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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Synthesis of Samarium Nitride Thin Films on Magnesium Oxide (001) Substrates Using Molecular Beam Epitaxy

Rare-earth nitrides are an exciting family of materials with a wide variety of properties desirable for new physics and applications in spintronics and superconducting devices. Among them, samarium nitride is an interesting compound reported to have ferromagnetic behavior coupled with the potential existence of p-wave superconductivity. Synthesis of high-quality thin films is essential in order to manifest these behaviors and understand the impact that vacancies, structural distortions, and doping can have on these properties. In this study, we report the synthesis of samarium nitride monocrystalline thin films on magnesium oxide (001) substrates with a chromium nitride capping layer using molecular beam epitaxy (MBE). We observed a high-quality monocrystalline SmN film with matching orientation to the substrate, then optimized the growth temperature. Despite the initial 2 nm of growth showing formation of a potential samarium oxide layer, the subsequent layers showed high-quality SmN, with semiconducting behavior revealed by an increase in resistivity with decreasing temperature. These promising results highlight the importance of studying diverse heteroepitaxial schemes and open the door for integration of rare-earth nitrides and transition metal nitrides for future spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3

The magnetic properties in two-dimensional van der Waals materials depend sensitively on structure. CrI 3 , as an example, has been recently demonstrated to exhibit distinct magnetic properties depending on the layer thickness and stacking order. Bulk CrI 3 is ferromagnetic (FM) with a Curie temperature of 61 K and a rhombohedral layer stacking, whereas few-layer CrI 3 has a layered antiferromagnetic (AFM) phase with a lower ordering temperature of 45 K and a monoclinic stacking. In this work, we use cryogenic magnetic force microscopy to investigate CrI 3 flakes in the intermediate thickness range (25–200 nm) and find that the two types of magnetic orders, hence the stacking orders, can coexist in the same flake with a layer of ~13 nm at each surface being in the layered AFM phase similar to few-layer CrI 3 and the rest in the bulk FM phase. The switching of the bulk moment proceeds through a remnant state with nearly compensated magnetic moment along the c-axis, indicating formation of c-axis domains allowed by a weak interlayer coupling strength in the rhombohedral phase. Our research provides a comprehensive picture on the magnetism in CrI 3 and point to the possibility of engineering magnetic heterostructures within the same material.

2D magnets↗
DOE OSTI2025

Spin-Forbidden Excitations in the Magneto-optical Spectra of CrI 3 Tuned by Covalency

Spin-forbidden (Δ⁢𝑆 ≠0) multiplet excitations and their coupling to magnetic properties are of increasing importance for magneto-optical studies of correlated materials. Nonetheless, the mechanisms for optically brightening these transitions and their generality remain poorly understood. Here, we report magnetic circular dichroism (MCD) spectroscopy on the van der Waals ferromagnet (FM) CrI 3 . Previously unreported spin-forbidden (Δ⁢𝑆 =1) 4 𝐴 2⁢g → 2 𝐸 g / 2 𝑇 1⁢g Cr 3+ 𝑑⁢𝑑 excitations are observed near the ligand-to-metal charge-transfer excitation threshold. The assignment of these excitations and their Cr 3+ multiplet character is established through complementary Cr 𝐿 3 -edge resonant inelastic x-ray scattering measurements along with charge-transfer multiplet calculations and chemical trends in the chromium trihalide series (Cr⁢𝑋 3 , 𝑋 = Cl, Br, I). We utilize the high sensitivity of MCD spectroscopy to study the thickness-dependent optical response. The spin-forbidden excitations remain robust down to the monolayer limit, and we observe a significant magnetic-field dependence of the MCD spectrum across the antiferromagnetic-to-FM transition in few-layer samples. We preliminarily attribute this behavior to changes in the metal-ligand covalency with magnetic state. Our results clarify the magneto-optical response of CrI 3 and identify covalency as a central mechanism for the brightening and field tunability of spin-forbidden multiplet excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Prediction of room-temperature half-metallicity in layered halide double perovskites

