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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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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Search for the Heisenberg spin glass on rewired square lattices with antiferromagnetic interaction

Spin glass (SG) is a typical magnetic system with frozen random spin orientation at low temperatures. The system exhibits rich physical properties, such as infinite number of ground states, memory effect, and aging phenomena. There are two main ingredients considered to be pivotal for the existence of SG behavior, namely, frustration and randomness. For the canonical SG system, frustration is led by the presence of competing interaction between ferromagnetic (FM) and antiferromagnetic (AF) couplings. Previously, Bartolozzi et al. [Phys. Rev. B73, 224419 (2006)], reported the SG properties of the AF Ising spins on scale free network (SFN). It is a new type of SG, different from the canonical one which requires the presence of both FM and AF couplings. In this new system, frustration is purely caused by the topological factor and its randomness is related to the irregular connectvity. Recently, Surungan et. al. [Journal of Physics: Conference Series, 640, 012001 (2015)] reported SG bahavior of AF Heisenberg model on SFN. We further investigate this type of system by studying an AF Heisenberg model on rewired square lattices. We used Replica Exchange algorithm of Monte Carlo Method and calculated the SG order parameter to search for the existence of SG phase.

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 OSTI2016

Thermal and temporal evolution of microstructure in polycrystalline ZnO

Tug between electronics and spintronics has opened up new area of research named as dilute magnetic semiconductors (DMS), ZnO is one of the most reliable candidates for spintronic devices and DMS. Since, pure and transition metal doped polycrystalline ZnO shows room temperature ferromagnetism, therefore it is very important to gain insight into its microstructure (MS) evolution. We report thermal evolution of MS of pure ZnO on sintering it at 200 °C, 400 °C, 600 °C, and 800 °C in ambient atmosphere for two hours. Temporal evolution at fixed temperature was analyzed using mean field model of internal energy and entropy. Grain size of ZnO MS were analyzed using integral breadth method of X-ray diffraction (XRD) lines using Voigt profile fit,. XRD line corresponding to [101] plane shifts from 36.17° to 36.28° whereas grain size increases from 67.5 nm to 93.7 nm with increase of temperature from 23°C to 800°C respectively. Grain growth with increase of temperature show Arrhenius type behavior with activation energy of 30.77 kJ-mol{sup −1} and temporal growth shows diffusive behavior with exponent 0.5.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

No Signature of Room Temperature Ferromagnetism in Fe-Doped ITO Thin Films

Iron (Fe)-doped indium tin oxide thin films (In{sub 0.95 − x}Fe{sub x}Sn{sub 0.05}){sub 2}O{sub 3} at x = 0.00, 0.05, 0.10, and 0.15 were deposited onto glass substrates using an electron beam evaporation technique. The coated thin films were characterized by X-ray diffractometer (XRD), UV-VIS-NIR spectrophotometer, photoluminescence spectrophotometer, scanning electron microscope, and vibrating sample magnetometer to study their structural, optical, morphological, and magnetic properties, respectively. The crystallite size of the films increased from 18 to 30 nm with Fe concentration (x). The morphology showed the formation of nanorods. The bandgap of the films decreased from 3.94 to 3.81 eV. The X-ray photoelectron spectroscopy (XPS) studies confirm the presence of Fe ions in two different oxidation states (Fe{sup 2+}, Fe{sup 3+}). The magnetic studies revealed the paramagnetic behavior of the films at room temperature and at low temperature (5 K). The effective magnetic moment (μ{sub eff}) of Fe-doped indium tin oxide thin films was calculated using Langevin theory of paramagnetism extending to Curie’s law. The effective magnetic moment (μ{sub eff}) was increased on increasing the dopant concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Metallic ferromagnetic films with magnetic damping under 1.4 × 10 -3

Low-damping magnetic materials have been widely used in microwave and spintronic applications because of their low energy loss and high sensitivity. While the Gilbert damping constant can reach 10 -4 to 10 -5 in some insulating ferromagnets, metallic ferromagnets generally have larger damping due to magnon scattering by conduction electrons. Meanwhile, low-damping metallic ferromagnets are desired for charge-based spintronic devices. In this article, we report the growth of Co 25 Fe 75 epitaxial films with excellent crystalline quality evident by the clear Laue oscillations and exceptionally narrow rocking curve in the X-ray diffraction scans as well as from scanning transmission electron microscopy. Remarkably, the Co 25 Fe 75 epitaxial films exhibit a damping constant <1.4 × 10 -3 , which is comparable to the values for some high-quality Y 3 Fe 5 O 12 films. This record low damping for metallic ferromagnets offers new opportunities for charge-based applications such as spin-transfer-torque-induced switching and magnetic oscillations.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2014

