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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Layer-Dependent Magnetism and Spin Fluctuations in Atomically Thin van der Waals Magnet CrPS 4

van der Waals (vdW) magnets, an emerging family of two-dimensional (2D) materials, have received tremendous attention due to their rich fundamental physics and significant potential for cutting-edge technological applications. In contrast to the conventional bulk counterparts, vdW magnets exhibit significant tunability of local material properties, such as stacking engineered interlayer coupling and layer-number dependent magnetic and electronic interactions, which promise to deliver previously unavailable merits to develop multifunctional microelectronic devices. As a further ingredient of this emerging topic, here we report nanoscale quantum sensing and imaging of atomically thin vdW magnet chromium thiophosphate CrPS 4 , revealing its characteristic layer-dependent 2D static magnetism and dynamic spin fluctuations. We also show a large tunneling magnetoresistance in CrPS 4 -based spin filter vdW heterostructures. Here, the excellent material stability, robust strategy against environmental degradation, in combination with tailored magnetic properties highlight the potential of CrPS 4 in developing state-of-the-art 2D spintronic devices for next-generation information technologies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

The Hard Ferromagnetism in FePS 3 Induced by Non‐Magnetic Molecular Intercalation

Abstract Manipulating the magnetic ground states of 2D magnets is a focal point of recent research efforts. Various methods have demonstrated efficacy in modulating the magnetic properties inherent to van der Waals (vdW) magnetic systems. Herein, the emergence of robust anisotropic ferromagnetism within antiferromagnetic FePS 3 is unveiled via intercalation with non‐magnetic pyridinium ions. A one‐step ion exchange reaction facilitates the formation of energetically favorable B‐phase and metastable P‐phase. Notably, both B‐ and P‐phases manifest hard ferromagnetic behavior, featuring substantial unsaturated coercive fields (>7 T) and high Curie temperatures (72–87 K). First‐principles calculations elucidate the pivotal role of electron transfer from pyridinium ions to FePS 3 in engineering magnetic exchange interactions. Calculated effective spin Hamiltonian corroborates the observed hard ferromagnetism in intercalated FePS 3 . This study offers crucial insights into hard magnetism in intercalated vdW materials, thereby presenting promising avenues for 2D vdW magnet‐based magnetic devices.

Ou, Yunbo↗
DOE OSTI2025

Manipulation of Emergent Collective Excitations via Composition Control in Mixed MPX 3 Correlated 2D Antiferromagnets

Transition metal (i.e., Mn, Fe, Cr) and chalcogen (Se) substituents are introduced into single-crystalline NiPS 3 , and the evolution of the two emergent quasi-particle excitations characteristic to the XXZ correlated antiferromagnetism of NiPS 3 (i.e., spin orbit entangled exciton (SOX) and two-magnon scattering (2M )) are investigated as functions of substituent concentration through comprehensive room- and low-temperature photoluminescence (PL) and Raman spectroscopy studies. These findings are further correlated with the magnetic properties of the same set of compounds reported in prior studies. The work revealed that the SOX emission intensities and linewidths are mainly controlled by the magnetic anisotropy and spin orientations, and are strongly suppressed by the introduction of substituents. The suppression depends on the type of substituent, with Fe affecting the SOX emission more than Mn and Cr. The 2 M scattering is linked to short-range correlations and exhibits greater resiliency against metal atom substitution. While the 2M peak at low temperature gets suppressed and red-shifted in frequency with increasing concentrations of all the substituents, Fe induces the weakest suppression compared to all other substituents. Altogether, these findings revealed the introduction of substituents as a powerful route to control the emergent collective excitations in NiPS 3 and mixed-MPX 3 materials.

2D magnet↗
DOE OSTI2025

Polymer-assisted transfer of MBE-grown van der Waals materials

Molecular beam epitaxy (MBE) has been used to create high-quality, large-scale two-dimensional van der Waals (2D vdW) materials. However, due to the strong adhesion between the substrate and deposited materials, the peel-off and dry transfer of MBE-grown vdW films onto other substrates has been challenging. This limits the study and use of MBE films for heterogeneous integration including stacked and twisted heterostructures. In this work, we develop a polymer-assisted dry transfer method and successfully perform full-film transfer of various MBE-grown 2D vdW materials including transition metal dichalcogenides (TMD), topological insulators (TI) and 2D magnets. In particular, we transfer air-sensitive 2D magnets, characterize their magnetic properties, and compare them with as-grown materials. The results show that the transfer technique does not degrade the magnetic properties, with the Curie temperature and hysteresis loops exhibiting similar behaviors after the transfer. Our results enable further development of heterogeneous integration of 2D vdW materials based on MBE growth.

