No abstract

Optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.

The x-ray transitions in Cu-and Ni-like tungsten ions in the 5.19-5.26Å wavelength range that are relevant as a high-temperature tokamak diagnostic, in particular for JET in the ITER-like wall configuration, have been studied. Tungsten spectra were measured at the upgraded Shanghai-Electron Beam Ion Trap operated with electron-beam energies from 3.16 to 4.55 keV. High-resolution measurements were performed by means of a flat Si 111 crystal spectrometer equipped by a CCD camera. The experimental wavelengths were determined with an accuracy of 0.3-0.4 mÅ. The wavelength of the ground-state transition in Cu-like tungsten from the 3p 5 3d 10 4s4d [(3/2,(1/2,5/2) 2 ] 1/2 level was measured. All measured wavelengths were compared with those measured from JET ITER-like wall plasmas and with other experiments and various theoretical predictions including COWAN, RELAC, multiconfigurational Dirac-Fock (MCDF), and FAC calculations. To obtain a higher accuracy from theoretical predictions, the MCDF calculations were extended by taking into account correlation effects (configuration-interaction approach). It was found that such an extension brings the calculations closer to the experimental values in comparison with other calculations. ‡ See author list of "X. Litaudon et al., Nucl. Fusion 97, 102001 (2017)." for highly ionized tungsten ions up to Cu-like (W 45+ ) and Na-like (W 63+ ), respectively [19,20].The experimental spectroscopic studies of tungsten ions were supported by extensive theoretical considerations (see, e.g., Ref.[21] and references therein). In the last few years a significant improvement has been achieved in the theoretical approaches, in particular by the development of large-scale relativistic configuration-interaction (CI) methods taking into account electron correlation effects. Such a technique was employed for M1 transitions in Ag-and Cd-like tungsten [17,22], electric-multipole transitions in Sn-like tungsten [23,24], and for transitions from low-lying levels in Ni-like tungsten [25]. An extended experimental and theoretical data base on the tungsten ions can be found in Refs. [26][27][28].Recently, in measurements on the Joint European Torus (JET) with the ITER-like wall (ILW) configuration (beryllium wall and tungsten divertor) [29,30], the W 45+ and W 46+ (3p-4d) x-ray lines were observed. From analysis of tungsten line intensities, it was found that the W concentration is ∼10 −5 for the ELMy H-mode JET plasmas (edge-localized-mode of plasma operation with high-energy confinement times) with 2.0-2.5 MA current, 2.7 T toroidal magnetic field and 14-18 MW neutral beam injection (NBI) power. Tungsten concentration determined from the W 45+ line was systematically lower than that obtained from W 46+ by ∼20% for different types of JET discharges [7]. It was further shown that, in order to reproduce the experimental spectra in the 5.192-5.232Å wavelength range at the measured electron 2469-9926/2018/97(5)/052501 (9) 052501-1