Magnetoelectric coupling of multiferroic chromium doped barium titanate thin film probed by magneto-impedance spectroscopy

Thin film of BaTiO{sub 3} doped with 0.1 at. % Cr (Cr:BTO) has been prepared by pulsed laser deposition technique. Film was deposited on Pt/SrTiO{sub 3} substrate at 500 °C in 50 mTorr Oxygen gas pressure using KrF (298 nm) laser. Polycrystalline growth of single phase Cr:BTO thin film has been confirmed by grazing angle X-ray diffraction. Cr:BTO film exhibited remnant polarization 6.4 μC/cm{sup 2} and 0.79 MV/cm coercivity. Magnetization measurement of Cr:BTO film showed magnetic moment 12 emu/cc. Formation of weakly magnetic domains has been captured by magnetic force microscopy. Theoretical impedance equation fitted to experimental data in Cole-Cole plot for thin film in presence of transverse magnetic field resolved the increase in grain capacitance from 4.58 × 10{sup −12} to 5.4 × 10{sup −11} F. Film exhibited high value 137 mV/cm-Oe magneto-electric (ME) coupling coefficient at room temperature. The high value of ME coupling obtained can reduce the typical processing steps involved in multilayer deposition to obtain multiferrocity in thin film. Barium titanate being best ferroelectric material has been tailored to be multiferroic by non ferromagnetic element, Cr, doping in thin film form opens an avenue for more stable and reliable spintronic material for low power magnetoelectric random excess memory applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Tailoring the energy level alignment at the Co/Alq{sub 3} interface by controlled cobalt oxidation

We have studied the influence of oxygen exposure at the prototypical interface between cobalt and the organic semiconductor tris(8-hydroxyquinoline)aluminum (III) (Alq{sub 3}) by photoemission spectroscopy. We find that oxidation of the cobalt leads to a gradual suppression of hybrid interface states, to a progressive change in the work function and to a continuous energetic shift of the molecular orbitals towards higher binding energies. Based on these observations, we propose controlled oxidation of the ferromagnetic electrode as an easy and effective possibility to tune the performance of organic spintronics devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Severe Dirac Mass Gap Suppression in Sb 2 Te 3 -Based Quantum Anomalous Hall Materials

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ~320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr 0.08 (Bi 0.1 Sb 0.9 ) 1.92 Te 3 to that of its nonmagnetic parent (Bi 0.1 Sb 0.9 ) 2 Te 3 , to explore the cause. In (Bi 0.1 Sb 0.9 ) 2 Te 3 , we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr 0.08 (Bi 0.1 Sb 0.9 ) 1.92 Te 3 with concurrent but uncorrelated Dirac mass gap disorder. Additionally, these two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb 2 Te 3 -based FMTI materials to very low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Nanocrystalline zinc ferrite films studied by magneto-optical spectroscopy

Ferrimagnetic Zn-ferrite (ZnFe{sub 2}O{sub 4}) films can be grown with the ferromagnetic resonance linewidth of 40 Oe at 9.5 GHz without going through a high temperature processing. This presents interest for applications. The work deals with laser ablated ZnFe{sub 2}O{sub 4} films deposited at O{sub 2} pressure of 0.16 mbar onto fused quartz substrates. The films about 120 nm thick are nanocrystalline and their spontaneous magnetization, 4πM{sub s}, depends on the nanograin size, which is controlled by the substrate temperature (T{sub s}). At T{sub s} ≈ 350 °C, where the grain distribution peaks around ∼20–30 nm, the room temperature 4πM{sub s} reaches a maximum of ∼2.3 kG. The films were studied by magnetooptical polar Kerr effect (MOKE) spectroscopy at photon energies between 1 and 5 eV. The complementary characteristics were provided by spectral ellipsometry (SE). Both the SE and MOKE spectra confirmed ferrimagnetic ordering. The structural details correspond to those observed in MgFe{sub 2}O{sub 4} and Li{sub 0.5}Fe{sub 2.5}O{sub 4} spinels. SE experiments confirm the insulator behavior. The films display MOKE amplitudes somewhat reduced with respect to those in Li{sub 0.5}Fe{sub 2.5}O{sub 4} and MgFe{sub 2}O{sub 4} due to a lower degree of spinel inversion and nanocrystalline structure. The results indicate that the films are free of oxygen vacancies and Fe{sup 3+}-Fe{sup 2+} exchange.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic and transport properties of cobalt doped La₀.₇Sr₀.₃MnO₃

