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Wu, Stephen M.

Publications and source records attributed to Wu, Stephen M..

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Millimeter-Scale Exfoliation of hBN with Tunable Flake Thickness for Scalable Encapsulation

As a two-dimensional (2D) dielectric material, hexagonal boron nitride (hBN) is in high demand for applications in photonics, nonlinear optics, and nanoelectronics. Unfortunately, the high-throughput preparation of macroscopic-scale, high-quality hBN flakes with controlled thickness is an ongoing challenge, limiting device fabrication and technological integration. In this report we present a metal thin-film exfoliation method to prepare hBN flakes with millimeter-scale dimension, near-unity yields, and tunable flake thickness distribution from 1-7 layers, a substantial improvement over scotch tape exfoliation. The single crystallinity and high quality of the exfoliated hBN are demonstrated with optical microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation. We further explore a possible mechanism for the effectiveness and selectivity based on thin-film residual stress measurements, density functional theory calculations, and transmission electron microscopy imaging of the deposited metal films. We find that the magnitude of the residual tensile stress induced by thin film deposition plays a key role in determining exfoliated flake thickness in a manner which closely resembles 3D semiconductor spalling. Lastly, we demonstrate that our exfoliated, large-area hBN flakes can be readily incorporated as encapsulating layers for other 2D monolayers. Altogether, this method brings us one step closer to the high throughput, mass production of hBN-based 2D photonic, optoelectronic, and quantum devices.

2D materials↗
DOE OSTI2018

Probing short-range magnetic order in a geometrically frustrated magnet by means of the spin Seebeck effect

The spin Seebeck effect (SSE) is a phenomenon where a spin current is generated from thermal excitations in magnetic materials. It is believed that magnetic long-range order (LRO) is not required for SSE, while short-ranged order (SRO) plays an important role. However, a definitive experimental demonstration of the connection between the SSE and SRO has been missing. Here, we show that the SSE is able to probe a specific SRO in a model geometrically frustrated magnet Gd 3 Ga 5 O 12 (GGG). A field-induced SRO in GGG is detected by the SSE in the temperature range 2-5 K. The origin of the SRO is verified by comparing the SSE data directly to existing neutron scattering measurements at much lower temperatures (T < 0.9 K), where field-induced LRO exists. Our theoretical calculations of the magnetic structure further confirm the anisotropic field dependence of the SRO in GGG. Furthermore, these findings establish that the SSE can serve as an effective probe of SRO in geometrically frustrated magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tunable Noncollinear Antiferromagnetic Resistive Memory through Oxide Superlattice Design

Antiferromagnets (AFMs) have recently gathered a large amount of attention as a potential replacement for ferromagnets (FMs) in spintronic devices due to their lack of stray magnetic fields, invisibility to external magnetic probes, and faster magneti- zation dynamics. Their development into a practical technology, however, has been hampered by the small number of materials where the antiferromagnetic state can be both controlled and read out. We show here, that by relaxing the strict criterion on pure antiferromagnetism, we can engineer a new class of magnetic materials that 1 overcome these limitations. This is accomplished by stabilizing a non-collinear mag- netic phase in LaNiO 3 /La 2/3 Sr 1/3 MnO 3 superlattices. This state can be continuously tuned between AFM and FM coupling through varying either superlattice spacing, strain, applied magnetic field, or temperature. By using this new “knob” to tune magnetic ordering, we take a nanoscale materials-by-design approach to engineering ferromagnetic-like controllability into antiferromagnetic synthetic magnetic structures. This approach can be used to trade-off between the favorable and unfavorable proper- ties of FMs and AFMs when designing realistic resistive antiferromagnetic memories. We demonstrate a memory device in one such superlattice, where the magnetic state of the non-collinear antiferromagnet is reversibly switched between different orientations using a small magnetic field and read out in real time with anisotropic magnetoresistance measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Nanoscale measurement of Nernst effect in two-dimensional charge density wave material 1T-TaS 2

Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS 2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS 2 , which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW! NCCDW transition.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Antiferromagnetic spin Seebeck effect.

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF2. A device scale on-chip heater is deposited on a bilayer of MnF2 (110) (30nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF2(110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF2 through the inverse spin Hall effect. The low temperature (2–80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9T) are applied parallel to the easy axis of the MnF2 thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

Wu, Stephen M.↗
DOE OSTI2015

Spin waves in micro-structured yttrium iron garnet nanometer-thick films

We investigated the spin-wave propagation in a micro-structured yttrium iron garnet waveguide of 40 nm thickness. Utilizing spatially-resolved Brillouin light scattering microscopy, an exponential decay of the spin-wave amplitude of (10.06 ± 0.83) μm was observed. This leads to an estimated Gilbert damping constant of α=(8.79±0.73)×10{sup −4}, which is larger than damping values obtained through ferromagnetic resonance measurements in unstructured films. The theoretically calculated spatial interference of waveguide modes was compared to the spin-wave pattern observed experimentally by means of Brillouin light scattering spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin Seebeck devices using local on-chip heating

A micro-patterned spin Seebeck device is fabricated using an on-chip heater. Current is driven through a Au heater layer electrically isolated from a bilayer consisting of Fe{sub 3}O{sub 4} (insulating ferrimagnet) and a spin detector layer. It is shown that through this method it is possible to measure the longitudinal spin Seebeck effect (SSE) for small area magnetic devices, equivalent to traditional macroscopic SSE experiments. Using a lock-in detection technique, it is possible to more sensitively characterize both the SSE and the anomalous Nernst effect (ANE), as well as the inverse spin Hall effect in various spin detector materials. By using the spin detector layer as a thermometer, we can obtain a value for the temperature gradient across the device. These results are well matched to values obtained through electromagnetic/thermal modeling of the device structure and with large area spin Seebeck measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Unambiguous separation of the inverse spin Hall and anomalous Nernst effects within a ferromagnetic metal using the spin Seebeck effect

In this paper, the longitudinal spin Seebeck effect is measured on the ferromagnetic insulator Fe 3 O 4 with the ferromagnetic metal Co 0.2 Fe 0.6 B 0.2 (CoFeB) as the spin detector. By using a non-magnetic spacer material between the two materials (Ti), it is possible to decouple the two ferromagnetic materials and directly observe pure spin flow from Fe 3 O 4 into CoFeB. It is shown that in a single ferromagnetic metal, the inverse spin Hall effect (ISHE) and anomalous Nernst effect (ANE) can occur simultaneously with opposite polarity. Using this and the large difference in the coercive fields between the two magnets, it is possible to unambiguously separate the contributions of the spin Seebeck effect from the ANE and observe the degree to which each effect contributes to the total response. Finally, these experiments show conclusively that the ISHE and ANE in CoFeB are separate phenomena with different origins and can coexist in the same material with opposite response to a thermal gradient.

36 MATERIALS SCIENCE↗