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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Relaxor-like dielectric response of spin liquid CuCrO{sub 2}

Broadband dielectric analysis (10{sup −2}-10{sup 7} Hz) of layered triangular lattice CuCrO{sub 2} is performed (123 K - 473 K) and analyzed in connection with recently observed spin frustration in this multiferroic [M. Poienar et al. Phys. Rev. B 81, 104411, (2010); M. Frontzek et al. Phys. Rev. B 84, 094448, (2011)]. Most unexpectedly, this well known delafossite has found to have nontrivial charge degrees of freedom, being characterized by a relaxor-like dielectric relaxation around 375 K with FWHM of ∼100K. The result strongly suggests the existence of intermolecular Coulomb interaction between charge disproportionation induced electric dipoles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural, optical and dielectric property of Co doped Bi{sub 2}Fe{sub 4}O{sub 9}

Multiferroic Bi{sub 2}Fe{sub 4}O{sub 9} and Co doped Bi{sub 2}Fe{sub 4}O{sub 9} are prepared by solid state route reaction method using bismuth oxide(Bi{sub 2}O{sub 3}), iron oxide(Fe{sub 2}O{sub 3}) and cobalt oxide (Co{sub 3}O{sub 4}). Their structural optical and dielectric properties are studied and compared. X-ray diffraction (XRD) results confirm that there is no change in crystal structure due to Co doping. From dielectric constant measurement we conclude that dielectric constant increases due to Co doping. UV-Visible plot shows due to Co doping bang gap energy increases.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural, electrical and magnetic properties of Bi{sub 0.90}La{sub 0.10}Fe{sub 0.90}Co{sub 0.10}O{sub 3} ceramics

Multiferroic Bi{sub 0.90}La{sub 0.10}Fe{sub 0.90}Co{sub 0.10}O{sub 3} ceramics have been synthesized via soft chemical co-precipitation method. From the X-ray diffraction analysis, ceramic powder sintered at 600°C was found to be single phase belonging to the rhombohedral structure with R3c space group confirmed by Rietveld analysis. Further, dielectric, Leakage and magnetic studies were performed at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phonons and magnetic excitation correlations in weak ferromagnetic YCrO{sub 3}

Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO{sub 3} over a temperature range of 20–300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a Néel transition at T{sub N} ∼ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below T{sub N}. Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near T{sub N}. An additional phonon anomaly was identified at T{sup *} ∼ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman frequencies between T{sub N} and T{sup *} suggest competing WFM and AFM interactions. A close match between the phonon frequency of B{sub 3g} (3)-octahedral rotation mode with the square of sublattice magnetization between T{sub N} and T{sup *} is indicative of the presence of spin-phonon coupling in multiferroic YCrO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Physics of the multi-functionality of lanthanum ferrite ceramics

In the present work, we have illustrated the physics of the multifunctional characteristics of nano-crystalline LaFeO{sub 3} powder prepared using auto-combustion synthesis. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. The temperature dependence of dielectric constant of pure LaFeO{sub 3} exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. The dielectric relaxation of LaFeO{sub 3} correlates well with small polaron conduction. The occurrence of polarization hysteresis in LaFeO{sub 3} (with centro-symmetric Pnma space group) is thought to be spin current induced type. The canting of the Fe{sup 3+} spins induce weak ferromagnetism in nano-crystalline LaFeO{sub 3}. Room temperature saturation magnetization of pure LaFeO{sub 3} is reported to be 3.0 emu/g. Due to the presence of both ferromagnetic as well as polarization ordering, LaFeO{sub 3} behaves like a single phase multiferroic ceramics. The magneto-electric coupling in this system has been demonstrated through the magneto-dielectric measurements which yield about 0.8% dielectric tuning (at 10 kHz) with the application of 2 T magnetic field. As a typical application of the synthesized nano-crystalline LaFeO{sub 3} powder, we have studied its butane sensing characteristics. The efficient butane sensing characteristics have been correlated to their catalytic activity towards oxidation of butane. Through X-ray photoelectron spectroscopy analyses, we detect the surface adsorbed oxygen species on LaFeO{sub 3} surface. Surface adsorbed oxygen species play major role in their low temperature butane sensing. Finally, we have hypothesized that the desorbed H{sub 2}O and O{sub 2} (originate from surface adsorbed hydroxyl and oxygen) initiate the catalytic oxidative dehydrogenation of n-butane resulting in weakening of the electrostatics of the gas molecules.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Exotic exchange bias at epitaxial ferroelectric-ferromagnetic interfaces