The magnetic and transport properties of La₀.₇Sr₀.₃Mn{sub 1–x}CoxO₃ (0≤x≤1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x>0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3<x≤0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ~0.1 for the typical La₀.₇Sr₀.₃Mn₀.₆Co₀.₄O₃ sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co³⁺ ions are in the intermediate spin state with the increase of Co-doping level (x ≥ 0.3), while the Co⁴⁺ ions show the spin state transition from the high spin state to the intermediate spin state at x=0.6. The temperature dependence of resistivity shows a metal-insulator transition character for the intermediate Co-doping samples, which can be well fitted by the three dimensional variable-range-hopping model in the whole temperature range. These facts suggest that the double exchange interaction in the perovskite La₀.₇Sr₀.₃MnO₃ is destroyed by the B site doping, due to the strong competition between the double exchange and superexchange interactions in Co/Mn ions as well as the frustration of spins in the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural, optical, and ferromagnetic characterization of Sm-doped LaOCl nanocrystalline synthesized by solvothermal route: Significant effect of hydrogen post treatment

Pure and Sm-doped lanthanum oxychloride (LaOCl) nanomaterials were synthesized by solvothermal route followed by a subsequent heat treatment process. The objective of the present work is to study and develop conditions required to create stable room-temperature ferromagnetic (RT-FM) properties in LaOCl. To achieve that aim, magnetic samarium Sm{sup 3+} ions were used as dopant sources for stable FM properties. Systematic structural, optical, and magnetic properties of undoped and Sm-doped LaOCl samples were investigated as function of post-annealing conditions (temperature and atmosphere). The optical absorption properties were studied by diffuse reflection spectroscopy (DRS). The magnetic measurements reveal that Sm-doped LaOCl nanopowders have partial RT-FM properties due to the doped ions. The variations of magnetic properties with pre-annealing temperature were investigated. Furthermore, the electronic medium of host LaOCl crystalline lattice, which carries the spin-spin (S.S) exchange interaction between localised dopant Sm{sup 3+}(4f{sup 5}) spins, was developed by annealing in hydrogen gas (hydrogenation). It was established that annealing in hydrogen atmosphere boosts the RT-FM properties so that the saturation magnetisation could be increased by more than 100%. Physical explanations and discussions were given in this paper. Thus, it was proved that the magnetic properties could be tailored to diamagnetic LaOCl compound by Sm-doping and post treatment under H{sub 2} atmosphere. Therefore, LaOCl nanocrystals could be used as a potential candidate for optical phosphor applications with magnetic properties. - Graphical abstract: M-H dependence of Sm-doped LaOCl powders. Study the effect of hydrogenation. - Highlights: • Synthesis of Sm-doped LaOCl nanoparticles. • DM LaOCl transforms to FM with dilute concentration of Sm doping. • Annealing under H{sub 2} atmosphere induces drastic boost in the FM properties. • Saturation magnetization attained 29 memu/g with little doping and hydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Towards additive manufacturing of magnetocaloric working materials

The feasibility of three-dimensional printing via the Directed Energy Deposition (DED) technique is demonstrated for the ferromagnetic AlFe 2 B 2 (aka 1-2-2) compound, which shows promise as a working material for room-temperature magnetocaloric devices. Like many intermetallic materials, the 1-2-2 compound is brittle at room temperature; not only is it difficult to synthesize from the melt, it is challenging to form it into useful geometries necessary for effective heat transfer in magnetocaloric devices. Using Laser Engineered Net Shaping (LENS TM ) synthesis from two powder precursors, AlB 2 and Fe, samples of simple (rod) and complex (honeycomb) architectures were successfully synthesized. Furthermore, the phase constitution and distribution, magnetic transition temperature, and saturation magnetization determined for the LENS TM -synthesized rod sample are all similar to those obtained for 1-2-2 materials made by conventional casting. Further, the measured magnetic entropy change, as determined from Maxwell's Relations applied to magnetic data, indicates that the magnetocaloric response of the annealed LENS TM -printed rod samples are comparable to those of the 1-2-2 compounds made by conventional techniques. These results confirm that complex magnetofunctional shapes, which are needed for efficient energy conversion in magnetocaloric devices, can be rapidly fabricated from the AlFe 2 B 2 compound.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Possible Origin of Ferromagnetism in Transition Metal Doped Zirconia

