SEARCH · Search NASA
Results for “MAGNETIC INSULATION”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Strong magnetic correlations to 900 K in single crystals of the trigonal antiferromagnetic insulators SrMn 2 As 2 and CaMn 2 As 2
Crystallographic, electronic transport, thermal, and magnetic properties are reported for SrMn 2 As 2 and CaMn 2 As 2 single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl 2 Si 2 -type structure (space group $P\bar{3}$ m1), in agreement with the literature. Electrical resistivity ρ versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn 2 As 2 and 61 meV for CaMn 2 As 2 . In a local-moment picture, the Mn +2 3d 5 ions are expected to have high-spin S=5/2 with spectroscopic splitting factor g≈2. Magnetic susceptibility χ and heat capacity Cp measurements versus T reveal antiferromagnetic (AFM) transitions at T N =120(2) K and 62(3) K for SrMn 2 As 2 and CaMn 2 As 2 , respectively. The anisotropic χ(T≤T N ) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. Finally, the χ(T) data for both compounds and the Cp(T) for SrMn 2 As 2 show strong dynamic short-range AFM correlations from T N up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.
Evolution of ferromagnetism in two-dimensional electron gas of LaTiO 3 /SrTiO 3
Understanding, creating, and manipulating spin polarization of two-dimensional electron gases at complex oxide interfaces present an experimental challenge. For example, despite almost a decade long research effort, the microscopic origin of ferromagnetism in LaAlO 3 /SrTiO 3 heterojunctions is still an open question. Here, by using a prototypical two-dimensional electron gas (2DEG) which emerges at the interface between band insulator SrTiO 3 and antiferromagnetic Mott insulator LaTiO 3 , the experiment reveals the evidence for magnetic phase separation in a hole-doped $\textbf{Ti}$ $d^1$ $\textbf{t}_{2_g}$ system, resulting in spin-polarized 2DEG. The details of electronic and magnetic properties of the 2DEG were investigated by temperature-dependent d.c. transport, angle-dependent X-ray photoemission spectroscopy, and temperature-dependent magnetoresistance. The observation of clear hysteresis in magnetotransport at low magnetic fields implies spin-polarization from magnetic islands in the hole rich LaTiO 3 near the interface. We present these findings to emphasize the role of magnetic instabilities in doped Mott insulators, thus providing another path for designing all-oxide structures relevant to spintronic applications.
Reconciling monolayer and bilayer Jeff = 1/2 square lattices in hybrid oxide superlattice
The number of atomic layers confined in a two-dimensional structure is crucial for the electronic and magnetic properties. Single-layer and bilayer J eff = 1/2 square lattices are well-known examples where the presence of the extra layer turns the XY anisotropy to the c-axis anisotropy. We report on experimental realization of a hybrid SrIrO 3 /SrTiO 3 superlattice that integrates monolayer and bilayer square lattices in one layered structure. By synchrotron x-ray diffraction, resonant x-ray magnetic scattering, magnetization, and resistivity measurements, we found that the hybrid superlattice exhibits properties that are distinct from both the single-layer and bilayer systems and cannot be explained by a simple addition of them. In particular, the entire hybrid superlattice orders simultaneously through a single antiferromagnetic transition at temperatures similar to the bilayer system but with all the J eff = 1/2 moments mainly pointing in the ab plane similar to the single-layer system. The results show that bringing monolayer and bilayer with orthogonal properties in proximity to each other in a hybrid superlattice structure is a powerful way to stabilize a unique state not obtainable in a uniform structure.
Electrical Interrogation of Thickness‐Dependent Multiferroic Phase Transitions in the 2D Antiferromagnetic Semiconductor NiI 2
Abstract 2D magnetic materials hold promise for quantum and spintronic applications. 2D antiferromagnetic materials are of particular interest due to their relative insensitivity to external magnetic fields and higher switching speeds compared to 2D ferromagnets. However, their lack of macroscopic magnetization impedes detection and control of antiferromagnetic order, thus motivating magneto‐electrical measurements for these purposes. Additionally, many 2D magnetic materials are ambient‐reactive and electrically insulating or highly resistive below their magnetic ordering temperatures, which imposes severe constraints on electronic device fabrication and characterization. Herein, these issues are overcome via a fabrication protocol that achieves electrically conductive devices from the ambient‐reactive 2D antiferromagnetic semiconductor NiI 2 . The resulting gate‐tunable transistors show band‐like electronic transport below the antiferromagnetic and multiferroic transition temperatures of NiI 2 , revealing a Hall mobility of 15 cm 2 V −1 s −1 at 1.7 K. These devices also allow direct electrical probing of the thickness‐dependent multiferroic phase transition temperature of NiI 2 from 59 K (bulk) to 28 K (monolayer).
