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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Neutron diffraction, Mössbauer effect and electron paramagnetic resonance studies on multiferroic Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3}

Multiferroic Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} ceramics were synthesized via a modified two-stage Columbite method. Single phase formation was confirmed from the analysis of x-ray and neutron diffraction patterns recorded at room temperature. Structural analysis of the diffraction data reveals cubic phase (space group Pm-3m) for the title compound. Magnetic structure of the title compound at room temperature exhibits G-type antiferromagnetic structure. The Mössbauer spectroscopy and Electron Paramagnetic Resonance (EPR) studies were carried out at 300 K. The isomer shift and quadrupole splitting of the Mössbauer spectra confirms the trivalent state of iron (Fe{sup 3+}). The Mössbauer spectra also suggest that the iron and tungsten are randomly distributed at the octahedral, B site. EPR spectra show a single broad line associated with Fe{sup 3+} ions. Both spectra clearly exhibit weak ferromagnetic behaviour of Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} ceramic at 300 K. Considering neutron diffraction, Mössbauer and EPR results together, it may be stated here that Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} exhibits antiferromagnetic behavior along with weak ferromagnetism at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin reorientation in multiferroic spinel Co{sub 0.5}Fe{sub 0.5}Cr{sub 2}O{sub 4} with Mössbauer spectroscopy

We studied the magnetic properties of multiferroic spinel Co{sub 0.5}Fe{sub 0.5}Cr{sub 2}O{sub 4}, especially focusing on the spin-ordering, spin-reorientation, and charge re-distribution effects. From the Rietveld refinement analysis, the crystal structure was identified to be a normal cubic spinel of Fd-3m. Based on the temperature-dependent measurements of the magnetization and magnetic hyperfine field (H{sub hf}), Curie temperature (T{sub C}) of Co{sub 0.5}Fe{sub 0.5}Cr{sub 2}O{sub 4} was determined to be around 86 K where H{sub hf} ≅ 0. From the change of slope in the M-T curve, the conical-spiral magnetic ordering temperature was determined to be 20 K, which coincides with H{sub hf} measurement. Also, we have observed the decrease in the slope of the electric quadrupole splitting (ΔE{sub Q}) curve above 20 K, suggesting that the change in ΔE{sub Q} around T{sub S} is originated from charge redistribution due to the spin-relocation associated with the distortion of each tetrahedral site around Fe{sup 2+} ion above T{sub S}. This indicates that Co{sub 0.5}Fe{sub 0.5}Cr{sub 2}O{sub 4} has the noncollinear conical-spiral spin ordering with incommensurate spin structure below T{sub S}, while above T{sub S}, it has ferrimagnetic spin ordering with commensuration in the collinear state. In addition, the Jahn-teller distortion temperature is measured to be around 155 K, since both ΔE{sub Q} and Δ{sub 1} of {sup 5}T{sub 2g} band decrease rapidly with increasing temperature and disappear around 155 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

On the thermodynamic efficiency of a nickel-based multiferroic thermomagnetic generator: From bulk to atomic scale

A model is developed to correlate the effects of size on the thermodynamic efficiency for a nickel-based multiferroic thermomagnetic generator device. Three existing models are combined in order to estimate this correlation, they are (1) thermodynamic efficiency relations, (2) a model of ferromagnetic transition behavior, and (3) the bond-order length strength correlation. At the smallest size considered, a monolayer of nickel atoms shows a reduction in Curie temperature from its bulk value of T{sub c,Bulk}=630 K to T{sub c,ML}=240 K. This difference is analytically shown to affect the thermodynamic efficiency values when compared to bulk. Various nickel nanofilms are considered as a working body, such that the combined model predicts relative efficiency values that are comparable to the bulk scale, but operating closer to room-temperature when compared to bulk form. This result is unexpected since the absolute efficiency is shown to increase as a function of decreasing size, this discrepancy is explained as a consequence of Curie point suppression. The combined model is also applied to a hypothetical composite made of separated layers of nickel with distinct thicknesses. This composite material is predicted to spread the ferromagnetic transition across a much larger temperature range as compared to bulk nickel, such that this material may be better suited for different applications; for example, as a sensor or thermal switch. Moreover, this combined model is also shown to give a lower-bound estimate for thermodynamic efficiency, since the actual performance depends on material characterizations that have yet to be determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO{sub 3}

Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO{sub 3}, have been reinvestigated in detail. The spin-reorientation transition of the Fe{sup 3+} magnetic phase and the low-temperature magnetic ordering of Er{sup 3+} were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe{sup 3+} align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Néel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe{sup 3+} moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er{sup 3+} moments align antiferromagnetically with a C{sub y} mode below 4.5 K. For the Fe{sup 3+} moments, an additional C{sub x} mode is induced by the ordering of the Er{sup 3+} moments. Namely, they change from G{sub y}F{sub z} mode into C{sub x}G{sub y}F{sub z} mode in the Pnma space-group setting. This study resolves the long-lasting dispute about the magnetic structure of ErFeO{sub 3} at low temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Antiferromagnetic ordering in spin-chain multiferroic Gd{sub 2}BaNiO{sub 5} studied by electronic spin resonance

High-field electron spin resonance (ESR) has been employed to study the antiferromagnetic (AFM) ordering state (T < T{sub N} = 55 K) of spin-chain multiferroic Gd{sub 2}BaNiO{sub 5}. The spin reorientation at T{sub SR} = 24 K is well characterized by the temperature-dependent ESR spectra. The magnetization data evidence a field-induced spin-flop transition at 2 K. The frequency-field relationship of the ESR data can be explained by conventional AFM resonance theory with uniaxial anisotropy, in good agreement with magnetization data. Related discussion on zero-field spin gap is presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetically tunable alternating current electrical properties of x La{sub 0.7}Sr{sub 0.3}MnO{sub 3}–(1 − x) ErMnO{sub 3} (x = 0.1, 0.3, and 0.5) multiferroic nanocomposite

Detailed magnetically tunable ac electrical properties of x La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (LSMO)–(1 − x) ErMnO{sub 3} (EMO) (x = 0.1, 0.3, and 0.5) multiferroic nanocomposites have been studied at 300 K in presence of varying magnetic field (H{sub appl}), applied both in parallel and perpendicular configuration with respect to the measuring electric field. AC electrical properties have exhibited significant variation with H{sub appl} for all composites, whereas for parallel configuration of H{sub appl} such effect is very feeble for x = 0.3 composite. We have attributed this anisotropic behavior to the demagnetization effect in the sample. In contrast, for x = 0.1 and 0.5 composites, no such anisotropy effect is experimentally evidenced. Impedance and real part of impedance have been found to decrease with H{sub appl} at low frequency (f) region. We attribute this observation to the depinning of the magnetic domain walls from the grain boundaries pinning centers and thereby enhancing the spin dependent transport in the composite. For x = 0.3 composite, Nyquist plots have been fitted considering dominant contributions of LSMO and EMO grain boundaries and the interface region between them. However, for x = 0.1 composite, it corresponds to EMO grain boundaries and grain boundary interface region. The relaxation frequency (f{sub R}) is observed to shift at higher/lower f region in perpendicular/parallel configuration of H{sub appl} for x = 0.3 composite. This opposite variation of f{sub R}s with H{sub appl} for perpendicular and parallel configurations has been attributed to two competing factors of H{sub appl} induced enhancement of inductive part and H{sub appl} enhanced spin dependent transport causing fast relaxation processes in the sample. For x = 0.1 composite, in both configurations of H{sub appl}, f{sub R}s is shifting towards high f region, which has been discussed in terms of dominant role of spin dependent transport.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Piezoresponse and magnetic properties of multiferroic (1−x)Bi{sub 0.9}Dy{sub 0.1}FeO{sub 3}–xPbTiO{sub 3} solid solution

