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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on GeTe7As4 by Materials Project

As2Te3(GeTe)As2Te3 is MAX Phase-like structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one As2Te3 sheet oriented in the (0, 0, 1) direction and one As2Te3(GeTe) sheet oriented in the (0, 0, 1) direction. In the As2Te3 sheet, As1+ is bonded to six Te+1.14- atoms to form a mixture of edge and corner-sharing AsTe6 octahedra. The corner-sharing octahedral tilt angles are 0°. There are three shorter (2.86 Å) and three longer (3.01 Å) As–Te bond lengths. There are two inequivalent Te+1.14- sites. In the first Te+1.14- site, Te+1.14- is bonded in a 3-coordinate geometry to three equivalent As1+ atoms. In the second Te+1.14- site, Te+1.14- is bonded to six equivalent As1+ atoms to form edge-sharing TeAs6 octahedra. In the As2Te3(GeTe) sheet, Ge4+ is bonded to six equivalent Te+1.14- atoms to form GeTe6 octahedra that share corners with six equivalent AsTe6 octahedra, edges with six equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. All Ge–Te bond lengths are 2.97 Å. As1+ is bonded to six Te+1.14- atoms to form AsTe6 octahedra that share corners with three equivalent GeTe6 octahedra, edges with three equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. There are three shorter (2.85 Å) and three longer (3.03 Å) As–Te bond lengths. There are two inequivalent Te+1.14- sites. In the first Te+1.14- site, Te+1.14- is bonded in a 3-coordinate geometry to three equivalent As1+ atoms. In the second Te+1.14- site, Te+1.14- is bonded to three equivalent Ge4+ and three equivalent As1+ atoms to form a mixture of edge and corner-sharing TeGe3As3 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Giant Modulation of Magnetoresistance in a Van Der Waals Magnet by In‐Plane Current Injection

Efficient magnetization control is a central issue in magnetism and spintronics. Particularly, there are increasing demands for manipulation of magnetic states in van der Waals (vdW) magnets with unconventional functionalities. However, the electrically induced phase transition between ferromagnetic-to-antiferromagnetic states without external magnetic field is yet to be demonstrated. Here, the current-induced magnetic phase transition in a vdW ferromagnet Fe 5 GeTe 2 is reported. Based on magneto-transport measurements and theoretical analysis, it is demonstrated that transition in the interlayer magnetic coupling occurs through vertical voltage drop between layers induced by current which is attributed to high anisotropy of the resistivity caused by the vdW gaps. Such magnetic phase transition results in giant modulation of the longitudinal magnetoresistance from 5% to 170%. The electrical tunability of the magnetic phase in Fe 5 GeTe 2 with current-in-plane geometry opens a path for electric control of magnetic properties, expanding the ability to use vdW magnets for spintronic applications.

2D magnet↗
DOE OSTI2024

Strong Surface-Enhanced Coherent Phonon Generation in van der Waals Materials

Terahertz (THz) coherent phonons have emerged as promising candidates for the next generation of high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. However, effectively manipulating THz coherent phonons remains a significant challenge. Here, in this study, we investigated THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of Fe 3 GeTe 2 , Fe 5 GeTe 2 , and FePS 3 . We successfully generated the THz A 1g coherent phonon mode in these vdW flakes. An innovative approach involved partially exfoliating vdW flakes on a gold substrate and partially on a silicon (Si) substrate to compare the THz coherent phonon generation between both sides. Interestingly, we observed a significantly enhanced THz coherent phonon in the vdW/gold area compared with that in the vdW/Si area. Frequency-domain Raman mapping across the vdW flakes corroborated these findings. Numerical simulations further indicated a stronger enhanced surface field in vdW/gold structures than in vdW/Si structures. Consequently, we attribute the observed enhancement in THz coherent phonon generation to the increased surface field on the gold substrate. This enhancement was consistent across the three different vdW materials studied, suggesting the universality of this strategy. Our results hold promise for advancing the design of THz phononic and phonon-integrated devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Out-of-Plane Magnetic Anisotropy Engineered via Band Distortion in Two-Dimensional Materials

We unveil the connection between magnetic anisotropy and band structures in Γ-specialized In 2 Se 3 /Fe 3 GeTe 2 heterostructure, through first-principles calculations. A significant band distortion occurs near the Γ point with a formed valley. As the band-distortion-caused valley moves relative to the quasivalence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such a rule applies to either the strained or polarization-switched In 2 Se 3 /Fe 3 GeTe 2 heterostructure. Furthermore, these findings demonstrate the feasibility of predicting the magnetic anisotropy by band structure modification in two-dimensional magnetic systems.