Half-metallic ferromagnets (HMFs) that possess intriguing physical properties with completely spin-polarized current are key candidates for high-efficiency spintronic devices. However, HMFs that could simultaneously have high Curie temperature ( T c ), wide half-metallic gap (Δ HM ), and large bulk magnetocrystalline anisotropy energy (MAE) are very rare, which significantly restrict their room-temperature (RT) applications. In this article, through materials screening in layered halide double perovskites (LHDPs), we have theoretically identified that Cs 4 FePb 2 Cl 12 , which has good crystallographic, dynamic and thermal stabilities, possesses an intrinsic half-metallic ground-state with a high T c ~ 450 K. Interestingly, the long-range ferromagnetic ordering in bulk Cs 4 FePb 2 Cl 12 is contributed by the strong super-superexchange interactions between the neighboring Fe d orbitals mediated by different anionic Cl p orbitals. The high T c of layered Cs 4 FePb 2 Cl 12 can be well maintained even in the monolayer limitation, i.e., T c ~ 370 K for Cs 4 FePb 2 Cl 12 monolayer, which is critical for nanoscale device applications. Moreover, both bulk and monolayer Cs 4 FePb 2 Cl 12 can exhibit wide Δ HM ~ 0.55 eV and large MAE >320 μeV/Fe, comparable to that of the best HMFs reported in the literature. Our findings can significantly extend the potentials of LHDPs for high-temperature spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Observation of the inverse spin Hall effect in ZnO thin films: An all-electrical approach to spin injection and detection

The inverse spin Hall effect (ISHE) is a newly discovered, quantum mechanical phenomenon where an applied spin current results in the generation of an electrical voltage in the transverse direction. It is anticipated that the ISHE can provide a more simple way of measuring spin currents in spintronic devices. The ISHE was first observed in noble metals that exhibit strong spin-orbit coupling. However, recently, the ISHE has been detected in conventional semiconductors (such as Si and Ge), which possess weak spin-orbit coupling. This suggests that large-spin orbit coupling is not a requirement for observing the ISHE. In this paper, we are reporting the observation of the ISHE in an alternative semiconductor material, zinc oxide (ZnO) using all-electrical means. In our study, we found that when a spin-polarized current is injected into the ZnO film from a NiFe ferromagnetic injector via an MgO tunnel barrier layer, a voltage transverse to both the direction of the current as well as its spin-polarization is generated in the ZnO layer. The polarity of this voltage signal was found to flip on reversing the direction of the injected current as well as on reversing the polarization of the current, consistent with the predictions of the ISHE process. Through careful analysis of the ISHE data, we determined a spin-Hall angle of approximately 1.651 × 10{sup −2} for ZnO, which is two orders of magnitude higher than that of silicon. Observation of a detectable room-temperature ISHE signal in ZnO via electrical injection and detection is a groundbreaking step that opens a path towards achieving transparent spin detectors for next-generation spintronic device technology.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Single pair of Weyl fermions in the half-metallic semimetal EuC d 2 A s 2 [A Single Pair of Weyl Fermions in Half-metallic EuCd 2 As 2 Semimetal]

Materials with the ideal case of a single pair of Weyl points (WPs) are highly desirable for elucidating the unique properties of Weyl fermions. EuCd 2 As 2 is an antiferromagnetic topological insulator or Dirac semimetal depending on the different magnetic configurations. Using first-principles band-structure calculations, we show that inducing ferromagnetism in EuCd 2 As 2 can generate a single pair of WPs from splitting the single pair of antiferromagnetic Dirac points due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that a single pair of WPs is obtained in EuCd 2 As 2 because the Dirac points are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at Γ while the other pair survives. As a result, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Exploring possible ferromagnetism of the LaAl O 3 / SrTi O 3 interface

We report on extensive investigations of magnetism at the n -type LaAl O 3 / SrTi O 3 interface performed by utilizing a spectrum of local and integrative analytical techniques: Scanning superconducting quantum interference device microscopy, polar Kerr magnetometry, ferromagnetic resonance, and magnetic torque magnetometry. The samples originated from the same wafers. For nominally fully oxidized samples, we find that the mere presence of the conducting interface does not induce magnetism to values exceeding already present magnetic signals originating from the substrates, irrespective of the measurement technique. With the controlled introduction of oxygen vacancies, however, the different analytical techniques with their inherently different sensitivities and potential interactions with possible magnetic moments in the samples yield different results concerning the existence of magnetism. These unexpected differences obtained with the various measurement techniques are a possible source of the disagreement in the literature about the existence of ferromagnetism at LaAl O 3 / SrTi O 3 interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Manipulation of Magnetic Skyrmion in a 2D van der Waals Heterostructure via Both Electric and Magnetic Fields

