Current driven magnetization dynamics in helical spin density waves

A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. In this paper, we show that a current through a bulk system with a helical spin density wave induces a spin transfer torque, giving rise to a rotation of the order parameter.The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. We demonstrate the effect by a quantitative calculation of the current induced magnetization dynamics of Erbium. Finally we propose a setup for experimental verification.Comment: In the previous version of this paper was a small numerical mistake made when evaluating equation 3 and 9. The number of digits given in the calculation of the torque current tensor is reduced to better represent the accuracy of the calculation. A slightly modified paper have been published in Phys. Rev. Lett. 96, 256601 (2006) 4 pages 3 figure

Mg–Ti nanoparticles with superior kinetics for hydrogen storage

open5siAcknowledgements The assistance of F. Corticelli and V. Morandi (IMM-CNR, Bologna) during FE-SEM observations is gratefully acknowledged. Part of this work was supported by the COST Action MP1103 “Nanostructured materials for solid-state hydrogen storage”. We are grateful to the beamline I711 at MAXlab, Lund, Sweden for the provision of beamtime.Mg nanoparticles (NPs) with addition of Ti catalysts were synthesised by inert gas condensation and in situ hydrogenation at 150 °C. The NPs size and composition were systematically investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy and powder X-ray diffraction (PXD), while time resolved in situ synchrotron radiation-PXD was used to monitor the mechanism for hydrogen uptake and release at 280 °C. The Mg–Ti NPs reveal activation energies of 68 kJ mol−1 for absorption and 78 kJ mol−1 for desorption by isothermal kinetics analysis, similar to the lowest values reported in the literature for MgH2 using Nb2O5 as a catalyst. Hence, hydrogen desorption (pdes = 8 mbar) and absorption (pabs = 260 mbar) is achieved at 200 °C in ∼2000 s, while keeping 5.3 wt% storage capacity. Thermodynamic data extracted from van ’t Hoff plots reveal unchanged values compared to bulk MgH2. Therefore, the improved hydrogen storage performances are assigned to the enhanced kinetics only.openCalizzi, Marco; Chericoni, Domizia; Jepsen, Lars H.; Jensen, Torben R.; Pasquini, LucaCalizzi, Marco; Chericoni, Domizia; Jepsen, Lars H.; Jensen, Torben R.; Pasquini, Luc

Uncovering Extreme Nonlinear Dynamics in Solids Through Time-Domain Field Analysis

Time-domain analysis of harmonic fields with sub-cycle resolution is now experimentally viable due to the emergence of sensitive, on-chip techniques for petahertz-scale optical-field sampling. We demonstrate how such a time-domain, field-resolved analysis uncovers the extreme nonlinear electron dynamics responsible for high-harmonic generation within solids. Time-dependent density functional theory was used to simulate harmonic generation from a solid-state band-gap system driven by near- to mid-infrared waveforms. Particular attention was paid to regimes where both intraband and interband emission mechanisms play a critical role in shaping the nonlinear response. We show that a time-domain analysis of the harmonic radiation fields identifies the interplay between intra- and interband dynamical processes underlying the nonlinear light generation. With further analysis, we show that changes to the dominant emission regime can occur after only slight changes to the peak driving intensity and central driving wavelength. Time-domain analysis of harmonic fields also reveals, for the first time, the possibility of rapid changes in the dominant emission mechanism within the temporal window of the driving pulse envelope. Finally, we examine the experimental viability of performing time-domain analysis of harmonic fields with sub-cycle resolution using realistic parameters

Evaluation of the anisotropic mechanical properties of reinforced polyurethane foams

The mechanical impact of adding milled glass fibers and nanoparticles at different mass fractions to low-density (relative density < 0.2) polyurethane (PU) foams is investigated. Tensile, compressive, and shear stress–strain curves are measured in the plane parallel to the foam-rise direction and the in-plane components of the elastic modulus are determined in order to assess the mechanical anisotropy of the foams. Power-law relationships between the moduli and apparent density are established for pure PU foams and used as a baseline to which the properties of composite foams are compared. Cellular mechanics models based on both rectangular and Kelvin unit-cell geometries are employed to estimate changes in the cell shape based on the mechanical anisotropy of composite foams, and the model results are compared with direct observations of the cellular structure from microscopy. A single measure of foam stiffness reinforcement is defined that excludes the effects of the apparent foam density and cell shape. The analysis reveals the large impact of cell shape on the moduli of the glass-fiber and nanocomposite foams. Nanocomposite foams exhibit up to an 11.1% degree of reinforcement, and glass-fiber foams up to 18.7% using this method for quantifying foam reinforcement, whereas a simple normalization to the in-plane modulus components of the pure PU foam would indicate from ?40.5% to 25.9% reinforcement in nanocomposite foams, and ?7.5 to 20.2% in glass-fiber foams