Li, Ziling [The Ohio State University]↗
DOE OSTI2025

Re-investigation of moment direction in a Kitaev material α -RuCl 3

Abstract We report x-ray diffraction and resonant elastic x-ray scattering studies on two α -RuCl 3 crystals with distinct magnetic transition temperatures: T N = 7.3 K and 6.5 K. We find that the sample with T N = 6.5 K exhibits a high degree of stacking faults and structural twinning at low temperature, whereas the T N = 7.3 K sample primarily comprises a single domain of R 3 ¯ . Notwithstanding, both samples exhibit an identical zigzag magnetic structure, with magnetic moments pointing away from the honeycomb plane by α = 31 ( 2 ) ∘ . We argue that the identical ordered moment directions in these samples suggest that the intralayer magnetic interactions remain mostly unchanged regardless of T N .

2D magnetism↗
DOE OSTI2022

Exchange bias and interface-related effects in two-dimensional van der Waals magnetic heterostructures: Open questions and perspectives

The exchange bias (EB) effect is known as a fundamentally and technologically important magnetic property of a magnetic bilayer film. It is manifested as a horizontal shift in a magnetic hysteresis loop of a film subject to cooling in the presence of a magnetic field. The EB effect in van der Waals (vdW) heterostructures offers a novel approach for tuning the magnetic properties of the newly discovered single-layer magnets, as well as adds a new impetus to magnetic vdW heterostructures. Indeed, intriguing EB effects have recently been reported in a variety of low-dimensional vdW magnetic systems ranging from a weakly interlayer-coupled vdW magnet (e.g., Fe 3 GeTe 2 ) to a bilayer composed of two different magnetic vdW materials (e.g., Fe 3 GeTe 2 /CrCl 3 , Fe 3 GeTe 2 /FePS 3 , Fe 3 GeTe 2 /MnPS 3 , Fe 3 GeTe 2 /CrSe, Fe 3 GeTe 2 /CrOCl, Fe 3 GeTe 2 /CoPc, Fe 5 GeTe 2 /FePS 3 ), to bilayers of two different vdW defective magnets (e.g., VSe 2 /MoS 2 ), or to metallic ferromagnet/vdW defective magnet interfaces (e.g., Fe/MoS 2 ). Despite their huge potential in spintronic device applications, the physical origins of the observed EB effects have remained elusive to researchers. Here, we present here a critical review of the EB effect and associated phenomena such as magnetic proximity (MP) in various vdW heterostructure systems and propose approaches to addressing some of the emerging fundamental questions.

2D magnets↗
DOE OSTI2025

Magnetic Proximity Effects in Iron Germanium Telluride/Platinum Heterostructures

Van der Waals (vdW) magnetic materials have attracted considerable attention for use in spintronic devices such as those controlled by spin–orbit torque (SOT). Such SOT-driven devices are typically fabricated by bringing a vdW magnet in proximity to a spin–charge conversion layer to achieve current-driven magnetization switching. Here, we show that such structures fabricated with iron germanium telluride (FGT) and platinum can exhibit emergent magnetic properties, which we attribute to magnetic proximity effects at the FGT/Pt interface. These changes manifest as increased perpendicular magnetic anisotropy and the emergence of additional magnetization reversal steps as probed by magneto-transport, with the most significant changes appearing in thinner flakes. The behavior was found to be robust and consistently appeared in samples made with crystals from different vendors. In conclusion, our results demonstrate the potential for engineering vdW spintronic systems through magnetic proximity effects.

