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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Antisite Defects in Layered Multiferroic CuCr 0.9 In 0.1 P 2 S 6

The CuCr 1-x In x P 2 S 6 system represents a large family of metal chalcogenophosphates that are unique and promising candidates for 2D materials with functionalities such as ferroelectricity. We carried out detailed microstructural and chemical characterization of these compounds using aberration-corrected STEM, in order to understand the origin of these different ordering phenomena. Quantitative STEM-HAADF imaging and analysis identified the stacking order of an 8-layer thin flake, which leads to the identification of anti-site In 3+ (Cu + ) doping. We believe that these findings will pave the way towards understanding the ferroic coupling phenomena in van der Waals lamellar compounds, as well as the potential applications in 2-D electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Calcium-doping effects on photovoltaic response and structure in multiferroic BiFeO{sub 3} ceramics

Photovoltaic (PV) effects, power-conversion efficiencies, and structures have been systematically measured in (Bi{sub 1−x}Ca{sub x})FeO{sub 3−δ} ceramics for x = 0.05, 0.10, and 0.15. The heterostructures of indium tin oxide (ITO) film/(Bi{sub 1−x}Ca{sub x})FeO{sub 3−δ} ceramics/Au film exhibit significant PV effects under illumination of λ = 405 nm. The maximum power-conversion efficiency in the ITO/(Bi{sub 0.90}Ca{sub 0.10})FeO{sub 2.95} (BFO10C)/Au can reach 0.0072%, which is larger than 0.0025% observed in the graphene/polycrystalline BFO/Pt films [Zang et al., Appl. Phys. Lett. 99, 132904 (2011)]. A theoretical model based on optically excited current in the depletion region between ITO film and Ca-doped BFO ceramics is used to describe the I-V characteristic, open-circuit voltage, and short-circuit current density as a function of illumination intensity. This work suggests that the Ca-substitution can reduce the rhombohedral distortion and stabilize the single-phase structure.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Reentrant spin-glass state in a geometrical frustrated multiferroic system: Role of disorder

We investigated the effect of magnetic (Mn) and nonmagnetic (Ga) doping at the Cr site of the layered geometrically frustrated antiferromagnetic compound LiCrO{sub 2}. 10% Ga doping at the Cr site does not invoke any metastability typical of a glassy magnetic state. However, similar amount of Mn doping drives the system to a spin glass (SG) state which is particularly evident from the magnetic memory and heat capacity studies. The onset of glassy state in 10% Mn doped sample is of reentrant type developing out of higher temperature antiferromagnetic state. The SG state in the Mn-doped sample shows a true reentry with the complete disappearance of the antiferromagnetic phase below the SG transition. The lack of SG state on Ga doping indicates the importance of random ferromagnetic/antiferromagnetic bonds for the glassy ground state in LiCrO{sub 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of diamagnetic barium substitution on magnetic and photovoltaic properties in multiferroic BiFeO{sub 3}

Spontaneous magnetization and photovoltaic (PV) effects have been measured in (Bi{sub 1-x}Ba{sub x})FeO{sub 3-δ} ceramics for x = 0.05, 0.10, and 0.15. The substitution of Ba{sup 2+} ion in the A site of the perovskite unit cell can effectively enhance the ferromagnetic magnetization. The heterostructure of indium tin oxide (ITO) film/(Bi{sub 1-x}Ba{sub x})FeO{sub 3-δ} ceramic/Au film exhibits significant PV effects under illumination of λ = 405 nm. The PV responses decrease with increasing Ba concentration. The maximum power-conversion efficiency in the ITO/(Bi{sub 0.95}Ba{sub 0.5})FeO{sub 2.95}/Au can reach 0.006%. A theoretical model based on optically excited current in the depletion region between ITO film and (Bi{sub 1-x}Ba{sub x})FeO{sub 3-δ} ceramics is used to describe the I-V characteristic, open-circuit voltage (V{sub oc}), and short-circuit current density (J{sub sc}) as a function of light intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The multiferroic properties of polycrystalline Bi{sub 1−x}Y{sub x}FeO{sub 3} films

Polycrystalline Bi{sub 1−x}Y{sub x}FeO{sub 3} films with varying x from 0 to 0.30 were prepared by pulsed laser deposition on surface oxidized Si (100) substrates with LaNiO{sub 3} as buffer layer. The influence of Y doping on the structure, ferroelectric properties, and exchange bias have been systematically investigated. X-ray diffraction and Raman spectroscopy studies revealed the structural transition from rhombohedral R3c to orthorhombic Pn2{sub 1}a with increasing x above 0.10. The leakage current density of BiFeO{sub 3} has been effectively suppressed by Y doping, and well saturated P–E loops have been observed in Bi{sub 1−x}Y{sub x}FeO{sub 3} (0.01 ≤ x ≤ 0.07). Exchange bias field with a 3.6 nm thick NiFe layer increases with increasing x to 0.01, then decreases with further increasing x.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effects of Cr doping on the magnetic properties of multiferroic YMnO{sub 3}