Multiferroics in spintronics have opened up opportunities for future technological developments, particularly in the field of ferroelectric (FE)-ferromagnetic (FM) oxide interfaces with functionalities. We find strong exchange bias shifts (up to 84 Oe) upon field cooling in metal-oxide (Fe/BaTiO{sub 3}) films combining FM and FE layers. The saturation magnetic moment of the FM layer is also significantly higher than in bulk (3.0 ± 0.2 μ{sub B}/atom) and the reversal mechanism occurs via a domain nucleation process. X-ray absorption spectroscopy at the Fe K-edge and Ba L3-edge indicate presence of few monolayers of antiferromagnetic FeO at the interface without the formation of any BaFeO{sub 3} layer. Polarized neutron reflectometry corroborates with our magnetization data as we perform depth profiling of the magnetic and structural densities in these bilayers. Our first principles density functional calculations support the formation of antiferromagnetic FeO layers at the interface along with an enhancement of Fe magnetic moments in the inner ferromagnetic layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A low temperature nonlinear optical rotational anisotropy spectrometer for the determination of crystallographic and electronic symmetries

Nonlinear optical generation from a crystalline material can reveal the symmetries of both its lattice structure and underlying ordered electronic phases and can therefore be exploited as a complementary technique to diffraction based scattering probes. Although this technique has been successfully used to study the lattice and magnetic structures of systems such as semiconductor surfaces, multiferroic crystals, magnetic thin films, and multilayers, challenging technical requirements have prevented its application to the plethora of complex electronic phases found in strongly correlated electron systems. These requirements include an ability to probe small bulk single crystals at the μm length scale, a need for sensitivity to the entire nonlinear optical susceptibility tensor, oblique light incidence reflection geometry, and incident light frequency tunability among others. These measurements are further complicated by the need for extreme sample environments such as ultra low temperatures, high magnetic fields, or high pressures. In this review we present a novel experimental construction using a rotating light scattering plane that meets all the aforementioned requirements. We demonstrate the efficacy of our scheme by making symmetry measurements on a μm scale facet of a small bulk single crystal of Sr{sub 2}IrO{sub 4} using optical second and third harmonic generation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Multifunctionalities driven by ferroic domains

Considerable attention has been paid to ferroic systems in pursuit of advanced applications in past decades. Most recently, the emergence and development of multiferroics, which exhibit the coexistence of different ferroic natures, has offered a new route to create functionalities in the system. In this manuscript, we step from domain engineering to explore a roadmap for discovering intriguing phenomena and multifunctionalities driven by periodic domain patters. As-grown periodic domains, offering exotic order parameters, periodic local perturbations and the capability of tailoring local spin, charge, orbital and lattice degrees of freedom, are introduced as modeling templates for fundamental studies and novel applications. We discuss related significant findings on ferroic domain, nanoscopic domain walls, and conjunct heterostructures based on the well-organized domain patterns, and end with future prospects and challenges in the field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The emergence of magnetic properties in (Pb{sub 0.845}Sm{sub 0.08}Fe{sub 0.035})(Ti{sub 0.98}Mn{sub 0.02})O{sub 3} and (Pb{sub 0.88}Nd{sub 0.08})(Ti{sub 0.98}Mn{sub 0.02})O{sub 3} perovskite ceramics