2-dimensional systems↗
DOE OSTI2020

Tuning magnetic order in the van der Waals metal $Fe_5GeTe_2$ by cobalt substitution

Fe 5 - x GeTe 2 is a van der Waals material with one of the highest reported bulk Curie temperatures, T C ≈ 310 K . Here, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to ≈ 30 % enhances T C and changes the magnetic anisotropy, while ≈ 50 % cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for ≈ 46 % cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe 5 - x GeTe 2 to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ge(Te2As)2 by Materials Project

As2Te3(GeTe) is MAX Phase-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three As2Te3(GeTe) sheets oriented in the (0, 0, 1) direction. Ge4+ is bonded to six equivalent Te2- atoms to form GeTe6 octahedra that share corners with six equivalent AsTe6 octahedra, edges with six equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. All Ge–Te bond lengths are 2.97 Å. As2+ is bonded to six Te2- atoms to form AsTe6 octahedra that share corners with three equivalent GeTe6 octahedra, edges with three equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. There are three shorter (2.85 Å) and three longer (3.03 Å) As–Te bond lengths. There are two inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a 3-coordinate geometry to three equivalent As2+ atoms. In the second Te2- site, Te2- is bonded to three equivalent Ge4+ and three equivalent As2+ atoms to form a mixture of corner and edge-sharing TeGe3As3 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TiGeTe6 by Materials Project

TiTe5GeTe crystallizes in the monoclinic C2/m space group. The structure is one-dimensional and consists of two GeTe ribbons oriented in the (0, 1, 0) direction and two TiTe5 ribbons oriented in the (0, 1, 0) direction. In each GeTe ribbon, Ge4+ is bonded in a 3-coordinate geometry to three equivalent Te1- atoms. There are two shorter (2.76 Å) and one longer (2.77 Å) Ge–Te bond lengths. Te1- is bonded in a 3-coordinate geometry to three equivalent Ge4+ atoms. In each TiTe5 ribbon, there are two inequivalent Ti2+ sites. In the first Ti2+ site, Ti2+ is bonded to seven Te1- atoms to form distorted face-sharing TiTe7 pentagonal bipyramids. There are three shorter (2.84 Å) and four longer (2.90 Å) Ti–Te bond lengths. In the second Ti2+ site, Ti2+ is bonded to seven Te1- atoms to form distorted face-sharing TiTe7 pentagonal bipyramids. There are three shorter (2.84 Å) and four longer (2.90 Å) Ti–Te bond lengths. There are ten inequivalent Te1- sites. In the first Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ atoms. In the second Te1- site, Te1- is bonded in a 12-coordinate geometry to five Te1- atoms. There are a spread of Te–Te bond distances ranging from 2.94–3.64 Å. In the third Te1- site, Te1- is bonded in a distorted single-bond geometry to one Ti2+ and two equivalent Te1- atoms. Both Te–Te bond lengths are 2.94 Å. In the fourth Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ atoms. In the fifth Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ atoms. In the sixth Te1- site, Te1- is bonded in a distorted single-bond geometry to one Ti2+ and two equivalent Te1- atoms. In the seventh Te1- site, Te1- is bonded in a 12-coordinate geometry to five Te1- atoms. The Te–Te bond length is 3.15 Å. In the eighth Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ and one Te1- atom. In the ninth Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ and one Te1- atom. In the tenth Te1- site, Te1- is bonded in a distorted L-shaped geometry to two equivalent Ti2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ge3Te by Materials Project

(GeGeGe)Te is alpha Niobium phosphide-like structured and crystallizes in the tetragonal I4/mmm space group. The structure is two-dimensional and consists of two Ge sheets oriented in the (0, 0, 1) direction and two GeTe sheets oriented in the (0, 0, 1) direction. In each Ge sheet, Ge is bonded in a square co-planar geometry to four equivalent Ge atoms. All Ge–Ge bond lengths are 2.92 Å. In each GeTe sheet, Ge is bonded in a square co-planar geometry to four equivalent Te atoms. All Ge–Te bond lengths are 2.92 Å. Te is bonded in a square co-planar geometry to four equivalent Ge atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on GaGeTe2 by Materials Project

GaTeGeTe is black P-derived structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of two GaTe sheets oriented in the (0, 0, 1) direction and two GeTe sheets oriented in the (0, 0, 1) direction. In each GaTe sheet, Ga2+ is bonded in a trigonal non-coplanar geometry to three equivalent Te2- atoms. All Ga–Te bond lengths are 2.74 Å. Te2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Ga2+ atoms. In each GeTe sheet, Ge2+ is bonded in a distorted trigonal non-coplanar geometry to three equivalent Te2- atoms. All Ge–Te bond lengths are 2.77 Å. Te2- is bonded in a 3-coordinate geometry to three equivalent Ge2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Acceleration of Crystallization Kinetics in Ge‐Sb‐Te‐Based Phase‐Change Materials by Substitution of Ge by Sn

