Engineering topics

Zhang, B. H.

Publications and source records attributed to Zhang, B. H..

6 records

DOE OSTI2021

First-principles studies of the unconventional spin-phonon coupling mediated by spin-orbit coupling in pyrochlore Cd 2 Os 2 O 7

Here, we perform systematic first-principles calculations to investigate the spin-phonon coupling (SPC) mediated by spin-orbit coupling in pyrochlore Cd 2 Os 2 O 7 which has the peculiar all-in-all-out noncollinear magnetic ground state. A large SPC coefficient up to 34 cm –1 is found, showing the potential of using this material in thermal spin devices.We further split the SPC coefficients to contributions from symmetric exchange interaction, Dzyaloshinskii-Moriya interaction (DMI), and the single ion anisotropy. Unlike what was assumed in previous studies, the DMI contributions are rather important for most modes. These results provide insights for the understanding of the SPC in 5d transition metal oxides and useful guidance for the search of excellent spin caloritronic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Tuning Dzyaloshinskii-Moriya interactions in magnetic bilayers with a ferroelectric substrate

Our group performed systematic first-principles calculations to explore the possibility of manipulating Dzyaloshinskii-Moriya interactions (DMIs) in heavy metal/ferromagnetic metal bilayers with ferroelectric (FE) polarization of a PbTiO 3 (PTO) substrate. Significantly, both the sign and magnitude of the DMI can be tailored by the FE polarization reversal in PTO. Moreover, the DMIs of Ir/Co/PTO and Ir/Fe/PTO can achieve large values (|d| > 10 meV) under the influence of an upward electric polarization of PTO. Our findings provided useful strategies for enhancing and tailoring DMIs in complex heterostructures and paved the way for the development of DMI-based devices with ultralow power consumption.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Unexpected dependence of the anomalous Hall angle on the Hall conductivity in amorphous transition metal thin films

The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films— Fe x S i –x , Fe x Ge 1-x , Co x Ge 1–x , Co x Si 1–x , and Fe 1–y Co y Si. The experimental results are compared with density functional theory calculations of the density of Berry curvature and intrinsic anomalous Hall conductivity. In all samples, the longitudinal conductivity (σ xx ), magnetization (M), and Hall resistivity (ρ xy ) increase with increasing transition metal concentration; due to the structural disorder σ xx is lower in all samples than a typical crystalline metal. In the systems with Fe as the transition metal (including Fe 1–y Co y Si), the magnetization and AHE are large and in some cases greater than the crystalline analog. In all samples, the AHE is dominated by the intrinsic mechanism, arising from a nonzero, locally derived Berry curvature. The anomalous Hall angle (AHA) (= σ xy /σ xx ) is as large as 5% at low temperature. These results are compared with the AHAs reported in a broad range of crystalline and amorphous materials. Previous work has shown that in a typical crystalline ferromagnet the Hall conductivity (σ xy ) and σ xx are correlated and are usually either both large or both small, resulting in an AHA that decreases with increasing σ xy . By contrast, the AHA increases linearly with increasing σ xy in the amorphous systems. This trend is attributed to a generally low σ xx , while σ xy varies and can be large. In the amorphous systems, σ xx and σ xy are not coupled, and there may thus exist the potential to further increase the AHA by increasing σ xy .

36 MATERIALS SCIENCE↗

DOE OSTI2019

First-principles studies of spin-phonon coupling in monolayer Cr 2 Ge 2 Te 6

Here, we perform systematic first-principles calculations to investigate the spin-phonon coupling (SPC) of Cr 2 Ge 2 Te 6 (CGT) monolayer (ML). It is found that the $E_g$ phonon mode at 211.8 cm –1 may have a SPC as large as 3.19 cm –1 , as it alters the direct exchange interaction across Cr-Cr pairs. The strength of SPC of the CGT ML can be decreased by an in-plane lattice stretch. These results provide useful insights for the understanding of SPC in two-dimensional magnetic semiconductors and may guide the design of spintronic and spin Seebeck materials and devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1990

Variations of global mesoscale eddy energy observed from Geosat

Data on oceanic-current variability were extracted from Geosat altimeter observations for 44 17-day repeat cycles, using the Sandwell and Zhang (1989) technique to process the altimeter data and to produce a sea-surface-slope profile having an estimated accuracy of 0.2 microrad. These were used to generate a series of global eddy kinetic energy maps, each averaged over 3 months, together with their mean. It was found that the maximum mean eddy kinetic energy per unit mass exceeds 2000 sq cm/sq sec for most of the western boundary currents; for the Antarctic Circumpolar Current, however, this value reaches only 500 sq cm/sq sec.

Shum, C. K.↗

NASA NTRS1990

Altimeter crossover methods for precision orbit determination and the mapping of geophysical parameters

Accurate orbit determination and the recovery of geophysical parameters are presently attempted via methodologies which use differenced height measurements at the points where the ground tracks of the altimetric satellite orbits intersect. Such 'crossover measurements' could significantly improve the earth's gravity field model. Attention is given to a novel technique employing crossover measurements from two satellites carrying altimeter instruments; this method can observe zonal harmonics of the earth's geopotential which are weakly observed through single-satellite crossovers. This dual-satellite crossover technique will be applicable to data from such future oceanographic satellites as ERS-1.

Shum, C. K.↗