2D magnet↗
DOE OSTI2022

Revealing room temperature ferromagnetism in exfoliated Fe 5 GeTe 2 flakes with quantum magnetic imaging

Van der Waals (vdW) material Fe 5 GeTe 2 , with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe 5 GeTe 2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe 5 GeTe 2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. Here, we investigate the magnetic properties of exfoliated thin flakes of vdW magnet Fe 5 GeTe 2 via quantum magnetic imaging technique based on nitrogen vacancy centers in diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe 5 GeTe 2 thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities is consistent with previously reported perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at ≈300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe 5 GeTe 2 . This work also demonstrates that the imaging technique enables rapid screening of multiple flakes simultaneously as well as time-resolved imaging for monitoring time-dependent magnetic behaviors, thereby paving the way towards high throughput characterization of potential two-dimensional (2D) magnets near room temperature and providing critical insights into the evolution of domain behaviors in 2D magnets due to degradation.

2D magnet↗
DOE OSTI2025

Charge Density Wave and Ferromagnetism in Intercalated CrSBr

In materials with 1D electronic bands, electron–electron interactions can produce intriguing quantum phenomena, including spin-charge separation and charge density waves (CDW). Most of these systems, however, are non-magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, in this study, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields Li 0.17(2) (tetrahydrofuran) 0.26(3) CrSBr, which possesses an electronically driven quasi-1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin-polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provide a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

2D magnets↗
DOE OSTI2025

Characterizing sample degradation from synchrotron based X-ray measurements of ultra-thin exfoliated flakes

It is undeniable that novel 2D devices and heterostructures will have a lasting impact on the advancement of future technologies. However, the inherent instability of many exfoliated van der Waals (vdW) materials is a well-known hurdle yet to be overcome. Thus, the sustained interest in exfoliated vdW materials underscores the importance of understanding the mechanisms of sample degradation to establish proactive protective measures. Here, the impact of prolonged synchrotron-based X-ray beam exposure on exfoliated flakes of two contemporary vdW materials, NiPS 3 and α-RuCl 3 , is explored using resonant inelastic X-ray scattering (RIXS) and total fluorescence yield X-ray absorption spectroscopy (XAS). In NiPS 3 , the resulting RIXS and XAS spectra show a suppression, then vanishing, of NiS 6 multiplet excitations coupled with an upward shift of the peak energy of the XAS as a function of X-ray dose. In α-RuCl 3 , the signs of beam damage from the RIXS spectra are less evident. However, the post-experiment characterization of both materials using Raman spectroscopy exhibits signals of an amorphous and disordered system compared to pristine flakes; in addition, energy-dispersive X-ray spectroscopy of NiPS 3 shows evidence of ligand vacancies. As synchrotron radiation is fast becoming a required probe to study 2D vdW materials, these findings lay the groundwork for the development of future protective measures for synchrotron-based prolonged X-ray beam exposure, as well as for X-ray free electron laser.

2D magnetism↗
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 OSTI2021

Thickness and Spin Dependence of Raman Modes in Magnetic Layered Fe 3 GeTe 2

2D layered Fe 3 GeTe 2 has attracted increasing attention due to its high magnetic ordering temperature and novel physical properties. Lattice dynamics is a fundamental property of Fe 3 GeTe 2 , and its relationships with the number of layers and interlayer spin ordering have not yet been explored in depth. Here, by first-principles density functional theory calculations, the phonon vibrations and Raman intensities of Fe 3 GeTe 2 are systematically studied from the bulk to monolayer structures. Furthermore, the spin-phonon coupling effect is investigated by considering different interlayer magnetic orderings: ferromagnetic and antiferromagnetic. It is found that the frequencies of Raman modes in Fe 3 GeTe 2 exhibit considerable dependence on the layer number and spin ordering. In this work, the results not only reveal the notable spin-phonon interactions in Fe 3 GeTe 2 , but also demonstrate that Raman modes can be utilized for characterizing the sample thickness and interlayer spin ordering in this 2D magnet.