We have synthesized a series of YMn{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 0.1) samples and study the effect of Cr-doping on their magnetic properties. The magnetic characterization indicates that with increasing Cr-content up to 0.1, the antiferromagnetic (AFM) transition temperature increases from 73 to 89 K. Our experiment results also indicate that the Cr-doped samples exhibit the characteristics of spin-glass state at low temperature. Moreover, the magnetic hysteresis curves of the doped samples show a weak ferromagnetic (FM) behavior. It is found that the spin-glass state of the Cr-doped samples is due to the competition between AFM superexchange and FM double-exchange interaction, induced by the Cr doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Origin of enhanced magnetization in rare earth doped multiferroic bismuth ferrite

We report structural and magnetic properties of rare earth doped Bi{sub 0.95}R{sub 0.05} FeO{sub 3} (R = Y, Ho, and Er) submicron particles. Rare earth doping enhances the magnetization and the magnetization shows an increasing trend with decreasing dopant ionic radii. In contrast to the x-ray diffraction pattern, we have seen a strong evidence for the presence of rare earth iron garnets R{sub 3}Fe{sub 5}O{sub 12} in magnetization measured as a function of temperature, in selected area electron diffraction, and in Raman measurements. Our results emphasised the role of secondary phases in the magnetic property of rare earth doped BiFeO{sub 3} compounds along with the structural distortion favoring spin canting by increase in Dzyaloshinskii-Moriya exchange energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Exchange interaction mediated ferroelectricity in multiferroic MnTiO{sub 3} with anisotropic orbital hybridization and hole delocalization

We present the orbital structure of MnTiO{sub 3} with polarization dependent x-ray absorption and resonant x-ray emission spectra accompanied with electronic structure calculations. The results clearly indicate a strongly anisotropic O 2p-Mn 3d orbital hybridization whereas the Mn 3d hole state shows a highly delocalized characteristic ascribed to the 3d-4p mixing. The extended Mn 4p orbital could enhance the exchange interaction between Mn (3d)-O (2p)-Mn (3d) leading to an asymmetric charge distribution in Mn-O bonds. The delocalized characteristic of Mn 3d holes is indispensable to the mechanism of spin-dependent-metal-ligand hybridization to explain magnetically induced ferroelectricity.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Epitaxial growth of metastable multiferroic AlFeO{sub 3} film on SrTiO{sub 3} (111) substrate

GaFeO{sub 3}-type AlFeO{sub 3} is consisted of oxygen octahedra and tetrahedra containing Al and Fe ions and is known to have a non-centrosymmetric polar structure with space group Pna2{sub 1}. We tried to grow epitaxial GaFeO{sub 3}-type AlFeO{sub 3} films on SrTiO{sub 3} (111) substrates by pulsed laser deposition technique. Both the atomic arrangement of close-packed and the atomic distance of the substrate surface played important roles in stabilizing GaFeO{sub 3}-type AlFeO{sub 3} on the substrate. Piezoresponse force microscopy measurements clearly showed that GaFeO{sub 3}-type AlFeO{sub 3} films have ferroelectricity at room temperature. In addition, AlFeO{sub 3} film also showed pinched-like hysteresis loop with T{sub N} ∼ 317 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ferromagnetic response of multiferroic TbMnO{sub 3} films mediated by epitaxial strain and chemical pressure

High quality Tb{sub 1−x}Al{sub x}MnO{sub 3} (x = 0, 0.3) films have been grown under different values of compressive/tensile strain using (001)-oriented SrTiO{sub 3} and MgO substrates. The films were grown by means of rf sputtering at substrate temperature of 800 °C. X-ray diffraction analysis shows that films are single phase, preferentially oriented in the (111) and (122) directions for films deposited on SrTiO{sub 3} and MgO substrates, respectively. Although the TbMnO{sub 3} target shows antiferromagnetic order, the films deposited on both substrates show weak ferromagnetic phase at low temperature coexisting with the antiferromagnetic phase. The introduction of Al in the films clearly enhances their ferromagnetic behavior, improving the magnetic performance of this material. Indeed, M(H) measurements at 5 K show a well-defined hysteresis for films grown on both substrates. However, a stronger magnetic signal (larger values of remanence and coercive field) is observed for films deposited on MgO substrates. The chemical pressure generated by Al doping together with the substrate-induced strain seem to modify the subtle competition between magnetic interactions in the system. It is speculated that such modification could lead to a non-collinear magnetic state that may be tuned by strain modifications. This may be performed by varying the thickness of the films and/or considering other substrate materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancement in the multiferroic properties of BiFeO{sub 3} by charge compensated aliovalent substitution of Ba and Nb