Abstract Thin films of (Ge 1– x Sn x ) 8 Sb 2 Te 11 are prepared to study the impact of Sn‐substitution on properties relevant for application in phase‐change memory, a next‐generation electronic data storage technology. It is expected that substitution decreases the crystallization temperature, but it is not known how the maximum crystallization rate is affected. Ge 8 Sb 2 Te 11 is chosen from the (GeTe) y (Sb 2 Te 3 ) 1– y system of phase‐change materials as a starting point due to its higher crystallization temperature as compared to the common material Ge 2 Sb 2 Te 5 . In situ X‐ray diffraction at 5 K min −1 heating rate is performed to determine the crystallization temperature and the resulting structure. To measure the maximum crystallization rate, femtosecond optical pulses that heat the material repetitively and monitor the resulting increase of optical reflectance are used. Glasses over the entire composition range are prepared using a melt‐quenching process. While at x = 0, 97, subsequent pulses are required for crystallization, one single pulse is enough to achieve the same effect at x = 0.5. The samples are further characterized by optical ellipsometry and calorimetry. The combined electrical and optical contrast and the ability to cycle between states with single femtosecond pulses renders Ge 4 Sn 4 Sb 2 Te 11 promising for photonics applications.

Zalden, Peter↗
DOE OSTI2025

Universal Superconductivity in FeTe and All‐Iron‐Based Ferromagnetic Superconductor Heterostructures

Abstract Ferromagnetism (FM) and superconductivity (SC) are two of the most famous macroscopic quantum phenomena. However, nature normally does not allow SC and FM to coexist without significant degradation. Here, the first fully iron‐based SC/FM heterostructures, composed of Fe(Te,Se) and Fe 3 GeTe 2 , are introduced, and it is shown that this system exhibits both strong FM and high‐temperature SC with an atomically sharp interface. From this study, it is also discovered that minute level of various cationic dopants can drive otherwise non‐superconducting FeTe films into a SC state. This suggests that the ground state of FeTe is so close to the SC state that it can be driven in and out of the SC state with various other perturbations. Altogether, this shows that Fe‐Te‐based heterostructures provide a unique opportunity to manipulate magnetism, superconductivity, and topological physics, paving the way toward new superconducting technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A Van der Waals Interface Hosting Two Groups of Magnetic Skyrmions

Multiple magnetic skyrmion phases add an additional degree of freedom for skyrmion-based ultrahigh-density spin memory devices. Extending the field to 2D van der Waals magnets is a rewarding challenge, where the realizable degree of freedoms (e.g., thickness, twist angle, and electrical gating) and high skyrmion density result in intriguing new properties and enhanced functionality. In this work, a van der Waals interface, formed by two 2D ferromagnets Cr 2 Ge 2 Te 6 and Fe 3 GeTe 2 with a Curie temperature of ≈65 and ≈205 K, respectively, hosting two groups of magnetic skyrmions, is reported. Two sets of topological Hall effect signals are observed below 6s0 K when Cr 2 Ge 2 Te 6 is magnetically ordered. These two groups of skyrmions are directly imaged using magnetic force microscopy, and supported by micromagnetic simulations. Interestingly, the magnetic skyrmions persist in the heterostructure with zero applied magnetic field. The results are promising for the realization of skyrmionic devices based on van der Waals heterostructures hosting multiple skyrmion phases.

2D magnetism↗
DOE OSTI2023

Room-Temperature, Current-Induced Magnetization Self-Switching in A Van Der Waals Ferromagnet

2D layered materials with broken inversion symmetry are being extensively pursued as spin source layers to realize high-efficiency magnetic switching. Such low-symmetry layered systems are, however, scarce. In addition, most layered magnets with perpendicular magnetic anisotropy show a low Curie temperature. Here, in this study, the experimental observation of spin–orbit torque magnetization self-switching at room temperature in a layered polar ferromagnetic metal, Fe 2.5 Co 2.5 GeTe 2 is reported. The spin–orbit torque is generated from the broken inversion symmetry along the c-axis of the crystal. These results provide a direct pathway toward applicable 2D spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Direct‐Write Printed Contacts to Layered and 2D Materials

Advancements in fabrication methods have shaped new computing device technologies. Among these methods, depositing electrical contacts to the channel material is fundamental to device characterization. Novel layered and 2D materials are promising for next-generation computing electronic channel materials. Direct-write printing of conductive inks is introduced as a surprisingly effective, significantly faster, and cleaner method to contact different classes of layered materials, including graphene (semi-metal), MoS 2 (semiconductor), Bi-2212 (superconductor), and Fe 5 GeTe 2 (metallic ferromagnet). Based on the electrical response, the quality of the printed contacts is comparable to what is achievable with resist-based lithography techniques. These devices are tested by sweeping gate voltage, temperature, and magnetic field to show that the materials remain pristine post-processing. This work demonstrates that direct-write printing is an agile method for prototyping and characterizing the electrical properties of novel layered materials.