Seebeck coefficient and thermal conductivity of doped armchair graphene nanoribbon in the presence of magnetic field
Highlights: • Theoretical calculation of Seebeck coefficient and thermal conductivity of electrons on grapheme nanoribbon. • The investigation of the effect of magnetic field and ribbon width on temperature dependence of thermoelectric properties of nanoribbon. • The study of thermopower as a function of magnetic field and temperature. - Abstract: We address the thermal conductivity and Seebeck coefficient of armchair graphene nanoribbon within tight binding model Hamiltonian in the presence of magnetic field. The possible effects of ribbon width, magnetic field chemical potential on thermoelectric properties are investigated. Green's function approach has been employed in order to calculate transport coefficients of the system. We have also found the temperature dependence of the thermal conductivity for various ribbon widths and chemical potentials. Our results show a peak appears in temperature dependence of thermal conductivity of insulator nanoribbon for low magnetic fields. Upon increasing magnetic field, thermal conductivity shows a monotonic decreasing behavior and peak disappears. Our results show the increase of chemical potential causes to decreasing behavior for thermal conductivity. Moreover we study the effects of magnetic field on temperature behavior of thermopower of insulator graphene nanoribbon in details.
The Spin Pumping Effect in a Ferromagnetic Insulator/Normal Metal System: A Simple Quantum Mechanical Model
A simple quantum mechanical model of the spin pumping effect upon the excitation of a ferromagnetic resonance in a magnetic dielectric having a flat interface with a nonmagnetic metal is constructed. The model is based on the solution of the Schrödinger equation for conduction electrons in metal. It is shown that the reflection of electrons from the insulator with an oscillating magnetization with a spin flip gives rise to both a direct and alternating (oscillating with the frequency of the resonance-exciting microwave field) spin fluxes from the ferromagnet to the insulator. In this case, the effect of direct current is small compared to the effect of the alternating current; the role of a small parameter is played by the ratio of the exchange constant of a ferromagnet to the potential barrier between the metal and the dielectric (which is higher than the Fermi energy). The result obtained is consistent with known experimental data. The model constructed provides a simple and clear picture of the origination of the phenomenon and enable one to analyze the dependence of the effect on the system parameters.
Unraveling structural, electronic, and magnetic ambiguities in Pb 1−𝛿 CrO 3 with an insulating charge-transfer band structure
As a recently identified Mott system, PbCrO 3 remains largely unexplored, especially for its band structure, leading to many contentious issues on its structural, electronic, and magnetic properties. Here we present a comprehensive study of two different Pb 1−𝛿 CrO 3 (δ = 0 and 0.15) samples involving atomic deficiency prepared under pressure. By means of state-of-the-art diffraction techniques, the crystal structure of PbCrO 3 is definitively determined to adopt the pristine 𝑃𝑚$\overline{3}$𝑚 symmetry, rather than other previously misassigned structures of M2-Pm$\overline{3}$𝑚 and Pmnm. The two materials exhibit a similar charge-transfer-type insulating band structure, and the charge-transfer effect splits both Cr 2𝑝 and Pb 4𝑓 orbitals, rationalizing doublet splitting of the associated spectral lines. Nearly identical nominal cationic valence states of Cr 4+ and Pb 2+ are identified for this oxide system, hence calling into question the validity of recently proposed charge disproportionation mechanisms. In addition, Pb 0.85 CrO 3 exhibits an anomalously higher Néel temperature of ∼240 K than that of PbCrO 3 (i.e., ∼200 K), likely due to the deficiency-induced enhancements of Cr3𝑑–O2𝑝 orbital overlap and magnetic exchange. In conclusion, these findings provide much solid evidence to look into the fundamental properties of this important material system.
Simulated thermonuclear performance of auto-magnetizing helical liner MagLIF target implosions
Magnetized liner inertial fusion (MagLIF) implosions on the Z accelerator require magnetization of the fuel to reduce thermal conduction losses of energy from the laser-preheated deuterium fusion fuel to the surrounding liner material. While external field coils traditionally used to axially magnetize MagLIF targets are limited to 10–20 T (or perhaps ∼ 30 T with technological development), calculations suggest that higher axial magnetic fields would improve thermal insulation of the fuel and improve MagLIF target performance. We present results from three-dimensional radiation-magnetohydrodynamic simulations of MagLIF implosions employing auto-magnetizing helical liners composed of discrete metallic helical conduction paths separated by electrically insulating material. These simulated auto-magnetizing (AutoMag) liners produce axial magnetic fields >30 T inside the fusion fuel prior to implosion. Simulations indicate that higher thermonuclear yields and burn-averaged fuel ion temperatures are attained for implosions using AutoMag liners compared to standard MagLIF implosions. Comparable implosion morphology is evident in synthetic x-ray images of AutoMag–MagLIF and standard MagLIF implosions.