The multiferroic solid solution of (1−x)[0.9BiFeO{sub 3}–0.1DyFeO{sub 3}]–xPbTiO{sub 3} with compositions around the morphotropic phase boundary has been synthesized in the form of ceramics and characterized by Piezoresponse Force Microscope (PFM) and Superconducting Quantum Interference Device. Both the original local polar domain structure and the domain evolution after poling have been studied by PFM. The PFM phase imaging has revealed some interesting details of poling and domain switching process: The out-of-plane phase image shows a uniform direction of polarization along the applied electric field, while the in-plane phase image indicates two kinds of domains with antiparallel polarizations. This kind of poled domain structure is explained based on the orientations of the polarization as permitted by the rhombohedral crystal symmetry in grains of different crystallographic orientations. The magnetic properties measured within the temperature range from 1.8 K to 300 K reveal an interesting sequence of magnetic transitions from a weakly ferromagnetic order (WFM{sub 1}) to an antiferromagnetic state (AFM), and then to another weak ferromagnetic phase (WFM{sub 2}), upon cooling. A preliminary magnetic phase diagram is proposed for BDF-34PT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Quantitative phase separation in multiferroic Bi{sub 0.88}Sm{sub 0.12}FeO{sub 3} ceramics via piezoresponse force microscopy

BiFeO{sub 3} (BFO) is a classical multiferroic material with both ferroelectric and magnetic ordering at room temperature. Doping of this material with rare-earth oxides was found to be an efficient way to enhance the otherwise low piezoelectric response of unmodified BFO ceramics. In this work, we studied two types of bulk Sm-modified BFO ceramics with compositions close to the morphotropic phase boundary (MPB) prepared by different solid-state processing methods. In both samples, coexistence of polar R3c and antipolar P{sub bam} phases was detected by conventional X-ray diffraction (XRD); the non-polar P{sub nma} or P{sub bnm} phase also has potential to be present due to the compositional proximity to the polar-to-non-polar phase boundary. Two approaches to separate the phases based on the piezoresponse force microscopy measurements have been proposed. The obtained fractions of the polar and non-polar/anti-polar phases were close to those determined by quantitative XRD analysis. The results thus reveal a useful method for quantitative determination of the phase composition in multi-phase ceramic systems, including the technologically most important MPB systems.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Polaron response dominated multiferroic property in 12R-type hexagonal Ba(Ti{sub 1/3}Mn{sub 2/3})O{sub 3-δ} ceramics

We report the multiferroic properties of 12R-type hexagonal Ba(Ti{sub 1/3}Mn{sub 2/3})O{sub 3-δ} found in Mn-doped BaTiO{sub 3} series samples. Hysteresis measurements reveal the coexistence of weak ferromagnetism and ferroelectricity at room temperature. Furthermore, frequency-driven dynamic ferroelectric phase transition is disclosed around a critical frequency of 220 Hz. Analyses on the dielectric relaxation, leakage current, crystal structure, and magnetic susceptibility lead us to conclude that the response of polarons dominates the observed physical properties, and the dynamic phase transition may ascribe to the response mode changes of the localized electrons. More importantly, we figure out the crucial factors leading to difference of the ferroelectric and magnetic properties of the 12R-type Ba(Ti{sub 1/3}Mn{sub 2/3})O{sub 3-δ} samples from that of the 6H-type Ba(Ti{sub 1-x}M{sub x})O{sub 3-δ} (M = Fe, Mn) samples.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Modified Heisenberg model for the zig-zag structure in multiferroic RMn{sub 2}O{sub 5}

The class of RMn{sub 2}O{sub 5} (R = Ho, Tb, Y, Eu) compounds offers multiferroic properties where the refined magnetic zig-zag order breaks the inversion symmetry. Varying the temperature, the system undergoes a magnetic and a subsequent ferroelectric phase transition where the ferroelectricity is magnetically induced. We propose a modified anisotropic Heisenberg model that can be used as a tractable analytical model studying the properties of those antiferromagnetic zig-zag spin chains. Based on a finite temperature Green's function method, it is shown that the polarization is induced solely by different exchange couplings of the two different Mn{sup 4+} and Mn{sup 3+} magnetic ions. We calculate the excitation energy of the spin system for finite temperatures, which for its part determines the temperature dependent magnetization and polarization. The ferroelectric phase transition is manifested as a kink in the excitation energy. The variation of the polarization by an external magnetic field depends strongly on the direction of that field. Whereas, the polarization in b-direction increases with an external magnetic field as well in b-direction it can be switched for strong fields in a-direction. The results based on that modified Heisenberg model are in qualitative agreement with experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Giant magnetoelectric coupling interaction in BaTiO{sub 3}/BiFeO{sub 3}/BaTiO{sub 3} trilayer multiferroic heterostructures