We have studied the electronic structure and magnetic properties of cubic zirconia (c-ZrO{sub 2}) with cobalt (Co) or nickel (Ni) doping using density functional theory (DFT) calculations. The pure c-ZrO{sub 2} is a nonmagnetic insulator with a wide bandgap. The calculated results reveal that isolated Co or Ni atom can both produce the local magnetic moment in c-ZrO{sub 2}. And the isolated Co atom can introduce a magnetic moment of about 0.98 μ{sub B}, while the Ni atom is 2.85 μ{sub B}. The impurity peaks can be formed in the bandgap. Our studies show that the magnetic moments mainly result from d orbitals of the impurity atoms. And the spin-up electrons will be arranged in t{sub 2g} orbitals under the ligand field of O{sub h} group in Co- or Ni-doped c-ZrO{sub 2}. Obviously, this will lead to a high-spin state (S = 1/2 or 1). The studies of magnetic coupling reveal that the two Co atoms in c-ZrO{sub 2} are not always coupled ferromagnetically at different distances. And the system will be in a spin singlet state (S = 0) when the distance is 6.209 or 7.170 Å between two Co atoms. However, the two Ni atoms in c-ZrO{sub 2} are always coupled ferromagnetically at all distances. So we can conclude that the Ni-doped c-ZrO{sub 2} is more suitable for spintronic material than Co doping. These results are significant for spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Design and installation of a ferromagnetic wall in tokamak geometry

Low-activation ferritic steels are leading material candidates for use in next-generation fusion development experiments such as a prospective component test facility and DEMO power reactor. Understanding the interaction of plasmas with a ferromagnetic wall will provide crucial physics for these facilities. In order to study ferromagnetic effects in toroidal geometry, a ferritic wall upgrade was designed and installed in the High Beta Tokamak–Extended Pulse (HBT-EP). Several material options were investigated based on conductivity, magnetic permeability, vacuum compatibility, and other criteria, and the material of choice (high-cobalt steel) is characterized. Installation was accomplished quickly, with minimal impact on existing diagnostics and overall machine performance, and initial results demonstrate the effects of the ferritic wall on plasma stability.

42 ENGINEERING↗
DOE OSTI2014

Structurally controllable spin spatial splitter in a hybrid ferromagnet and semiconductor nanostructure

We theoretically investigate modulation of a tunable δ-potential to the lateral displacement of electrons across a magnetically modulated semiconductor nanostructure. Experimentally, this nanostructure can be produced by depositing a nanosized ferromagnetic stripe with in-plane magnetization on top of a semiconductor heterostructure, while the δ-potential can be realized by means of the atomic layer doping technique. Theoretical analysis reveals that this δ-doping can break the intrinsic symmetry in nanostructure and a considerable spin polarization in the lateral displacement will appear. Numerical calculations demonstrate that both magnitude and sign of spin polarization can be manipulated by changing the height and/or position of the δ-doping, giving rise to a structurally tunable spin spatial splitter.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tailoring the Structural, Optical and Magnetic Properties of NiFe{sub 2}O{sub 4} by Varying Annealing Temperature

Magnetic nanocrystalline NiFe{sub 2}O{sub 4} nanoparticles of size ranging between 13 and 19 nm were synthesized by the hydrothermal method using polyethylene glycol (PEG) as surfactant. The role of annealing on phase, crystallite size, and morphology was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDX) was used for qualitative analysis. X-ray diffraction confirmed the single phase of as-obtained particles. TEM revealed a spherical morphology having an average particle size of 13.73 nm at 523 K and rises to 14.89, 16.42, and 21.14 nm at 623, 723, and 823 K respectively, which is approximately the same as the crystallite size determined from XRD. EDX results confirmed that elemental ratios were in stoichiometry. Fourier transform infrared (FTIR) spectral analysis demonstrates the formation of M-O bond in NiFe{sub 2}O{sub 4} nanoparticles. The optical spectra obtained from UV-visible spectroscopy revealed that nanoparticles have direct allowed energy band gap of 2.34, 2.47, 2.70, and 2.20 eV at 523, 623, 723, and 823 K respectively. A vibrating sample magnetometer (VSM) was used at room temperature for magnetic measurements which further showed ferromagnetic behavior of as-synthesized material. In addition, it was found that saturation magnetization increases with an increase of annealing temperature while coercivity decreases with temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Pressure–Temperature Phase Diagram Reveals Spin–Lattice Interactions in Co[N(CN) 2 ] 2

Diamond anvil cell techniques, synchrotron-based infrared and Raman spectroscopies, and lattice dynamics calculations are combined with prior magnetic property work to reveal the pressure–temperature phase diagram of Co[N(CN)2]2. The second-order structural boundaries converge on key areas of activity involving the spin state exposing how the pressure-induced local lattice distortions trigger the ferromagnetic → antiferromagnetic transition in this quantum material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI 3

Here, we examine the crystallographic and magnetic properties of single crystals of CrI 3 , an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210–220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R$\bar{3}$, BiI 3 -type) and the high-temperature structure is monoclinic (C2/m, AlCl 3 -type). Evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI 3 was found; we observed an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Our calculations suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI 3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.

36 MATERIALS SCIENCE↗