2D magnets↗
DOE OSTI2025

Effect of Ni deposition on CrI 3 monolayer: First-principles study

In our recent experimental work, we demonstrated that Ni thin film deposition can be used to tune the surface magnetic states in bulk CrI 3 . Here, we present the results of a computational study of Ni deposition on a CrI 3 monolayer. In particular, we compare structural, electronic, and magnetic properties of three different systems: a pristine CrI 3 monolayer, a CrI 3 monolayer with partial Ni coverage, and a CrI 3 monolayer with full Ni coverage. The corresponding cells used in our calculations are Cr 2 I 6 , Cr 2 I 6 Ni 1 , and Cr 2 I 6 Ni 3 . In Cr 2 I 6 Ni 1 , the Ni atom is positioned over the hollow site, which corresponds to the lowest energy configuration. In Cr 2 I 6 Ni 3 , the three Ni atoms are positioned over the single hollow site, and over the two Cr sites. Our calculations indicate that the partial coverage of CrI 3 monolayer with Ni retains the semiconducting nature of the parent compound, but results in a strong reduction of the energy band gap. At the same time, the full coverage of CrI 3 monolayer with Ni results in a metallic transition, with a highly spin-polarized (possibly half-metallic) electronic structure. The magnetic alignment of the CrI 3 monolayer with the deposited Ni is ferrimagnetic, due to the anti-aligned magnetic moments of Cr and Ni atoms. While the partial coverage of the CrI 3 monolayer with Ni has a moderate impact on the lattice parameter of the parent compound, full coverage by Ni results in a significant increase of the lattice constant. For both partial and full coverages, the lowest energy configuration corresponds to the Ni atoms being approximately aligned with the out-of-plane coordinate of the surface iodine layer. In addition, in the full coverage mode, the Cr layer exhibits a significant shift away from the Ni atoms, effectively “detaching” the Ni / I atomic layer from the rest of the cell.

2D magnetism↗
DOE OSTI2025

Magnetism at the interface of MoSe 2 /V 2 O 5 heterostructures

In this study, we report the existence of magnetism in a heterostructure made up of an MoSe 2 transition metal dichalcogenide monolayer and a V 2 O 5 substrate. Using density functional theory simulations, we find that ferromagnetic ordering can be found in the MoSe 2 /V 2 O 5 heterostructure even though the individual components are nonmagnetic. The role of van der Waals interactions is examined.

2D magnet↗
DOE OSTI2022

Anomalous Second Harmonic Generation from Atomically Thin MnBi 2 Te 4

MnBi 2 Te 4 is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic (AFM) interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature dependent SHG measurements on MnBi 2 Te 4 flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi 2 Te 4 is unexpectedly unrelated to both magnetic state and layer number. As a result, we suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi 2 Te 4 -vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry breaking in van der Waals centrosymmetric magnets.

2D magnets↗
DOE OSTI2021

Covalent 2D Cr 2 Te 3 ferromagnet

To broaden the scope of van der Waals 2D magnets, we report the synthesis and magnetism of covalent 2D magnetic Cr 2 Te 3 with a thickness down to one-unit-cell. The 2D Cr 2 Te 3 crystals exhibit robust ferromagnetism with a Curie temperature of 180 K, a large perpendicular anisotropy of 7 × 10 5 J m -3 , and a high coercivity of ~4.6 kG at 20 K. First principles calculations further show a transition from canted to collinear ferromagnetism, a transition from perpendicular to in-plane anisotropy, and emergent half-metallic behavior in atomically-thin Cr 2 Te 3 , suggesting its potential application for injecting carriers with high spin polarization into spintronic devices.

2D magnets↗
DOE OSTI2024

Understanding and tuning magnetism in van der Waals-type metal thiophosphates

Over the past two decades, significant progress in two-dimensional (2D) materials has invigorated research in condensed matter and material physics in low dimensions. While traditionally studied in three-dimensional systems, magnetism has now been extended to the 2D realm. Recent breakthroughs in 2D magnetism have attracted substantial interest from the scientific community, owing to the stable magnetic order achievable in atomically thin layers of the van der Waals (vdW)-type layered magnetic materials. These advances offer an exciting platform for investigating related phenomena in low dimensions and hold promise for spintronic applications. Consequently, vdW magnetic materials with tunable magnetism have attracted significant attention. Specifically, antiferromagnetic metal thiophosphates MPX 3 (M = transition metal, P = phosphorus, X = chalcogen) have been investigated extensively. These materials exhibit long-range magnetic order spanning from bulk to the 2D limit. The magnetism in MPX 3 arises from localized moments associated with transition metal ions, making it tunable via substitutions and intercalations. In this review, we focus on such tuning by providing a comprehensive summary of various metal- and chalcogen-substitution and intercalation studies, along with the mechanism of magnetism modulation, and a perspective on the development of this emergent material family.

42 ENGINEERING↗