As a promising candidate for the much-desired low power consumption spintronic devices, 2D magnetic van der Waals material also provides a versatile platform for the design and control of topological spin textures. In this work on WTe 2 /CrCl 3 bilayer van der Waals heterostructures, a complete Néel-type skyrmion–bimeron–ferromagnet phase transition is demonstrated, accompanied by the evolution of the topological number. This cyclic transition, mediated by a perpendicular magnetic field, is largely driven by the competition between the out-of-plane magnetocrystalline anisotropy and magnetic dipole–dipole interaction. In the presence of a driving current, the Néel-type skyrmion gains a higher velocity yet larger skyrmion Hall angle, in comparison to the bimeron. By incorporating a ferroelectric CuInP 2 S 6 monolayer as a substrate, writing and erasing of skyrmions may be regulated using a ferroelectric polarization. Here this work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.

multiferroic↗
DOE OSTI2014

Half-metallicity and tetragonal distortion in semi-Heusler alloy FeCrSe

Full-potential linearized augmented plane wave methods are carried out to investigate the electronic structures and magnetic properties in semi-Heusler alloy FeCrSe. Results show that FeCrSe is half-metallic ferromagnet with the half-metallic gap 0.31 eV at equilibrium lattice constant. Calculated total magnetic moment of 2.00μ{sub B} per formula unit follows the Slater-Pauling rule quite well. Two kinds of structural changes are used to investigate the sensitivity of half-metallicity. It is found that the half-metallicity can be retained when lattice constant is changed by −4.56% to 3.52%, and the results of tetragonal distortion indicate the half-metallicity can be kept at the range of c/a ratio from 0.85 to 1.20. The Curie temperature, cohesive energy, and heat of formations of FeCrSe are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Brownmillerite Ca 2 Co 2 O 5 : Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions

We synthesized Ca 2 Co 2 O 5 in the brownmillerite form using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4×0.8×0.5 mm 3 were obtained. At room temperature, Ca 2 Co 2 O 5 crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a=5.28960(10) Å, b=14.9240(2) Å, and c=10.9547(2) Å. Furthermore, with decreasing temperature, it undergoes re-entrant sequence of first-order structural phase transitions (Pcmb→ P2/c11→ P121/m1→ Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca 2 Co 2 O 5 occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, Ca 2 Co 2 O 5 shows a weak ferromagnetic component directed primarily along the b axis, and it also exhibits thermal and magnetic history dependence in magnetization.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two-dimensional ferromagnetism in few-layer van der Waals crystals: Renormalized spin-wave theory and calculations

Dimensionality shapes the behavior of magnetism. Recent experiments on quasi two-dimensional (2D) van der Waals crystals have demonstrated unusual ferromagnetic orders and phase transitions distinct from their bulk counterparts. In this work, we present a method using renormalized spin-wave theory to study the ferromagnetic behavior in Heisenberg-type 2D ferromagnets, and apply our method to few-layer Cr 2 Ge 2 Te 6 crystals. Here we present the details of the method, which includes a self-consistent updating scheme of the spin-wave spectra at the Hartree-Fock level. The theoretical results obtained quantitatively explain the experimental measurements, showing strong dimensionality effects and sensitive external magnetic-field dependence which leads to easily tunable magnetism in this class of 2D materials. This method can be generalized to study various magnetic phenomena in other systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The low-temperature heat capacity and thermodynamic properties of greigite (Fe 3 S 4 )

Heat capacity measurements provide important insights into the energetic, thermodynamic, and magnetic properties of materials. Herein we report the heat capacity of greigite (Fe 3 S 4 ) from 1.8 to 300 K. Greigite is a magnetic spinel mineral and through a ferromagnetic magnon term, C fsw = B fsw T 3/2 , ferrimagnetic ordering is observed in the low-temperature heat capacity. Using a set of theoretical fits of the experimental data, we calculate the thermodynamic functions, including the standard entropy ($Δ_{0}^{T}$S m °). Greigite is important in iron sulfide formation and reaction pathways in environmental, ore-forming, and technological settings and previous work has measured enthalpies $ΔH_{r}°$ of formation and decomposition to neighboring phases. In this work, the stability of greigite relative to the elements is demonstrated with a negative Gibbs energy ($ΔG_{r}°$) of formation and the stability relative to decomposition products of pyrrhotite (FeS 1.092 ) and pyrite (FeS 2 ) is demonstrated with a positive Gibbs energy ($ΔG_{r}°$) of decomposition. Values of the standard thermodynamic functions C p,m °, $Δ_{0}^{T}$S m °, $Δ_{0}^{T}$H m °, and Φ m ° are tabulated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Investigations on the temperature dependent electrical and magnetic properties of NiTiO{sub 3} by molten salt synthesis

