Spin polarization in half-metals probed by femtosecond spin excitation

Rare-earth surface alloying: a new phase for GdAu2

Surface alloying is a powerful way of varying physical and chemical properties of metals, for a number of applications from catalysis to nuclear and green technologies. Surfaces offer many degrees of freedom, giving rise to new phases that do not have a bulk counterpart. However, the atomic characterization of distinct surface compounds is a major task, which demands powerful experimental and theoretical tools. Here we illustrate the process for the case of a GdAu2 surface phase of extraordinary crystallinity. The combined use of surface-sensitive techniques and state-of-the-art ab initio calculations disentangles its atomic and electronic properties. In particular, the stacking of the surface layers allows for gadolinium's natural ferromagnetic state, at variance with the bulk phase, where frustration leads to antiferromagnetic interlayer coupling.

Time-resolved demagnetization of Co2MnSi observed using x-ray magnetic circular dichroism and an ultrafast streak camera

The demagnetization dynamics of the Heusler alloy Co(2)MnSi was studied using picosecond time-resolved x-ray magnetic circular dichroism. The sample was excited using femtosecond laser pulses. In contrast to the sub-picosecond demagnetization of the metal ferromagnet Ni, substantially slower demagnetization with a time constant of 3.5 ± 0.5 ps was measured. This could be explained by a spin-dependent band gap inhibiting the spin-flip scattering of hot electrons in Co(2)MnSi, which is predicted to be half-metallic. A universal demagnetization time constant was measured across a range of pump power levels.

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization

Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

Observation of elastic doublon decay in the Fermi-Hubbard model

We investigate the decay of highly excited states of ultracold fermions in a three-dimensional optical lattice. Starting from a repulsive Fermi-Hubbard system near half filling, we generate additional doubly occupied sites (doublons) by lattice modulation. The subsequent relaxation back to thermal equilibrium is monitored over time. The measured absolute doublon lifetime covers 2 orders of magnitude. In units of the tunneling time h/J it is found to depend exponentially on the ratio of on-site interaction energy U to kinetic energy J. We argue that the dominant mechanism for the relaxation is a simultaneous many-body process involving several single fermions as scattering partners. A many-body calculation is carried out using diagrammatic methods, yielding fair agreement with the data.

Bulk spin polarization of Co(1-x)Fe(x)S2

We report on a new method to determine the degree of bulk spin polarization in single crystal Co(1-x)Fe(x)S2 by modeling magnetic Compton scattering with ab initio calculations. Spin-dependent Compton profiles were measured for CoS2 and Co0.9Fe0.1S2. The ab initio calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, P, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the refined calculations. The values were found to be P=-72+/-6% and P=18+/-7% for CoS2 and Co0.9Fe0.1S2, respectively. Furthermore, determinations of P weighted by the Fermi velocity (v(F) or v(F)2) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of P on v(F).