Engineering topics
Ma, L.
Publications and source records attributed to Ma, L..
Tuning magnetotransport in PdPt/Y{sub 3}Fe{sub 5}O{sub 12}: Effects of magnetic proximity and spin-orbit coupling
We report that anisotropic magnetoresistance (AMR) and anomalous Hall conductivity (AHC) in the Pd{sub 1−x}Pt{sub x}/Y{sub 3}Fe{sub 5}O{sub 12} (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.
Evolution of H3K27me3-marked chromatin is linked to gene expression evolution and to patterns of gene duplication and diversification
Histone modifications are critical for the regulation of gene expression, cell type specification, and differentiation. However, evolutionary patterns of key modifications that regulate gene expression in differentiating organisms have not been examined. Here we mapped the genomic locations of the repressive mark histone 3 lysine 27 trimethylation (H3K27me3) in four species of Drosophila, and compared these patterns to those in C. elegans. We found that patterns of H3K27me3 are highly conserved across species, but conservation is substantially weaker among duplicated genes. We further discovered that retropositions are associated with greater evolutionary changes in H3K27me3 and gene expression than tandem duplications, indicating that local chromatin constraints influence duplicated gene evolution. These changes are also associated with concomitant evolution of gene expression. Our findings reveal the strong conservation of genomic architecture governed by an epigenetic mark across distantly related species and the importance of gene duplication in generating novel H3K27me3 profiles.
Magneto-optical Kerr effect in L1{sub 0} FePdPt ternary alloys: Experiments and first-principles calculations
We have studied the magneto-optical Kerr effect (MOKE) of L1{sub 0} Fe{sub 0.5}(Pd{sub 1−x}Pt{sub x}){sub 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{sub ↓,x{sup 2}−y{sup 2}} 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.
Spin-orbit interaction tuning of perpendicular magnetic anisotropy in L1{sub 0} FePdPt films
The dependence of perpendicular magnetic anisotropy K{sub u} on spin-orbit coupling strength ξ is investigated in L1{sub 0} ordered FePd{sub 1−x}Pt{sub x} films by time-resolved magneto-optical Kerr effect measurements and ab initio density functional calculations. Continuous tuning of K{sub u} 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 K{sub u} nearly unchanged. Ab initio calculations predict a nearly quadratic dependence of K{sub u} on ξ, consistent with experimental data. K{sub u} 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 K{sub u} utilizing its sensitivity on ξ, which will help fabricate magnetic systems with desirable magnetic anisotropy.
Neutron diffraction study of MnNiGa{sub 2}—Structural and magnetic behaviour
MnNiGa{sub 2} crystallizes in the L21 (Heusler) structure and has a ferromagnetic ordering temperature T{sub C} ∼ 192 K. Rietveld refinement of the neutron diffraction patterns indicates that the Ga atoms occupy the equivalent 8c position, while Mn and Ni share the 4a (0, 0, 0) and 4b (0.5, 0.5, 0.5) sites with a mixed occupancy of Mn and Ni atoms. It is found that that ∼83% of Mn and ∼17% Ni are located at the 4a site while ∼83% of Ni and ∼17% Mn occupy the 4b site. There is no evidence of a magneto-volume effect around T{sub C}. In agreement with this finding, our detailed critical exponent analyses of isothermal magnetization curves and the related Arrott plots confirm that the magnetic phase transition at T{sub C} is second order.