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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Spin Frustration and Unconventional Surface Spin Canting State in Van der Waals Ferromagnet/Antiferromagnet Heterostructures

Atomically flat surfaces of van der Waals (vdW) materials pave an avenue for addressing a long-standing fundamental issue of how a compensated antiferromagnet (AFM) surface frustrates a ferromagnetic (FM) overlayer in FM/AFM heterostructures. We investigate Fe5GeTe2/NiPS3 vdW heterostructures by characterizing AFM and FM spins separately. We find that in-plane zig-zag AFM NiPS3 develops three equivalent AFM domains, which are robust against external magnetic field and magnetic coupling with Fe5GeTe2. Moreover, evidence is provided of in-plane-AFM-induced perpendicular magnetic anisotropy (PMA) in adjacent Fe5GeTe2, and an unconventional out-of-plane surface spin canting state with the Fe5GeTe2 spins spatially turn from out-of-plane direction near the interface to in-plane direction away from the interface in Fe5GeTe2/NiPS3. The out-of-plane surface spin canting is a unique property of spin frustration in vdW magnetic heterostructures.

Wang, Tianye↗
DOE OSTI2024

Uncovering the phonon spectra and lattice dynamics of plastically deformable InSe van der Waals crystals

Stacking two-dimensional (2D) van der Waals (vdW) materials in a layered bulk structure provides an appealing platform for the emergence of exotic physical properties. As a vdW crystal with exceptional plasticity, InSe offers the opportunity to explore various effects arising from the coupling of its peculiar mechanical behaviors and other physical properties. Here, we employ neutron scattering techniques to investigate the correlations of plastic interlayer slip, lattice anharmonicity, and thermal transport in InSe crystals. Not only are the interlayer slip direction and magnitude well captured by shifts in the Bragg reflections, but we also observe a deviation from the expected Debye behaviour in the heat capacity and lattice thermal conductivity. Combining the experimental data with first-principles calculations, we tentatively attribute the observed evidence of strong phonon-phonon interactions to a combination of a large acoustic-optical frequency resonance and a nesting effect. These findings correlate the macroscopic plastic slip and the microscopic lattice dynamics, providing insights into the mechano-thermo coupling and modulation in 2D vdW materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A magnetic sensor using a 2D van der Waals ferromagnetic material

Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. We define sensitivity as the slope of the characteristic curve of our sensor, df0/dH, where f0 is the resonance frequency and H is the external magnetic field. The sensitivity of the sensor reaches a large value of 16×106Hz/Oe, making it a potential candidate for a wide range of magnetic sensing applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI 3

We show that Van der Waals magnet VI 3 demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI 3 that revealed a ferromagnetic order with the moment direction tilted from the c-axis by ~36° at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed, before the magnetic order completely disappears at $T_C$ = 50 K. The refined magnetic moment of ~1.3$μ_B$ at 4 K is much lower than the fully ordered spin moment of 2$μ_B$/V 3+ , suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin–orbit coupling in VI 3 . This results in strong magnetoelastic interactions that make the magnetic properties of VI 3 easily tunable via strain and pressure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Systems and methods for disassembling two-dimensional van der Waals crystals into macroscopic monolayers and reassembling into artificial lattices

Systems and methods for generating one or more single crystal monolayers from two-dimensional van der Waals crystals are disclosed herein. Example methods include providing a bulk material including a plurality of van der Waals crystal layers, and exfoliating one or more single crystal monolayers of van der Waals crystal from the bulk material by applying a flexible and flat metal tape to a surface of the bulk material. In certain embodiments, the one or more single crystal monolayers can be assembled into an artificial lattice. The present disclosure also provides techniques for manufacturing flexible and flat metal tape for generating one or more single crystal monolayers from two-dimensional van der Waals crystals. The present disclosure also provides compositions for creating a macroscopic artificial lattice. In certain embodiments, the composition can include two or more macroscopic single crystal monolayers adapted from a bulk van der Waals crystal, where the single crystal monolayers are configured for assembly into an artificial lattice based on one or more properties.