Multiferroic trilayer thin films of BaTiO{sub 3}/BiFeO{sub 3}/BaTiO{sub 3} were prepared by RF-magnetron sputtering technique at different thicknesses of BiFeO{sub 3} layer. A pure phase polycrystalline growth of thin films was confirmed from X-ray diffraction results. The film showed maximum remnant electric polarization (2P{sub r}) of 13.5 μC/cm{sup 2} and saturation magnetization (M{sub s}) of 61 emu/cc at room temperature. Thermally activated charge transport dominated via oxygen vacancies as calculated by their activation energy, which was consistent with current–voltage characteristics. Magnetic field induced large change in resistance and capacitance of grain, and grain boundary was modeled by combined impedance and modulus spectroscopy in the presence of varied magnetic fields. Presence of large intrinsic magnetoelectric coupling was established by a maximum 20% increase in grain capacitance (C{sub g}) with applied magnetic field (2 kG) on trilayer having 20 nm BiFeO{sub 3} layer. Substantially higher magnetoelectric coupling in thinner films has been observed due to bonding between Fe and Ti atoms at interface via oxygen atoms. Room temperature magnetoelectric coupling was confirmed by dynamic magnetoelectric coupling, and maximum longitudinal magnetoelectric coupling of 515 mV/cm-Oe was observed at 20 nm thickness of BiFeO{sub 3}. The observed magnetoelectric properties are potentially useful for novel room temperature magnetoelectric and spintronic device applications for obtaining higher voltage at lower applied magnetic field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Spin dynamics in driven composite multiferroics

A spin dynamics approach has been used to study the behavior of the magnetic spins and the electric pseudo-spins in a 1-D composite multiferroic chain with a linear magneto-electric coupling at the interface. The response is investigated with either external magnetic or electric fields driving the system. The spin dynamics is based on the Landau-Lifshitz-Gilbert equation. A Gaussian white noise is later added into the dynamic process to include the thermal effects. The interface requires a closer inspection of the magneto-electric effects. Thus, we construct a 2-D ladder model to describe the behavior of the magnetic spins and the electric pseudo-spins with different magneto-electric couplings.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Multicaloric effect in bi-layer multiferroic composites

The multicaloric effect was theoretically proposed in 2012 and, despite numerous follow up studies, the effect still awaits experimental confirmation. The main limitation is the fact that the multicaloric effect is only observed at a temperature equal to the transition temperature of the magnetic and electric phases coexisting within a multiferroic (MF) (i.e., T ≈ T{sub c}{sup m} ≈ T{sub c}{sup e}). Such condition is hard to fulfill in single phase MFs and a solution is to develop suitable composite MF materials. Here, we examine the multicaloric effect in a bi-layer laminated composite MF in order to determine the optimal design parameters for best caloric response. We show that magnetically induced multicaloric effect requires magnetic component of heat capacity smaller than that of the electric phase, while the layer thickness of the magnetic phase must be at least 5 times the thickness of the electric phase. The electrically induced multicaloric effect requires the magnetic layer to be 10% of the electric phase thickness, while its heat capacity must be larger than that of the electric phase. These selection rules are generally applicable to bulk as well as thin film MF composites for optimal multicaloric effect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dispersion characteristics of spin-electromagnetic waves in planar multiferroic structures

A method of approximate boundary conditions is used to derive dispersion relations for spin-electromagnetic waves (SEWs) propagating in thin ferrite films and in multiferroic layered structures. A high accuracy of this method is proven. It was shown that the spin-electromagnetic wave propagating in the structure composed of a thin ferrite film, a thin ferroelectric film, and a slot transmission line is formed as a result of hybridization of the surface spin wave in the ferrite film and the electromagnetic wave in the slot-line. The structure demonstrates dual electric and magnetic field tunability of the SEW spectrum. The electric field tunability is provided by the thin ferroelectric film. Its efficiency increases with an increase in the thicknesses of the ferrite and ferroelectric films and with a decrease in the slot-line gap width. The theory is confirmed by experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Room temperature magnetoelectric coupling in BaTi{sub 1−x}Cr{sub x}O{sub 3} multiferroic thin films