Polycrystalline ceramics, Bi{sub 1-2x}Ba{sub 2x}Fe{sub 1-x}Nb{sub x}O{sub 3} (x = 0.00–0.15), were synthesized by solid state reactions method. X-ray diffraction data have revealed elimination of impurity phases and an increase in unit cell volume with Ba and Nb substitution. Diffraction peak splitting is found to be suppressed which indicates a decrease in octahedral distortion. The Mössbauer spectra demonstrate the suppression of spiral spin modulation of the magnetic moments resulting in enhanced ferromagnetism with increasing dopant concentration. The leakage current density of the sample with x = 0.10 is found to be greatly reduced up to six orders of magnitude as compared to the undoped sample. Ohmic conduction is found to be dominant mechanism in all the samples, however, undoped sample showed space charge limited conduction in high electric filed region, while the sample with x = 0.15 exhibited grain boundary limited conduction in low electric field region.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural and dielectric properties of Nd/Ca co-doped bi-ferrite multiferroics

Single-phase polycrystalline samples of Bi{sub 0.8}Nd{sub 0.2−x}Ca{sub x}FeO{sub 3} (x = 0.0, 0.1) were synthesized by chemical coprecipitation method. X-ray diffraction patterns accompanied by Rietveld-refined crystal structure parameters reveal the phase transition of Bi{sub 0.8}Nd{sub 0.1}Ca{sub 0.1}FeO{sub 3} with rhombohedral R3c symmetry to triclinic structure of Bi{sub 0.8}Nd{sub 0.2}FeO{sub 3} sample. Frequency dependence of dielectric constant (ε') and dielectric loss (tanδ), infers enhancement of both ε' (tanδ) in narrow band gap of Nd/Ca co-doped BFO as compare to wide band pristine BFO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Influence of La substitution on ferromagnetic properties of lead free Na{sub 0.5}Bi{sub 0.5}Ti{sub 0.988}Fe{sub 0.012}O{sub 3} multiferroic system

To study the effect of La doping and processing temperature, (Na{sub 0.5}Bi{sub 0.5}){sub 1−x}La{sub x}Ti{sub 0.988}Fe{sub 0.012}O{sub 3} (NBLTF) samples have been prepared by sol-gel method by employing two approaches namely with and without mol% excess of Na and Bi. X-ray diffraction analysis confirms the formation of rhombohedral perovskite structure in these samples with minor impurity phases. However, the impurity phases of stochiometric NBLTF samples disappear on increasing the sintering temperature from 650°C to 850°C. On other hand, NBLTF samples prepared with mol% excess of Na and Bi have shown pure perovskite structure only at lower sintering temperature 650°C and also at low concentration of La ∼ 1%. Transmission electron microscope investigation shows nano crystalline formation for all NBLTF samples sintered at 850°C. The enhancement in room temperature ferromagnetism is observed in La doped NBTF samples prepared at 850°C by taking stoichiometric amount of Na and Bi during synthesis.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Optical properties of Y and Ti co-substituted BiFeO{sub 3} multiferroics

Pure and co substituted Bi{sub 1−x}Y{sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} (x ≤ 0.24) ceramics were synthesized by solid state reaction method. X-ray diffraction patterns of Y and Ti codoped samples have shown single phase formation. Increasing Y and Ti concentration reveals structural transition from rhombohedral phase (R3c) for x ≤ 0.16 to orthorhombic phase (Pnma) for x = 0.24. FT-IR spectra exhibit broad absorption bands, which may be due to the overlapping of Fe-O and Bi-O vibrations. UV-visible spectroscopy results show strong absorption of light in the spectral range of 400-720 nm, indicating optical band gap in the visible region for these samples. These interesting optical properties of co-substituted BFO samples in visible region may find potential applications in optoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of Gd and Zr co-substitution on multiferroic properties of BiFeO{sub 3}

Single phase Bi{sub 1−x}Gd{sub x}Fe{sub 1−y}Zr{sub y}O{sub 3} (x=0.06;y=0, x= 0.10, y=0 and x=0.10,y= 0.03) nanoparticles were prepared to study the combined effect of Gd and Zr substitution on structural, magnetic and optical properties of BiFeO3. Rietveld refinement of X-ray diffraction patterns predicted that sample with x=0.06, y=0 was indexed to rhombohedral structure while samples with x=0.10, y=0 and x=0.10, y=0.03 were crystallized in multiphase (rhombohedral + orthorhombic). Magnetic properties were significantly enhanced with increasing concentration of Gd ions, however, magnetization reduced slightly with addition of 3% Zr ions in the system. Electron spin Resonance analysis indicated that breaking of spin cycloid is the main reason for enhancement of magnetization. Optical properties were investigated and direct band gap was found for all the samples.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Unusual multiferroicity in Cu{sub 3}Nb{sub 2}O{sub 8}

Cu{sub 3}Nb{sub 2}O{sub 8} is an anti-ferromagnetic - ferroelectric material in which the non-collinear spin alignment leads to breaking of spatial inversion symmetry. We confirm that the ferroelectricity and magnetic anomaly both occur at T{sub N1} = 25K. The emergence of ferroelectricity due to complex spin ordering is investigated by the temperature dependent low temperature measurements of magnetization, dielectric constant and electric polarization. The microscopic origin to explain magneto-electric coupling in this compound is based on a model based on ferroaxial coupling rather than inverse Dzyalonshkii-Moria interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