Shenlei Zhou

Anomalous Hall effect in epitaxial permalloy thin films

Anomalous Hall effect (AHE) of epitaxial permalloy thin films grown on MgO (001) substrates is investigated. The longitudinal conductivity independent term (i.e., the sum of intrinsic and side-jump contributions) of the anomalous Hall conductivity (AHC) is found to be much smaller than those of Fe and Ni films. Band theoretical calculations of the intrinsic AHC as a function of the number of valence electrons (band filling) indicate that the AHC of the permalloy is in the vicinity of sign change, thus resulting in the smallness of the intrinsic AHC. The contribution of the phonon scattering is found to be comparable to that of the impurity scattering. This work suggests that the permalloy films are ideal systems to understand the AHE mechanisms induced by impurity scattering.

Spin-orbit interaction tuning of perpendicular magnetic anisotropy in L10 FePdPt films

The dependence of perpendicular magnetic anisotropy Ku on spin-orbit coupling strength ξ is investigated in L10 ordered FePd1−xPtx films by time-resolved magneto-optical Kerr effect measurements and ab initio density functional calculations. Continuous tuning of Ku over a wide range of magnitude is realized by changing the Pt/Pd concentration ratio, which strongly modifies ξ but keeps other leading parameters affecting Ku nearly unchanged. Ab initio calculations predict a nearly quadratic dependence of Ku on ξ, consistent with experimental data. Ku increases with increasing chemical order and decreasing thermal spin fluctuations, which becomes more significant for samples with higher Pt concentration. The results demonstrate an effective method to tune Ku utilizing its sensitivity on ξ, which will help fabricate magnetic systems with desirable magnetic anisotropy.

Improved uniformity of target illumination by combining a lens array and the technique of spectral dispersion

A scheme using a lens array and the technique of spectral dispersion is presented to improve target illumination uniformity in laser produced plasmas. Detailed two-dimensional simulation shows that a quasi-near-field target pattern, of steeper edges and without side lobes, is achieved with a lens array, while interference stripes inside the pattern are smoothed out by the use of the spectral dispersion technique. Moving the target slightly from the exact focal plane of the principal focusing lens can eliminate middle-scale-length intensity fluctuation further. Numerical results indicate that a well-irradiated laser spot with small nonuniformity and great energy efficiency can be obtained in this scheme.

Magneto-optical Kerr effect in L10 FePdPt ternary alloys: Experiments and first-principles calculations

We have studied the magneto-optical Kerr effect (MOKE) of L10 Fe0.5(Pd1−xPtx)0.5 alloy films with both experiments and first-principles calculations. In the visible region, negative Kerr rotation and ellipticity peaks are, respectively, observed in the regions of 1.5–2.0 eV and 1.7–2.6 eV. These peaks are shifted towards higher energies, and their magnitudes are enhanced for larger x. The MOKE evolution is mainly ascribed to the anomalous Hall conductivity contributed by the spin-down d↓,x2−y2 bands from Pd and Pt. We established a close correlation among the MOKE spectra, the spin orbit coupling strength, and the band feature for this prototypical system.

Two-dimensional performance of uniform irradiation with the use of an edge-softened lens array and spectral dispersion

A lens array composed of edge-softened elements is used to improve on-target irradiation uniformity in the Shenguang II Laser Facility, with which a Fresnel pattern of suppressed diffraction peaks is obtained. Additional uniformity can be reached by reducing short-wavelength interference speckles inside the pattern when the technique of smoothing by spectral dispersion is also used. Two-dimensional performance of irradiation is simulated and the results indicate that a pattern of steeper edges and a flat top can be achieved with this joint technique.

Anomalous Hall effect in magnetic disordered alloys: Effects of spin orbital coupling

For disordered ternary Fe0.5(Pd1−xPtx)0.5 alloy films, the anomalous Hall effect obeys the conventional scaling law ρAH=aρxx+bρxx2 with the longitudinal resistivity ρxx and anomalous Hall resistivity ρAH. Contributed by the intrinsic term and the extrinsic side-jump one, the scattering-independent anomalous Hall conductivity b increases with increasing Pt/Pd concentration. In contrast, the skew scattering parameter a is mainly influenced by the residual resistivity. The present results will facilitate the theoretical studies of the anomalous Hall effect in magnetic disordered alloys.