We report on room temperature (RT) magnetoelectric coupling in tetragonal BaTi{sub 1−x}Cr{sub x}O{sub 3} thin film multiferroics (BTCO) sputter deposited on (100) SrTiO{sub 3} (where x = 0.005, 0.01, 0.02, and 0.03). As-deposited thin films are vacuum annealed by electron beam rapid thermal annealing technique. 50 nm thick BTCO with “x = 0.01” shows RT ferromagnetic and ferroelectric response with saturation magnetic moment of 1120 emu/cc and polarization of 14.7 microcoulomb/cm{sup 2}. Piezoresponse/magnetic force microscope images shows RT magnetoelectric coupling in BTCO with “x = 0.01,” which is confirmed using magnetocapacitance measurement where an increase in capacitance from 17.5 pF to 18.4 pF is observed with an applied magnetic field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Dynamic response in a finite size composite multiferroic thin film

Composite multiferroics, heterostructures of ferromagnetic and ferroelectric materials, are characterized by a remarkable magnetoelectric effect at the interface. Previous work has supported the ferromagnetic structure with magnetic spins and the ferroelectric with pseudospins which act as electric dipoles in a microscopic model, coupled with a magnetoelectric interaction [Wang and Grimson, J. Appl. Phys. 118, 124109 (2015)]. In this work, by solving the stochastic Landau-Lifshitz-Gilbert equation, the electric-field-induced magnetization switching in a twisted boundary condition has been studied, and a behavior of domain wall in the ferromagnetic structure is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Self-assembly of multiferroic core-shell particulate nanocomposites through DNA-DNA hybridization and magnetic field directed assembly of superstructures

Multiferroic composites of ferromagnetic and ferroelectric phases are of importance for studies on mechanical strain mediated coupling between the magnetic and electric subsystems. This work is on DNA-assisted self-assembly of superstructures of such composites with nanometer periodicity. The synthesis involved oligomeric DNA-functionalized ferroelectric and ferromagnetic nanoparticles, 600 nm BaTiO{sub 3} (BTO) and 200 nm NiFe{sub 2}O{sub 4} (NFO), respectively. Mixing BTO and NFO particles, possessing complementary DNA sequences, resulted in the formation of ordered core-shell heteronanocomposites held together by DNA hybridization. The composites were imaged by scanning electron microscopy and scanning microwave microscopy. The presence of heteroassemblies along with core-shell architecture is clearly observed. The reversible nature of the DNA hybridization allows for restructuring the composites into mm-long linear chains and 2D-arrays in the presence of a static magnetic field and ring-like structures in a rotating-magnetic field. Strong magneto-electric (ME) coupling in as-assembled composites is evident from static magnetic field H induced polarization and low-frequency magnetoelectric voltage coefficient measurements. Upon annealing the nanocomposites at high temperatures, evidence for the formation of bulk composites with excellent cross-coupling between the electric and magnetic subsystems is obtained by H-induced polarization and low-frequency ME voltage coefficient. The ME coupling strength in the self-assembled composites is measured to be much stronger than in bulk composites with randomly distributed NFO and BTO prepared by direct mixing and sintering.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Sintering time effect on crystal structure and magnetic properties of Bi{sub 0.8}La{sub 0.2}FeO{sub 3} multiferroics

Effect of sintering time over the structure and magnetic properties has been studied in Bi{sub 0.8}La{sub 0.2}FeO{sub 3} multiferroic ceramics prepared by solid state reaction technique. The structure changes with the advent mixed phase rhombohedral and orthorhombic symmetry to immaculate orthorhombic structure with sintering time from 2 to 3 hour, as revealed by means of the simulation of XRD patterns via Rietveld analysis through FullProf software. The M – H plots depict decent enhancement in magnetization with values of remnant magnetization (Mr) from 0.01868emu/g to 0.09357emu/g while the sintering time is varied from 2 to 3 hour. The metamagnetic transition may be attributed to the crumpling of the modulated spin cycloid existing inherently in the pristine compound. The presented study may have considerable impact in commercial as well as advanced electronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