that the latter persists also into thermal equilibrium which is reached on the ps time scale. Cluster calculations identify a reduction in 3d-4sp hybridization possibly associated with phonon-driven spin-flip excitations

The graphene n Ge 110 interface structure, doping, and electronic properties

The implementation of graphene in semiconducting technology requires the precise knowledge about the graphene-semiconductor interface. In our work the structure and electronic properties of the graphene/$n$-Ge(110) interface are investigated on the local (nm) and macro (from $\mu\mathrm{m}$ to mm) scales via a combination of different microscopic and spectroscopic surface science techniques accompanied by density functional theory calculations. The electronic structure of freestanding graphene remains almost completely intact in this system, with only a moderate $n$-doping indicating weak interaction between graphene and the Ge substrate. With regard to the optimization of graphene growth it is found that the substrate temperature is a crucial factor, which determines the graphene layer alignment on the Ge(110) substrate during its growth from the atomic carbon source. Moreover, our results demonstrate that the preparation routine for graphene on the doped semiconducting material ($n$-Ge) leads to the effective segregation of dopants at the interface between graphene and Ge(110). Furthermore, it is shown that these dopant atoms might form regular structures at the graphene/Ge interface and induce the doping of graphene. Our findings help to understand the interface properties of the graphene-semiconductor interfaces and the effect of dopants on the electronic structure of graphene in such systems.Comment: submitted on 21.12.2017; manuscript and supplementary inf