Sika Zhou

Comparison of Mg-based multilayers for solar He II radiation at 30.4 nm wavelength

Mg-based multilayers, including SiC/Mg, Co/Mg, B(4)C/Mg, and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a 30.4 nm wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of B(4)C/Mg and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a 30.4 nm wavelength.

Torque approach for tuning exchange bias training effect in polycrystalline NiFe/FeMn bilayers

In order to tune the exchange bias training effect, a torque approach is proposed. Torque loops are performed between two consecutive hysteresis loops by clockwise and counterclockwise rotation of the sample within an angular range under an in-plane external magnetic field. For the angular range from 0° to 180°, the coercivity HC and the exchange field HE are both reduced and the pinning direction (PD) orientation is changed, exhibiting the first type training effect. For the angular range from −20° to 380°, HC and the PD orientation are almost unchanged although HE is reduced, exhibiting the second type training effect.

Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

In this work, disordered-IrMn3/insulating-Y3Fe5O12 exchange-biased bilayers are studied. The behavior of the net magnetic moment ΔmAFM in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The ΔmAFM is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis loss are induced by partial rotation and irreversible switching of the ΔmAFM. In the athermal training effect, the state of the ΔmAFM cannot be recovered after one cycle of hysteresis loop. This work highlights the fundamental role of the ΔmAFM in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices.

Tuning magnetotransport in PdPt/Y3Fe5O12: Effects of magnetic proximity and spin-orbit coupling

We report that anisotropic magnetoresistance (AMR) and anomalous Hall conductivity (AHC) in the Pd1−xPtx/Y3Fe5O12 (YIG) bilayers could be tuned by varying the Pt concentration (x) and also temperature (T). In particular, the AHC at low T changes its sign when x increases from 0 to 1, agreeing with the negative and positive AHC predicted by our ab initio calculations for the magnetic proximity (MP)-induced ferromagnetic Pd and Pt, respectively. The AMR ratio is enhanced by ten times when x increases from 0 to 1. Furthermore, the AMR of PdPt/YIG bilayers shows similar T-dependence as the magnetic susceptibility of the corresponding bulk Pd/Pt, also indicating the MP effect as the origin of the AMR. The present work demonstrates that the alloying of Pt and Pd not only offers tunable spin-orbit coupling but also is useful to reveal the nature of the AMR and AHC in Pt/YIG bilayers, which are useful for spintronics applications.

Thermal stability of Mg/Co multilayer with B 4 C, Mo or Zr diffusion barrier layers

The efficiency of B(4)C, Mo and Zr barrier layers to improve thermal stability of Mg/Co multilayer up to 400 °C is investigated. Multilayers were deposited by direct current magnetron sputtering and characterized using X-ray and extreme ultraviolet reflection. The results suggest that B(4)C barrier layer is not effective due to drastic diffusion at Mg-B(4)C interface. Although introducing Mo barriers improves the thermal stability from 200 to 300 °C, it increases the interface roughness and thus degrades the optical performances. On the contrary, Zr barriers can significantly increase the thermal stability of Mg/Co up to 400 °C without optical performance degradation. Thus, Mg/Zr/Co/Zr is suitable for EUV applications requiring both optimal optical performances and heat resistance.

Zr/Mg multilayer mirror for extreme ultraviolet application and its thermal stability

Zr/Mg multilayer mirror was proposed for extreme ultraviolet (EUV) spectral range and deposited by magnetron sputtering. Its thermal stability during annealing up to 600 °C was evaluated by EUV reflection measurements, x-ray analyses, and transmission electron microscopy and found superior to that of Y2O3/Mg, SiC/Mg, and Co/Mg. The reflectance of as-deposited Zr/Mg multilayer is 30.6% at wavelength of 30.4 nm. The reflectance slightly decreases with annealing temperature when not above 500 °C and eventually drops to 15.1% at 600 °C. The degradation of performance is attributed to roughening induced by strain relaxation rather than interdiffusion or alloy compound formation.

Lateral transport properties of thermally excited magnons in yttrium iron garnet films

Spin information carried by magnons is attractive for computing technology, and the development of magnon-based computing circuits is of great interest. However, magnon transport in insulators has been challenging, different from the clear physical picture for spin transport in conductors. Here, we investigate the lateral transport properties of thermally excited magnons in yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of detected spins in non-local geometry devices have been experimentally observed and are strongly dependent on temperature, YIG film thickness, and injector-detector separation distance. A competing two-channel transport model for thermally excited magnons is proposed, which is qualitatively consistent with the spin signal behavior. In addition to the fundamental significance for thermal magnon transport, our work furthers the development of magnonics by creating an easily accessible magnon source with controllable transport.