The emergence of magnetic properties and quadratic magnetoelectric coupling in a material with outstanding ferroelectric properties has been pursued. Thus, the multiferroicity driven by the substitution of rare earth R{sup 3+} ions (R = Sm, Nd) on the A-site of PbTiO{sub 3} perovskite ceramics has been investigated. In some samples, a transition element with large ionic radius, like Fe{sup 2+}, has been also added on the A site. Polycrystalline ceramic samples with composition (Pb{sub 1-3x/2-y}R{sub x}Fe{sub y}) (Ti{sub 0.98}Mn{sub 0.02})O{sub 3} (R = Sm; x = 0.08, y = 0.035) and (Pb{sub 1−3x/2}R{sub x})(Ti{sub 0.98}Mn{sub 0.02})O{sub 3} (R = Nd, x = 0.08) have been prepared by conventional solid state ceramic processing, starting from reagent grade oxide powders. X-ray diffraction analysis confirmed the obtaining of a crystalline phase with tetragonal P4 mm symmetry. Transmission electron microscopy and high resolution transmission electron microscopy investigations have been carried out in order to analyze the local structure. Temperature dependence of dielectric permittivity revealed a strong anomaly at the temperature T{sub c} of the paraelectric-ferroelectric phase transition. Transition temperature values depend on the R ion type and are lower than in pure lead titanate by 80–100 K. A broad anomaly of dielectric permittivity is found below 140–180 K, where magnetization also increases due to the quadratic magnetoelectric coupling between ferroelectric and magnetic states. For the (Pb{sub 0.845}Sm{sub 0.08}Fe{sub 0.035})(Ti{sub 0.98}Mn{sub 0.02})O{sub 3} composition, the piezoelectric charge coefficient at room temperature was d{sub 31} ∼−6.5 pC/N and the remanent magnetization M{sub r} at room temperature was about 0.1 emu/g.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Photovoltaic properties of Aurivillius phase Bi{sub 5}FeTi{sub 3}O{sub 15} thin films grown by pulsed laser deposition

We report a remarkable photovoltaic effect in pulsed laser deposited multiferroic aurivillius phase Bi{sub 5}FeTi{sub 3}O{sub 15} (BFTO) thin films sandwiched between ZnO:Al transparent conductive oxide top electrode and SrRuO{sub 3} bottom electrode fabricated on amorphous fused silica substrates. The structural and micro structural properties of these films were analysed by X-ray diffraction and atomic force microscopy techniques. The films were showing a photo sensitive ferroelectric behaviour with a notable apparent polarization in the range of 10–15 μC/cm{sup 2}. These films also exhibited a switchable photo-response and this parameter was observed to be sensitive to polarisation field and the polarization direction. The device shows a large ON/OFF photo current ratio with an open circuit voltage of 0.14 V. The photo response at zero bias of this BFTO based heterostructures showed rapid increase to a saturation value of 6 μA at zero bias.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic and charge ordering in nanosized manganites

Perovskite manganites exhibit a wide range of functional properties, such as colossal magneto-resistance, magnetocaloric effect, multiferroic property, and some interesting physical phenomena including spin, charge, and orbital ordering. Recent advances in science and technology associated with perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. The nanoscale perovskite manganites display novel magnetic and electronic properties that are different from their bulk and film counterparts. Understanding the size effects of perovskite manganites at the nanoscale is of importance not only for the fundamental scientific research but also for developing next generation of electronic and magnetic nanodevices. In this paper, the current understanding and the fundamental issues related to the size effects on the magnetic properties and charge ordering in manganites are reviewed, which covers lattice structure, magnetic and electronic properties in both ferromagnetic and antiferromagnetic based manganites. In addition to review the literatures, this article identifies the promising avenues for the future research in this area.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Origin of superstructures in (double) perovskite thin films

