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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Topologically Allowed Nonsixfold Vortices in a Sixfold Multiferroic Material: Observation and Classification

Here, we report structural transformation of sixfold vortex domains into two-, four-, and eightfold vortices via a different type of topological defect in hexagonal manganites. Combining high-resolution electron microscopy and Landau-theory-based numerical simulations, we also investigate the remarkable atomic arrangement and the intertwined relationship between the vortex structures and the topological defects. The roles of their displacement field, formation temperature, and nucleation sites are revealed. All conceivable vortices in the system are topologically classified using homotopy group theory, and their origins are identified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic Field Control of Cycloidal Domains and Electric Polarization in Multiferroic BiFeO 3

The magnetic field induced rearrangement of the cycloidal spin structure in the room-temperaturemultiferroic BiFeO3 is studied using small-angle neutron scattering (SANS). The cycloid propagationvectors are observed to rotate when magnetic fields applied perpendicular to the polar axisexceed a pinning threshold value of ~5T in the studied crystals. In light of these experimentalresults, a phenomenological model is proposed that describes our observations, and we revisit themicroscopic origin of the magnetoelectric effect. A new term describing a coupling between themagnetic anisotropy and the polarization is proposed to explain the recently discovered magneticfield induced in-plane electric polarization.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dimensionality-Induced Change in Topological Order in Multiferroic Oxide Superlattices

We construct ferroelectric (LuFeO 3 ) m /(LuFe 2 O 4 ) superlattices with varying index m to study the effect of confinement on topological defects. We observe a thickness-dependent transition from neutral to charged domain walls and the emergence of fractional vortices. In thin LuFeO 3 layers, the volume fraction of domain walls grows, lowering the symmetry from P6 3 cm to P3c1 before reaching the nonpolar P6 3 /mmc state, analogous to the group-subgroup sequence observed at the high-temperature ferroelectric to paraelectric transition. Our study shows how dimensional confinement stabilizes textures beyond those in bulk ferroelectric systems.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Raman Vibrations, Domain Structures, and Photovoltaic Effects in A‐Site La‐Modified BiFeO 3 Multiferroic Ceramics

Micro‐Raman spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy ( TEM ), oxygen vacancies, synchrotron X‐ray absorption spectroscopy, magnetizations, optical band gaps, and photovoltaic ( PV ) effects have been studied in (Bi 1− x La x )FeO 3 ( BFO 100 x L) ceramics for x = 0.0, 0.05, 0.10, and 0.15. XRD , Raman spectra, and TEM confirm a rhombohedral R 3 c symmetry with the tilted FeO 6 oxygen octahedra in all compounds. The low‐frequency Raman vibrations become broader and shift toward higher frequency as La 3+ increases. Fe K‐edge synchrotron X‐ray absorptions reveal that Fe 3+ valence and Fe–O–Fe bond angle are not modified by the La 3+ substitution. All compounds exhibit a linear antiferromagnetic feature. Optical transmission reveals band gaps in the range of 2.22–2.24 eV. The heterostructures of indium tin oxide ( ITO ) film/(Bi 1− x La x )FeO 3 ceramics/Au film show a p – n junction‐like I – V characteristic behavior. The maximal PV power conversion efficiency can reach 0.19% in ITO / BFO 15L/Au under illumination of λ = 405 nm. A junction‐like theoretical model can reasonably describe open‐circuit voltage and short‐circuit current as a function of illumination intensity.

Tu, Chi‐Shun↗
DOE OSTI2015

53 Cr NMR study of CuCrO 2 multiferroic

The results of a study of the magnetically ordered phase of a CuCrO 2 single crystal by the NMR method on 53 Cr nuclei in the absence of an external magnetic field are presented for the first time. The 53 Cr NMR spectrum is observed in the frequency range v res = (61 - 66) MHz, and its shape depends on the delay between pulses tdel in the pulse sequence τ π / 2 – t del – τ π – t del – echo. Spin-spin and spin-lattice relaxation times were measured, components of the electric field gradient, hyperfine fields, and magnetic moment on chromium atoms were estimated.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