Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL.

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.

Efficient terahertz anti-reflection properties of metallic anti-dot structures.

We report the use of micrometer-sized copper (Cu) anti-dot structures as a novel terahertz (THz) anti-reflection coating (ARC) material and their superior performance over conventionally used metallic (Cu) thin films. Cu anti-dot structures of two different thicknesses (7 and 10 nm) with varying anti-dot diameters (100, 200, and 300 µm, inter-anti-dot separation fixed at 100 µm) are deposited on silicon substrates by RF magnetron sputtering and e-beam evaporation. The anti-reflection performance of these samples is studied in the frequency range of 0.3-2.2 THz. While continuous metallic (Cu) thin film minimizes the Fabry-Perot (FP) peak, it also suppresses the primary transmission peak, reducing the advantage due to the former effect. On the contrary, the anti-dot arrays reduce both the absolute amplitude of the FP peak and the amplitude ratio (AR) of the FP peak to the primary peak, making them a superior material for ARC applications. The AR can be further manipulated by varying the anti-dot size. A universal conductivity phase-matching condition, which is a prerequisite for the disappearance of the FP peak, is observed in these samples. The enhanced anti-reflection performance promotes these anti-dot structures as an efficient terahertz ARC material.

Terahertz conductivity engineering in surface decorated carbon nanotube films by gold nanoparticles.

We report the controllable conductivity of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes with their surface walls decorated by gold nanoparticles (Au NPs) with varying concentration in terahertz (THz) frequency range. Colloidal Au NPs of nominal diameter ∼15 nm are synthesized by the reduction of gold chloride solution using tri-sodium citrate. A simple chemical route is followed to attach Au NPs on the surfaces of both types of carbon nanotubes (CNTs). The attachment of Au NPs on the sidewalls of CNTs is confirmed by UV-visible spectroscopy and scanning electron microscope images. THz spectroscopic measurements are carried out at room temperature in transmission geometry in the frequency range of 0.3-2.0 THz. It is found that the THz conductivity of the surface decorated SWNT composites can either be increased or decreased by ±15% than that of the as-prepared SWNT composites by carefully choosing the Au NP concentration. The conductivity variation is qualitatively explained in terms of carrier trapping potential for low Au NP density, and alternative carrier conduction pathways at higher Au NP density and analyzed with the help of a modified universal dielectric relaxation model.

EMI shielding and conductivity of carbon nanotube-polymer composites at terahertz frequency.

We investigate the shielding effectiveness and complex conductivity of single-walled carbon nanotubes (SWNT) distributed in a polyvinyl alcohol (PVA) matrix in the THz frequency range. SWNTs are dispersed in PVA matrices with varying SWNT content (keeping the thickness of SWNT/PVA film constant) using a slow-drying method, and terahertz time-domain spectroscopy (THz-TDS) is performed at room temperature in transmission geometry in the frequency range of 0.3-2.1 THz. The transmittance spectra show a possible application of SWNT/PVA composites as low-bandpass filters in the THz frequency region. Shielding effectiveness of all the samples is measured, and, at a particular probing frequency, they tend to follow a linear relationship with SWNT weight fraction in the polymer matrices. THz conductivity of the composite system is described in the light of a.c. hopping conduction.

Polarizing effect of aligned nanoparticles in terahertz frequency region.

We report the polarizing behavior of aligned Ni nanoparticles (NPs) having average diameter of 165±15 nm in ~210 µm thick polyvinyl alcohol (PVA) matrix in the frequency range of 0.2-1.6 THz. The NPs have been prepared via a wet chemical route and then aligned in PVA film by using an external magnetic field. When the polarization of THz electric field is parallel to the NPs alignment direction, a strong THz absorption is observed whereas a minimum THz absorption is noticed for the corresponding perpendicular configuration. Degree of polarization is calculated to be 0.9±0.08. Considering the good polarizing performance, ease of preparation, durability, and low maintenance, this aligned NP system is a perfect candidate to emerge as a potential THz polarizer.

Picosecond spin-orbit torque-induced coherent magnetization switching in a ferromagnet.

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices.

Accelerated Ultrafast Magnetization Dynamics at Graphene/CoGd Interfaces.

Atomically thin graphene layers can act as a spin-sink material when adjacent to a nanoscale magnetic surface. The enhancement in the extrinsic spin-orbit coupling (SOC) strength of graphene plays an important role in absorbing the spin angular momentum injected from the magnetic surface after perturbation with an external stimulus. As a result, the dynamics of the excited spin system is modified within the magnetic layer. In this paper, we demonstrate the modulation of ultrafast magnetization dynamics at graphene/ferrimagnet interfaces using the time-resolved magneto-optical Kerr effect (TRMOKE) technique. Magnetically modified interfaces with a systematic increase in the number of graphene layers coupled with the 10 nm-thick Co74Gd26 layer are studied. We find that the variation in the dynamical parameters, i.e., ultrafast demagnetization time, remagnetization times, decay time, effective damping, precessional frequency, etc., observed at different time scales is interconnected. The demagnetization time and decay time for the ferrimagnet become approximately two times faster than the corresponding intrinsic values. We found a possible correlation between the demagnetization time and damping. The effect is more pronounced for the interfaces with monolayer graphene and graphite. The spin-mixing conductance is found to be approximately 0.8 × 1015 cm-2. The effect of SOC, pure spin current, the appearance of structural defects, and thermal properties at the graphene/ferrimagnet interface are responsible for the modifications of several dynamical parameters. This work demonstrates some important properties of the graphene/ferrimagnet interface which may unravel the possibilities of designing spintronic devices with elevated performance in the future.