Atomic-Scale Characterization of Dilute Dopants in Topological Insulators via STEM–EDS Using Registration and Cell Averaging Techniques
Magnetic dopants in three-dimensional topological insulators (TIs) offer a promising avenue for realizing the quantum anomalous Hall effect (QAHE) without the necessity for an external magnetic field. Understanding the relationship between site occupancy of magnetic dopant elements and their effect on macroscopic property is crucial for controlling the QAHE. By combining atomic-scale energy-dispersive X-ray spectroscopy (EDS) maps obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM) and novel data processing methodologies, including semi-automatic lattice averaging and frame registration, we have determined the substitutional sites of Mn atoms within the 1.2% Mn-doped Sb 2 Te 3 crystal. More importantly, the methodology developed in this study extends beyond Mn-doped Sb 2 Te 3 to other quantum materials, traditional semiconductors, and even electron irradiation sensitive materials.
Micro-fabricated integrated coil and magnetic circuit and method of manufacturing thereof
A micro-fabricated electromagnetic device is provided for on-circuit integration. The electromagnetic device includes a core. The core has a plurality of electrically insulating layers positioned alternatingly between a plurality of magnetic layers to collectively form a continuous laminate having alternating magnetic and electrically insulating layers. The electromagnetic device includes a coil embedded in openings of the semiconductor substrate. An insulating material is positioned in the cavity and between the coil and an inner surface of the core. A method of manufacturing the electromagnetic device includes providing a semiconductor substrate having openings formed therein. Windings of a coil are electroplated and embedded in the openings. The insulating material is coated on or around an exposed surface of the coil. Alternating magnetic layers and electrically insulating layers may be micro-fabricated and electroplated as a single and substantially continuous segment on or around the insulating material.
High-Frequency Transformer Design for Modular Power Conversion From Medium-Voltage AC to 400 VDC
This paper presents a high-frequency modular medium-voltage AC (4160 VAC or 13.8 kVAC) to low-voltage DC (400 VDC) system that is scalable in order to be used for different scale microgrids. A 15 kW, 500 kHz DC/DC converter is demonstrated as the most important stage of the system overall, which can be scalable to a 225 kW 4160 VAC to 400 VDC system. Motivation of a CLLC resonant converter and its design parameters determination is carefully illustrated. The high-frequency transformer is the key element for the DC/DC converter. Then, the paper focuses on high-frequency transformer design to realize high-voltage insulation, high efficiency, and high density at the same time. Based on a split winding UU core transformer structure, transformer insulation materials and dimension parameters are determined referring to the IEEE insulation standard. The transformer magnetic loss model is reviewed based on which loss design tradeoff is carefully analyzed. Different working frequency impact on transformer design over three different core materials is also presented. Finally, a 500 kHz transformer prototype has been developed and demonstrated with the IEEE standard required insulation tests. A whole CLLC resonant converter is also present with experiments results. Here, the converter holds outstanding 98% efficiency and 2.9 kW/L power density.
Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr 2 IrO 4
Sr 2 IrO 4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at T N = 240 K. We report results of a comprehensive study of single-crystal Sr 2 Ir 1-x Tb x O 4 (0≤x≤0.03). This study found that mere 3% (x=0.03) tetravalent Tb 4+ (4f 7 ) substituting for Ir 4+ (rather than Sr 2+ ) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x = 0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund’s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. Furthermore, this work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.
Electronic density of states of a U ( 1 ) quantum spin liquid with spinon Fermi surface. I. Orbital magnetic field effects
A quantum spin liquid with a spinon Fermi surface is an exotic insulator that hosts neutral Fermi surfaces inside the insulating gap. In an external magnetic field, it has been pointed out that the neutral Fermi surfaces are Landau quantized to form Landau levels due to the coupling to the induced emergent gauge magnetic field. In this paper, we calculate the electronic density of states (as observed in tunneling experiments) of the quantum spin liquid in an orbital magnetic field. Here, we find that the Landau levels from the neutral Fermi surfaces give rise to a set of steps emerging at the upper and lower Hubbard band edges. Each of the Hubbard band edge steps further develop into a band edge resonance peak when a weak gauge binding arising from the gauge field fluctuations is taken into account. Importantly, each Hubbard band edge step and its resulting resonance peak in the weak gauge binding are found to have a corresponding Landau level from the neutral Fermi surfaces, so the Hubbard band edge steps and the band edge resonance peaks provide signatures to the unique feature that characterizes the Landau quantization of the in-gap neutral Fermi surfaces in the spin liquid. We further consider the strong gauge-binding regime where the band edge resonance peaks move into the Mott gap and develop into true in-gap bound states. In the strong gauge binding regime, we solve the Landau-level spectrum of the in-gap bound states in an orbital magnetic field. For the in-gap bound state with a Mexican-hat-like band dispersion, we find that the envelop energy to have a state excited from the bound state Landau levels decreases quadratically with the magnetic field. The quadratic decrease behavior of the envelop energy is consistent with the intuition that applying magnetic field localizes the states and energetically promotes the in-gap bound states formation. Finally, we discuss the connection of our results to the electronic density of states spectra measured in the layered 1T-TaS 2 . We point out that a quantum spin liquid with a quasibound state in the upper Hubbard band can give the density of states spectra similar to the one measured in the experiment.