Source record↗
DOE OSTI2025

Quantum Imaging of Ferromagnetic van der Waals Magnetic Domain Structures at Ambient Conditions

Recently discovered 2D van der Waals magnetic materials, and specifically iron–germanium–telluride (Fe5GeTe2), have attracted significant attention both from a fundamental perspective and for potential applications. Key open questions concern their domain structure and magnetic phase transition temperature as a function of sample thickness and external field, as well as implications for integration into devices such as magnetic memories and logic. Here we address key questions using a nitrogen-vacancy center based quantum magnetic microscope, enabling direct imaging of the magnetization of Fe5GeTe2 at submicrometer spatial resolution as a function of temperature, magnetic field, and thickness. This quantum imaging technique provides noninvasive, high-sensitivity measurements with high spatial resolution under ambient conditions, making it particularly well suited for probing 2D magnets. We employ spatially resolved measures, including magnetization variance and cross-correlation, and find a significant spread in transition temperature yet with no clear dependence on thickness down to 15 nm. We also identify previously unknown stripe features in the optical as well as magnetic images, which we attribute to modulations of the constituting elements during crystal synthesis and subsequent oxidation. Our results suggest that the magnetic anisotropy in this material does not play a crucial role in their magnetic properties, leading to a magnetic phase transition of Fe5GeTe2 which is largely thickness-independent down to 15 nm. Our findings could be significant in designing future spintronic devices, magnetic memories, and logic with 2D van der Waals magnetic materials.

Bindu, Bindu [Hebrew University of Jerusalem, Isra↗
DOE OSTI2020

Two-Dimensional van der Waals Nanoplatelets with Robust Ferromagnetism

We have synthesized unique colloidal nanoplatelets of the two-dimensional (2D) van der Waals ferromagnet CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The CrI3 nanoplatelets have lateral dimensions of ~25 nm and thicknesses of only ~4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering with Curie temperatures similar to bulk CrI3, despite their small size. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets associated with concerted spin-reversal of individual CrI3 layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr3 and anion-alloyed Cr(I1–xBrx)3 nanoplatelets. These results represent the first example of lateral nanostructures of 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.

De Siena, Michael C.↗
DOE OSTI2020

Van der Waals Nanowires with Continuously Variable Interlayer Twist and Twist Homojunctions

Abstract Moiré patterns at van der Waals interfaces between twisted 2D crystals give rise to distinct optoelectronic excitations, as well as, narrowly dispersive bands responsible for correlated electron phenomena. Contrasting with the conventional, mechanically stacked planar twist moirés, recent work shows twisted van der Waals interfaces spontaneously formed in nanowires of layered crystals, where Eshelby twist due to axial screw dislocations stabilizes a chiral structure with small interlayer rotation. Here, the realization of tunable twist in germanium(II) sulfide (GeS) van der Waals nanowires is reported. Tapered nanowires host continuously variable interlayer twist. Homojunctions between dislocated (chiral) and defect‐free (achiral) segments are obtained by triggering the emission of axial dislocations during growth. Measurements across such junctions, implemented here using local absorption and luminescence spectroscopy, provide a convenient tool for detecting twist effects. The results identify a versatile system for 3D twistronics, probing moiré physics, and for realizing moiré architectures without equivalent in planar systems.

Sutter, Peter↗
DOE OSTI2024

Highly Efficient Room‐Temperature Spin‐Orbit‐Torque Switching in a Van der Waals Heterostructure of Topological Insulator and Ferromagnet

Abstract All‐Van der Waals (vdW)‐material‐based heterostructures with atomically sharp interfaces offer a versatile platform for high‐performing spintronic functionalities at room temperature. One of the key components is vdW topological insulators (TIs), which can produce a strong spin‐orbit‐torque (SOT) through the spin‐momentum locking of their topological surface state (TSS). However, the relatively low conductance of the TSS introduces a current leakage problem through the bulk states of the TI or the adjacent ferromagnetic metal layers, reducing the interfacial charge‐to‐spin conversion efficiency ( q ICS ). Here, a vdW heterostructure is used consisting of atomically‐thin layers of a bulk‐insulating TI Sn‐doped Bi 1.1 Sb 0.9 Te 2 S 1 and a room‐temperature ferromagnet Fe 3 GaTe 2, to enhance the relative current ratio on the TSS up to ≈20%. The resulting q ICS reaches ≈1.65 nm −1 and the critical current density J c ≈0.9 × 10 6 Acm −2 at 300 K, surpassing the performance of TI‐based and heavy‐metal‐based SOT devices. These findings demonstrate that an all‐vdW heterostructure with thickness optimization offers a promising platform for efficient current‐controlled magnetization switching at room temperature.