2D materials↗
DOE OSTI2021

Recent developments on 2D magnetic materials: challenges and opportunities

The emergence of two-dimensional (2D) magnetic materials exhibiting strong magnetization at ultrathin limits above room temperature are promising for miniaturization of devices beyond Moore’s law for future energy efficient nano-electronic devices. Here, the current status, different mechanisms for the existence of magnetism, spin current injection and other magnetic properties of monolayer to few-layers of various 2D magnetic materials are reviewed. Some of the promising applications of these materials are spintronics devices such as spin valves, spin tunnel field-effect transistors, and spin filtering magnetic tunnel junctions. Due to the tunable electronic properties of these 2D materials, it's quite interesting to inject the spin current with suitable ferromagnetic contacts. For instance,black phosphorus is a layered material with a small Schottky barrier height capable of injecting spin current. This review includes many recently explored 2D magnetic materials ranging from exfoliated 2D crystals to CVD grown materials from single to several layers, demonstrating tunable layer dependent magnetic properties. We also explore some of the promising theoretical study based on 2D magnetic compounds such as 2D alkali-based chromium chalcogenides, which shows ferromagnetic as well as semiconducting behavior. The layer-dependent magnetic ordering has been observed in layered compounds like 1T-CrTe 2 , VSe 2 , CrI 3 , and Fe 3 GeTe 2 , which have great potential for the future applications in magnetic based electronic devices. Finally, we emphasize the challenges, opportunities and future directions of the 2D magnetic materials, where new discoveries might have outstanding impact in transformational scientific breakthroughs towards memory, spintronics, optoelectronics and other multifunctional device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Structural and electronic characteristics of amorphous Ge 8 Sb 2 Te 11

GeTe-rich phase-change materials can be utilized in rewriteable optical memory due to the large contrast in reflectivity between amorphous and crystalline phases. Here we explored the structure and electronic properties of amorphous Ge 8 Sb 2 Te 11 using ab initio molecular dynamics simulations. The results indicate that amorphous Ge 8 Sb 2 Te 11 is dominantly composed of Ge-, Sb- and Te-centered octahedrons with distortions, while 30.4 % of Ge-centered clusters are in the form of tetrahedrons which are randomly distributed. The 5-fold rings possess a large proportion, and the Gesingle bondTe and Sbsingle bondTe bonds present larger formation energies than other bonds, leading to the ABAB bonding sequence (A: Ge and Sb, B: Te). The lone pair electrons locating at the opposite direction of bonds possess a large fraction of 14.8 %, which may enhance the distortions of local clusters. Importantly, these structural properties lead to the high stability of amorphous Ge 8 Sb 2 Te 11 and thus long data retention in the information storage.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ferroelectric Switching of Pure Spin Polarization in Two-Dimensional Electron Gas

Two-dimensional electron gas (2DEG) created at compound interfaces can exhibit a broad range of exotic physical phenomena, including quantum Hall phase, emergent ferromagnetism, and superconductivity. Although electron spin plays key roles in these phenomena, the fundamental understanding and application prospects of such emergent interfacial states have been largely impeded by the lack of purely spin-polarized 2DEG. In this paper, by first-principles calculations of the multiferroic superlattice GeTe/MnTe, we find the ferroelectric polarization of GeTe is concurrent with the half-metallic 2DEG at interfaces. Remarkably, the pure spin polarization of the 2DEG can be created and annihilated by polarizing and depolarizing the ferroelectrics and can be switched (between pure spin-up and pure spin-down) by flipping the ferroelectric polarization. Given the electric-field amplification effect of ferroelectric electronics, we envision multiferroic superlattices could open up new opportunities for low-power, high-efficiency spintronic devices such as spin field-effect transistors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Thermal Hysteresis and Ordering Behavior of Magnetic Skyrmion Lattices

The physics of phase transitions in two-dimensional (2D) systems underpins research in diverse fields including statistical mechanics, nanomagnetism, and soft condensed matter. However, many aspects of 2D phase transitions are still not well understood, including the effects of interparticle potential, polydispersity, and particle shape. Magnetic skyrmions are chiral spin-structure quasi-particles that form two-dimensional lattices. Here, in this study, we show, by real-space imaging using in situ cryo-Lorentz transmission electron microscopy coupled with machine learning image analysis, the ordering behavior of Néel skyrmion lattices in van der Waals Fe 3 GeTe 2 . We demonstrate a distinct change in the skyrmion size distribution during field-cooling, which leads to a loss of lattice order and an evolution of the skyrmion liquid phase. Remarkably, the lattice order is restored during field heating and demonstrates a thermal hysteresis. This behavior is explained by the skyrmion energy landscape and demonstrates the potential to control the lattice order in 2D phase transitions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