Terahertz imaging of Landau levels in HgTe-based topological insulators
We report on sub-terahertz photoconductivity under the magnetic field of a two dimensional topological insulator based on HgTe quantum wells. We perform a detailed visualization of Landau levels by means of photoconductivity measured at different gate voltages. This technique allows one to determine a critical magnetic field, corresponding to topological phase transition from inverted to normal band structure, even in almost gapless samples. The comparison with realistic calculations of Landau levels reveals a smaller role of bulk inversion asymmetry in HgTe quantum wells than it was assumed previously.
Anomalous electron transport in epitaxial NdNiO 3 films
The origin of simultaneous electronic, structural, and magnetic transitions in bulk rare-earth nickelates (RENiO 3 ) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple the metal-insulator transition (MIT) from the magnetic transition. Yet, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have calculated Hall effect on a series of epitaxial NdNiO 3 films, spanning a variety of electronic and magnetic phases. We find that the MIT results in only a partially gapped Fermi surface, whereas the full insulating phase forms below the magnetic transition. Moreover, we also find a systematic reduction of the Hall coefficient R H in the metallic phase of these films with epitaxial strain and also a surprising transition to a negative value at large compressive strain. The partially gapped, weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high-T c cuprates. The precursor metallic phase, which undergoes transition to the insulating phase, is a non-Fermi liquid with a temperature exponent n of resistivity of 1, whereas the exponent increases to 4/3 in the noninsulating samples. Such a nickelate phase diagram with sign reversal of R H , a pseudogap phase, and non-Fermi-liquid behavior is intriguingly similar to high-T c cuprates, giving crucial guidelines to engineer unconventional superconductivity in oxide heterostructures.
Metal-insulator transition in antiferromagnetic Ba 1 - x K x Mn 2 As 2 ( 0 ≤ x ≤ 0.4 ) single crystals studied by 55 Mn and 75 As NMR
The magnetic structure and metal-insulator transition in antiferromagnetic (AFM) BaMn 2 As 2 and Ba 1-x K x Mn 2 As 2 single crystals have been investigated by 55 Mn and 75 As nuclear magnetic resonance (NMR) measurements. In the parent AFM insulator BaMn 2 As 2 with a Néel temperature TN=625 K, we observed a 55 Mn zero-field NMR (ZFNMR) spectrum and confirmed the G-type AFM structure from the field dependence of the 55 Mn spectra and 75 As NMR spectra below TN. In hole-doped crystals with x>0.01, similar 55 Mn ZFNMR spectra were observed and the AFM state was revealed to be robust up to x=0.4 with the ordered moment nearly independent of x. The nuclear spin-lattice relaxation rates (1/T 1 ) for both nuclei in the doped samples follow the Korringa relation T 1 T=const, indicating a metallic state. This confirms the coexistence of AFM ordered localized Mn spins and conduction carriers from a microscopic point of view. From the x dependence of (T 1 T) -1/2 for both nuclei, we conclude that this transition is caused by vanishing of the hole concentration as the transition is approached from the metallic side.
Iridium valence variation and carrier sign tuning in ( Ca , Ba ) x La 2 – x CuIrO 6 double perovskites
Here, we report the structure and properties of ( Ca , Ba ) x La 2 – x CuIrO 6 . The rock-salt double perovskite structure goes through a P 1 ¯ to P 2 1 / n phase transition with increasing Ba content but not with Ca content. In both cases, the Ir ion is oxidized from 4 + to 5 + with increasing substitution. Transport and magnetic properties measurements reveal that all compositions are insulating with hysteretic spin-freezing magnetic behavior. Seebeck coefficient measurements reveal a large thermopower change from + 176 μ V /K in La 2 CuIrO 6 to – 223 μ V /K with increasing Ba content in Ba x La 2 – x CuIrO 6 . This behavior is modeled well by the Heikes formula for correlated hopping conduction. ( Ca , Ba ) x La 2 – x CuIrO 6 provides a model system for investigations of other systems with similar carrier sign tuning, with applications toward semiconductors and thermoelectric materials.