2D ferromagnet↗
DOE OSTI2021

Magnetism and spin dynamics in room-temperature van der Waals magnet Fe 5 GeTe 2

Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe 5 GeTe 2 with a Curie temperature T c = 332 K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c -axis (H∥ c ) and the ab -plane (H∥ ab ), with temperatures ranging from 300 to 10 K. We have found a sizable Landé g -factor difference between the H∥ c and H∥ ab cases. In both cases, the Landé g -factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe 5 GeTe 2 has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe 5 GeTe 2 transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe 5 GeTe 2 , which promote the understanding of magnetism in Fe 5 GeTe 2 , leading to implementations of Fe 5 GeTe 2 based room-temperature spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Origins of heat transport anisotropy in MoTe2 and other bulk van der Waals materials

Knowledge of how heat flows anisotropically in van der Waals (vdW) materials is crucial for thermal management of emerging 2D materials devices and design of novel anisotropic thermoelectric materials. Despite the importance, anisotropic heat transport in vdW materials is yet to be systematically studied and is often presumably attributed to anisotropic speeds of sound in vdW materials due to soft interlayer bonding relative to covalent in-plane networks of atoms. In this work, we investigate the origins of the anisotropic heat transport in vdW materials, through time-domain thermoreflectance (TDTR) measurements and first-principles calculations of anisotropic thermal conductivity of three different phases of MoTe 2 . MoTe 2 is ideal for the study due to its weak anisotropy in the speeds of sound. We find that even when the speeds of sound are roughly isotropic, the measured thermal conductivity of MoTe 2 along the c-axis is 5–8 times lower than that along the in-plane axes. We derive meaningful characteristic heat capacity, phonon group velocity, and relaxation times from our first principles calculations for selected vdW materials (MoTe 2 , BP, h-BN, and MoS 2 ), to assess the contributions of these factors to the anisotropic heat transport. Interestingly, we find that the main contributor to the heat transport anisotropy in vdW materials is anisotropy in heat capacity of the dominant heat-carrying phonon modes in different directions, which originates from anisotropic optical phonon dispersion and disparity in the frequency of heat-carrying phonons in different directions. Further, the discrepancy in frequency of the heat-carrying phonons also leads to ~2 times larger average relaxation times in the cross-plane direction, and partially explains the apparent dependence of the anisotropic heat transport on the anisotropic speeds of sound. This work provides insight into understanding of the anisotropic heat transport in vdW materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Directing Charge Carriers and Ferroelectric Domains at Lateral Interfaces in van der Waals Heterostructures

Emergent phenomena in traditional ferroelectrics are frequently observed at heterointerfaces. Accessing such functionalities in van der Waals ferroelectrics requires the formation of layered heterostructures, either vertically stacked (similar to oxide ferroelectrics) or laterally stitched (without equivalent in 3D-crystals). Here, we investigate lateral heterostructures of the ferroelectric van der Waals semiconductors SnSe and SnS. A two-step process produces ultrathin crystals comprising an SnSe core laterally joined to an SnS edge-band, as confirmed by Raman spectroscopy, transmission electron microscopy (TEM) imaging, and electron diffraction. TEM shows a moiré pattern across the SnSe core due to coverage by an ultrathin SnS layer. The ability of the lateral interface (IF) to direct excited carriers, probed by cathodoluminescence, shows electron transfer over 560 nm diffusion length from the SnS edge-band. Large, thin flakes supporting ferroelectricity allow investigating domains and domain wall interactions in uniform crystals and lateral heterostructures. Polarized optical microscopy of sub-20 nm flakes consistently shows ⟨110⟩ oriented stripe domains with mirror-twin domain walls. Heterostructures adopt two domain configurations, with domains either constrained to the SnSe core or propagating across the entire SnSe–SnS flakes. Furthermore, the combined results demonstrate multifunctional van der Waals heterostructures with high-quality IFs presenting extraordinary opportunities for manipulating carrier flows and ferroelectric domain patterns.