Highlights: ► NiTiO{sub 3} particles were synthesized by simple molten salt technique. ► The single phase compound formation was confirmed through XRD analysis. ► The detailed impedance properties of NiTiO{sub 3} were studied at elevated temperature. ► The conductance spectra were fitted using Jonscher Power Law. ► Low temperature magnetization behaviour was explained by core shell model. - Abstract: Submicron-sized NiTiO{sub 3} particles were successfully synthesized by simple molten salt method. The phase purity and crystal structure was confirmed through X-ray diffraction. The characteristics bands of Ni-O and Ti-O were identified using Fourier Transform Infra-red spectrum. The SEM and TEM images shows that the particles are lying in the range of 150–300 nm and selected area electron diffraction pattern (SAED) infers the single crystalline nature of NiTiO{sub 3}. The impedance spectra elucidate the grain interior conducting mechanism and semi-conducting nature of NiTiO{sub 3} at elevated temperatures. The frequency dependent conductance spectra were found to obey Jonscher's power law and a maximum value of conductivity observed was 2 × 10{sup −5} S cm{sup −1} at 520 °C. The magnetic susceptibility measurement infers the antiferromagnetic behavior with the Neel temperature (T{sub N}) of 14.9 K. The increase in coercivity below T{sub N} reveals the coexistence of antiferromagnetic with superparamagnetic behavior. B–H curve represents the first order phase transition from antiferromagnetic to ferromagnetic transition.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Room temperature multiferroic properties of (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film

Graphical abstract: - Highlights: • Chemical solution deposition of (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}–NiFe{sub 2}O{sub 4} double layered thin film. • Studies on structural, electrical and multiferroic properties. • NiFe{sub 2}O{sub 4} acts as both resistive buffer layer and magnetic source. - Abstract: (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film was prepared on a Pt(111)/Ti/SiO{sub 2}/Si(100) substrate by a chemical solution deposition method. X-ray diffraction and Raman scattering spectroscopy studies confirmed the formation of the distorted rhombohedral perovskite and the inverse spinel cubic structures for the (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film. The (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film exhibited well saturated ferromagnetic (2 M{sub r} of 18.1 emu/cm{sup 3} and 2H{sub c} of 0.32 kOe at 20 kOe) and ferroelectric (2P{sub r} of 60 μC/cm{sup 2} and 2E{sub c} of 813 kV/cm at 866 kV/cm) hysteresis loops with low order of leakage current density (4.5 × 10{sup −6} A/cm{sup 2} at an applied electric field of 100 kV/cm), which suggest the ferroelectric and ferromagnetic multi-layers applications in real devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Tunable high Néel temperature and large anomalous Hall response in antiferromagnetic Weyl semimetal Mn 3 Sn 1− x Ga x thin films

Antiferromagnetic Weyl semimetals based on Mn 3 X(X = Ge, Sn, Ga) kagome compounds exhibit the same ferromagnetic-like responses, including anomalous Hall, Nernst, and magneto-optical effects, as recently discussed for altermagnets. Driven by the Berry curvature due to Weyl fermions, these materials show a disproportionately large magnitude of electromagnetic effects even in the absence of large magnetization. For applications it is crucial to realize these responses in a wide range of temperatures both below and above 300 K. While stoichiometric Mn 3 X materials do not offer optimal performance, we show that Mn 3 Sn 1−x Ga x sputtered films with a variable composition offers a tunable Néel temperature, T N ≈ 425 ± 6–500 ± 15 K, which is crucial for device applications, together with a large tunable anomalous Hall effect. Our thin film growth method enables continuous and precise control over the film composition between x = 0 and x = 1. Through a detailed magnetization and Hall transport, we establish the magnetic phase diagram for the hexagonal Mn 3 Sn 1−x Ga x . Our results reveal an enhanced T N and antichiral magnetic phase in Ga-doped Mn 3 Sn and an enhanced anomalous Hall magnitude in Sn-doped Mn 3 Ga compared to their stoichiometric undoped forms. Our work demonstrates a route to optimize the technologically relevant antiferromagnets for various applications.

Magnetic properties and materials↗
DOE OSTI2018

Giant negative thermal expansion in Fe-Mn-Ga magnetic shape memory alloys

Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (Delta T) and large negative linear coefficients of thermal expansion (alpha(l)) have been achieved in Fe 44-x Mn 28 Ga 28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE Delta T of 81 K and alpha(l =) -50.2 x 10(-6) K-1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects for use as advanced NTE materials.

36 MATERIALS SCIENCE↗