Tunable magnetization dynamics in disordered FePdPt ternary alloys: Effects of spin orbit coupling

The magnetization dynamics of disordered Fe0.5(Pd1−xPtx)0.5 alloy films was studied by time-resolved magneto-optical Kerr effect and ferromagnetic resonance. The intrinsic Gilbert damping parameter α0 and the resonance linewidth change linearly with the Pt atomic concentration. In particular, the induced in-plane uniaxial anisotropy constant KU also increases for x increasing from 0 to 1. All these results can be attributed to the tuning effect of the spin orbit coupling. For the disordered ternary alloys, an approach is proposed to control the induced in-plane uniaxial anisotropy, different from conventional thermal treat methods, which is helpful to design and fabrications of spintronic devices.

High power glass laser research progresses in NLHPLP

A new high power laser facility with 8 beams and maximum output energy of one beam 5kJ/3.4ns/3ω has been performed and operated since 2015. Combined together the existing facilities have constructed a multifunction experimental platform including multi-pulse width of ns, ps and fs and active probing beam, which is an effective tool for Inertial Confinement Fusion (ICF) and High Energy Density (HED) researches. In addition another peculiar high power laser prototype pushes 1ω maximum output energy to 16kJ in 5ns and 17.5kJ in 20ns in flat-in-time pulse, this system is based on large aperture four-pass main amplifier architecture with 310mm×310mm output beam aperture. Meanwhile the near field and far field have good quality spanning large energy scope by use of a wide range of technologies, such as reasonable overall design technique, the integrated front end, cleanness class control, nonlinear laser propagation control, wave-front adaptive optics and precision measurement. Based on this excellent backup, 3ω damage research project is planning to be implemented. To realize the above aims, the beam expanding scheme in final transport spatial filter could be adopted considering tradeoff between the efficient utilization of 1ω output and 3ω damage threshold. Besides for deeply dissecting conversion process for beam characteristic influence of 1ω beam, WCI (Wave-front Code Image) instrument with refined structure would be used to measure optical field with simultaneous high precision amplitude and phase information, and what’s more WCI can measure the 1ω, 2ω and 3ω optical field in the same time at same position, so we can analyze the 3ω beam quality evolution systematically, and ultimately to improve the 3ω limited output. In a word, we need pay attention to some aspects contents with emphasis for future huger laser facility development. The first is to focus the new technology application. The second is to solve the matching problem between 1ω beam and the 3ω beam. The last is to build the whole effective design in order to improve efficiency and cost performance.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.

Performance of smoothing by spectral dispersion combined with distribute phase plate on SG-II

Experimental performance of one-dimensional (1D) smoothing by spectral dispersion (SSD) combined with distributed phase plate (DPP) on the ninth beam of SG-II is presented. Without the application of SSD, normalized focal-spot non-uniformity of an 85% energy concentration is about 60%. Then, spectral bandwidth of the 3-ns, 1053-nm laser pulse is broadened to 0.3 nm (as 270GHz in 3ω) by a 3-GHz modulator and a 10-GHz modulator integrated in the front-end system. Spectral dispersion of 236 μrad/Å is achieved by a Littrow-configuration, 1480-l/mm grating placed between the Φ40mm faraday isolator and the third Φ40mm rod-amplifier. By using such SSD, normalized non-uniformity with the same energy concentration is decreased to 16%. A scheme of spatial power spectral density (PSD) in different directions is adopted to analyze the intensity distribution of the far-field irradiation. Based on the spatial PSD analysis, theoretical predictions of spectral peak caused by SSD’s color cycles is in excellent agreement with the experimental result. With double-frequency modulation, the amplitude of the spectral peak is reduced by ~10dB. The temporal waveform of the 1ω laser is measured. Waveform distortion criterion defining the frequency modulation to amplitude modulation conversion (FM-to-AM) is about 6% with 1ω laser energy of ~1.8kJ.