2D ferroelectrics↗
DOE OSTI2021

van der Waals interaction as the starting point for an effective field theory

We consider the system of three 4 He atoms to assess whether a pure van der Waals potential can be used as a starting point for an effective field theory to describe three-body processes in ultracold atomic systems. Using a long-range van der Waals interaction in combination with short-distance two-body counterterms, we analyze the dependence of two- and three-body observables on the short-distance regulator that is required due to the singular nature of the van der Waals interaction. We benchmark our approach with results obtained with the realistic 4 He- 4 He LM2M2 potential and find good agreement. We furthermore show that in this effective field theory approach no three-body force is required at leading order and that universal van der Waals physics leads to a universal correlation between three-body observables in the absence of an Efimov three-body parameter.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Tunable Few-Layer van der Waals Crystals and Heterostructures as Emerging Energy and Quantum Materials (Final Technical Report)

2D and layered (van der Waals) semiconductors offer extraordinary opportunities for manipulating optically excited charge carriers, many-body excitations, and non-charge based quantum numbers. To date, research has focused on a limited group of materials, mostly transition metal dichalcogenides in the monolayer limit. Other van der Waals semiconductors, and especially few-layer to multilayer crystals and their heterostructures, carry large potential for the discovery of phenomena of interest for future energy and information technologies. But they remain largely unexplored, often due to a lack of access to high-quality materials and approaches for measuring their properties at the relevant scales. The goal of this project was to develop an EPSCoR-State/National Laboratory Partnership that addresses the challenges of preparing high-quality van der Waals semiconductors and of probing their structure, composition, and especially their optoelectronic and photonic properties, near the atomic scale using electron microscopy techniques. A central component of the project was the development of advanced methods for electron microscopy and electron-excited spectroscopy, taking advantage of unique samples as well as leading capabilities and expertise at the partner institutions. Efforts to advance leading-edge techniques was supported by ancillary developments, such as precision sample preparation for electron microscopy/spectroscopy, coordinated chemical imaging, and analytical electron microscopy. Experiments in materials synthesis and technique development were closely linked to theory and computation. The results obtained under this project yielded multifaceted benefits to the involved partners and their institutions, DOE-BES, the wider scientific community, and society at large, particularly in the State of Nebraska through dividends from knowledge and human capital generated under the project.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Interlayer Interactions in 1D Van der Waals Moiré Superlattices.

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlattices. In this review, experimental observations and theoretical perspectives related to interlayer interactions in 1D vdW moiré superlattices are summarized. The discussion focuses on double-walled carbon nanotubes (DWNTs), a model 1D vdW moiré system, and the authors highlight the new optical features emerging from the non-trivial strong interlayer coupling effect and the unique physics in 1D DWNTs. Future directions and questions in probing the intriguing physical phenomena in 1D vdW moiré superlattices such as, correlated physics in different 1D moiré systems beyond DWNTs are proposed and discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Controllable Skyrmionic Phase Transition between Néel Skyrmions and Bloch Skyrmionic Bubbles in van der Waals Ferromagnet Fe 3-δ GeTe 2

The van der Waals (vdW) ferromagnet Fe 3-δ GeTe 2 has garnered significant research interest as a platform for skyrmionic spin configurations, that is, skyrmions and skyrmionic bubbles. However, despite extensive efforts, the origin of the Dzyaloshinskii–Moriya interaction (DMI) in Fe 3-δ GeTe 2 remains elusive, making it challenging to acquire these skyrmionic phases in a controlled manner. In this study, it is demonstrated that the Fe content in Fe 3-δ GeTe 2 has a profound effect on the crystal structure, DMI, and skyrmionic phase. For the first time, a marked increase in Fe atom displacement with decreasing Fe content is observed, transforming the original centrosymmetric crystal structure into a non-centrosymmetric symmetry, leading to a considerable DMI. Additionally, by varying the Fe content and sample thickness, a controllable transition between Néel-type skyrmions and Bloch-type skyrmionic bubbles is achieved, governed by a delicate interplay between dipole–dipole interaction and the DMI. The findings offer novel insights into the variable skyrmionic phases in Fe 3-δ GeTe 2 and provide the impetus for developing vdW ferromagnet-based spintronic devices.

2D ferromagnet Fe3-δGeTe2↗
DOE OSTI2022

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

36 MATERIALS SCIENCE↗