We have investigated the origin of superstructure peaks as observed by X-ray diffraction of multiferroic Bi(Fe{sub 0.5}Cr{sub 0.5})O{sub 3} thin films grown by pulsed laser deposition on single crystal SrTiO{sub 3} substrates. The photon energy dependence of the contrast between the atomic scattering factors of Fe and Cr is used to rule out a chemically ordered double perovskite Bi{sub 2}FeCrO{sub 6} (BFCO). Structural calculations suggest that the experimentally observed superstructure occurs due to unequal cation displacements along the pseudo-cubic [111] direction that mimic the unit cell of the chemically ordered compound. This result helps to clarify discrepancies in the correlations of structural and magnetic order reported for Bi{sub 2}FeCrO{sub 6}. The observation of a superstructure in itself is not a sufficient proof of chemical order in double perovskites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ferroelectric-ferromagnetic multilayers: A magnetoelectric heterostructure with high output charge signal

Multiferroic composites and heterostructures comprising ferroelectric and ferromagnetic materials exhibit room-temperature magnetoelectric (ME) effects greatly exceeding those of single-phase magnetoelectrics known to date. Since these effects are mediated by the interfacial coupling between ferroic constituents, the ME responses may be enhanced by increasing the density of interfaces and improving their quality. A promising material system providing these features is a ferroelectric-ferromagnetic multilayer with epitaxial interfaces. In this paper, we describe theoretically the strain-mediated direct ME effect exhibited by free-standing multilayers composed of single-crystalline ferroelectric nanolayers interleaved by conducting ferromagnetic slabs. Using a nonlinear thermodynamic approach allowing for specific mechanical boundary conditions of the problem, we first calculate the polarization states and dielectric properties of ferroelectric nanolayers in dependence on the lattice mismatch between ferroic constituents and their volume fractions. In these calculations, the ferromagnetic component is described by a model which combines linear elastic behavior with magnetic-field-dependent lattice parameters. Then the quasistatic ME polarization and voltage coefficients are evaluated using the theoretical strain sensitivity of ferroelectric polarization and measured effective piezomagnetic coefficients of ferromagnets. For Pb(Zr₀.₅Ti₀.₅)O₃-FeGaB and BaTiO₃-FeGaB multilayers, the ME coefficients are calculated numerically as a function of the FeGaB volume fraction and used to evaluate the output charge and voltage signals. It is shown that the multilayer geometry of a ferroelectric-ferromagnetic nanocomposite opens the way for a drastic enhancement of the output charge signal. This feature makes biferroic multilayers advantageous for the development of ultrasensitive magnetic-field sensors for technical and biomedical applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} ferroelectrics

Structural phase transition, narrow band gap (E{sub g}), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it is noted that KBNNO with compositions x = 0.1–0.3 have quite narrow E{sub g} of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO{sub 3} (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetism of the samples develops from diamagnetism to ferromagnetism and paramagnetism, originating from the ferromagnetic–antiferromagnetic competition. These results are helpful in the deeper understanding of phase transitions, band gap tunability, and magnetism variations in perovskite oxides and show the potential role, such materials can play, in perovskite solar cells and multiferroic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetization-steps in Y₂CoMnO₆ double perovskite: The role of antisite disorder

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y²CoMnO₆ which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T c ≈ 75 K. At 2 K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H c ≈ 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8 K. In the temperature range 2 K ≤ T ≤ 5 K, the hysteresis loops are anomalous as the virgin curve lies outside the main loop. The field-cooling conditions as well as the rate of field-sweep are found to influence the steps. Quantitative analysis of the neutron diffraction data shows that at room temperature, Y₂CoMnO₆ consists of 62% of monoclinic P2₁/n with nearly 70% antisite disorder and 38% Pnma. The bond valence sums indicate the presence of other valence states for Co and Mn which arise from disorder. We explain the origin of steps by using a model for pinning of magnetization at the antiphase boundaries created by antisite disorder. The steps in magnetization closely resemble the martensitic transformations found in intermetallics and display first-order characteristics as revealed in the Arrott's plots.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Determination of effective mechanical properties of a double-layer beam by means of a nano-electromechanical transducer