Impact of ultrafast demagnetization process on magnetization reversal in L10 FePt revealed using double laser pulse excitation

Ultrafast laser induced magnetization reversal in L10 FePt films with high perpendicular magnetic anisotropy was investigated using single- and double-pulse excitations. Single-pulse excitation beyond 10 mJ cm−2 caused magnetization (M) reversal at the applied fields much smaller than the static coercivity of the films. For double-pulse excitation, both coercivity reduction and reversal percentage showed a rapid and large decrease with the increasing time interval (Δt) of the two pulses in the range of 0–2 ps. In this Δt range, the maximum demagnetization (ΔMp) was also strongly attenuated, whereas the integrated demagnetization signals over more than 10 ps, corresponding to the average lattice heat effect, showed little change. These results indicate that laser induced M reversal in FePt films critically relies on ΔMp. Because ΔMp is determined by spin temperature, which is higher than lattice temperature, utilizing an ultrafast laser instead of a continuous-wave laser in laser-assisted M reversal may reduce the overall deposited energy and increase the speed of recording. The effective control of M reversal by slightly tuning the time delay of two laser pulses may also be useful for ultrafast spin manipulation.Ultrafast laser induced magnetization reversal in L10 FePt films with high perpendicular magnetic anisotropy was investigated using single- and double-pulse excitations. Single-pulse excitation beyond 10 mJ cm−2 caused magnetization (M) reversal at the applied fields much smaller than the static coercivity of the films. For double-pulse excitation, both coercivity reduction and reversal percentage showed a rapid and large decrease with the increasing time interval (Δt) of the two pulses in the range of 0–2 ps. In this Δt range, the maximum demagnetization (ΔMp) was also strongly attenuated, whereas the integrated demagnetization signals over more than 10 ps, corresponding to the average lattice heat effect, showed little change. These results indicate that laser induced M reversal in FePt films critically relies on ΔMp. Because ΔMp is determined by spin temperature, which is higher than lattice temperature, utilizing an ultrafast laser instead of a continuous-wave laser in laser-assisted M reversal may re...

Improved thermal stability of Mg/Co multilayer by introducing Zr barrier layer

High reflective multilayer mirrors are widely used as optical elements for applications such as extreme ultraviolet (EUV) microscopy, high harmonic femtosecond chemistry, solar astrophysics imaging, and synchrotron radiation. Mg-based multilayers, such as Mg/SiC, Mg/Y2O3 and Mg/Co are promising in the wavelength of 25-40 nm for Mg L3 absorption edge is located at 25 nm. Mg/Co has narrower bandwidth and better thermal stability. In applications such as synchrotron radiation and solar imaging, multilayers mirrors must endure high heat loads. Thus, we investigated the thermal stability of Mg/Co multilayer and then introduced barrier layer to improve thermal stability in this paper. The interface structures evolution was studied by using X-ray reflection/scatter technique. Mg/Co multilayer can be stable when not heated above 300°C. B4C and Zr layers were inserted into Mg/Co multilayer as interface barrier layer to improve thermal stability. According to the measured results, B4C barrier layer is not suitable for Mg/Co multilayer, mainly due to the diffusion between Mg and B4C. The introduction of Zr can significantly improve the thermal stability of Mg/Co up to 400°C without reducing EUV reflectance. Thus, introduction of Zr barrier layer is an efficient method to improve the thermal stability of Mg/Co multilayer for EUV applications such as astronomical observation and synchrotron radiation.

Mg/B4C EUV multilayer by introducing Co as barrier layer

Mg/B4C multilayer provides very high theoretical reflectivity in extreme ultraviolet range near 30.4nm wavelength, while the interface between Mg and B4C layer is very poor. In this paper, Co was introduced into the interface between Mg and B4C as a barrier layer. Mg/B4C and Co/Mg/Co/B4C multilayers were designed, fabricated, and measured for the wavelength of 30.4nm. The thickness of Co barrier layer was optimized. Grazing incidence x-ray reflectance measurements show that the structural quality of Co/Mg/Co/B4C multilayer is improved significantly after Co barrier layer inserting, and the optimum thickness of the barrier layer is 1.5nm.