We investigate the mechanical properties of a doubly clamped, double-layer nanobeam embedded into an electromechanical system. The nanobeam consists of a highly pre-stressed silicon nitride and a superconducting niobium layer. By measuring the mechanical displacement spectral density both in the linear and the nonlinear Duffing regime, we determine the pre-stress and the effective Young's modulus of the nanobeam. An analytical double-layer model quantitatively corroborates the measured values. This suggests that this model can be used to design mechanical multilayer systems for electro- and optomechanical devices, including materials controllable by external parameters such as piezoelectric, magnetostrictive, or in more general multiferroic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Evidence of improved ferroelectric phase stabilization in Nd and Sc co-substituted BiFeO{sub 3}

Polycrystalline BiFeO{sub 3} and Bi{sub 0.9}Nd{sub 0.1}Fe{sub 1−x}Sc{sub x}O{sub 3} (x = 0, 0.05, and 0.10) multiferroic compounds were prepared using conventional solid-state route. X-ray diffraction studies and Raman measurements indicated that the compounds were crystallized in rhombohedral structure with R3c space group. Weak ferromagnetism was induced due to the suppression of canted spin structure in the substituted compounds. Both remanent magnetization (M{sub r}) and coercive field (H{sub c}) were enhanced in Nd and Sc substituted compounds. Further, Néel temperature T{sub N} was decreased from 644 K for BiFeO{sub 3} to 550 K for Bi{sub 0.9}Nd{sub 0.1}Fe{sub 0.9}Sc{sub 0.1}O{sub 3} compound due to weakening of magnetic exchange interactions between B-site cations in the substituted compounds. Enhanced and nearly well saturated electrical polarizations were observed in Sc substituted compounds, which are attributed to the strengthening of covalent hybridization between Bi and O ions and reduction in oxygen vacancies. The remnant polarization was enhanced to 12.5 μC/cm{sup 2} in Bi{sub 0.9}Nd{sub 0.1}Fe{sub 0.9}Sc{sub 0.1}O{sub 3} compound. Impedance studies revealed that insulating character of substituted compounds is enhanced and electrical relaxations are of non-Debye type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Room temperature p-type conductivity and coexistence of ferroelectric order in ferromagnetic Li doped ZnO nanoparticles

Memory and switching devices acquired new materials which exhibit ferroelectric and ferromagnetic order simultaneously. We reported multiferroic behavior in Zn{sub 1−y}Li{sub y}O(0.00≤y≤0.10) nanoparticles. The analysis of transmission electron micrographs confirmed the hexagonal morphology and wurtzite crystalline structure. We investigated p-type conductivity in doped samples and measured hole carriers in range 2.4 × 10{sup 17}/cc to 7.3 × 10{sup 17}/cc for different Li contents. We found that hole carriers are responsible for long range order ferromagnetic coupling in Li doped samples. Room temperature ferroelectric hysteresis loops were observed in 8% and 10% Li doped samples. We demonstrated ferroelectric coercivity (remnant polarization) 2.5 kV/cm (0.11 μC/cm{sup 2}) and 2.8 kV/cm (0.15 μC/cm{sup 2}) for y = 0.08 and y = 0.10 samples. We propose that the mechanism of Li induced ferroelectricity in ZnO is due to indirect dipole interaction via hole carriers. We investigated that if the sample has hole carriers ≥5.3 × 10{sup 17}/cc, they can mediate the ferroelectricity. Ferroelectric and ferromagnetic measurements showed that higher electric polarization and larger magnetic moment is attained when the hole concentration is larger and vice versa. Our results confirmed the hole dependent coexistence of ferromagnetic and ferroelectric behavior at room temperature, which provide potential applications for switchable and memory devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