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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Pressure-induced charge-transfer and structural transition in hexagonal multiferroic HoMnO 3

The structural properties of the hexagonal multiferroic ℎ-HoMnO 3 under high pressure have been explored using synchrotron x-ray diffraction and x-ray absorption spectroscopy in diamond anvil cells. The structure was found to undergo a pressure-induced phase transition at ~24 GPa to a rhombohedrally distorted superstructure, which is isostructural to the oxygen-loaded h-RMnO 3+δ (R = Y,Dy,Ho,Er; δ ≈ 0.28) phases found in the same systems. The driving force behind the phase transition is the highly compressible ab plane which facilitates a gradual charge disproportionation of Mn(III) with pressure. We speculate this stabilizes the spin-liquid phase due to ferromagnetic coupling between neighboring Mn(II)/Mn(IV) and Mn(III). In addition, we demonstrate that the structural behavior is highly susceptible to nonhydrostatic conditions and the choice of pressure medium should be carefully made.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Charge order driven multiferroic behavior in Sr 4 Fe 6 O 12 : An ab-initio study

Here, in this paper, we report the structural, electronic, and ferroelectric properties of the layered mixed-valent transition-metal compound Sr 4 ⁢ Fe 6 ⁢O 12 (SFO). We demonstrate how SFO undergoes a phase transition from a high-temperature (𝑇) centrosymmetric tetragonal phase (𝑃⁢4 2 /𝑚⁢𝑛⁢𝑚) to a low-𝑇 polar orthorhombic phase (𝑃⁢𝑚⁢𝑛⁢2 1 ). The transition is primarily driven by charge ordering at tetrahedral Fe-layer creating Fe 3+ and Fe 2+ cations between two edge-sharing tetrahedra. This charge ordering induces electronic polarization, which is remarkably larger (3.5 times) in magnitude than ionic polarization and oppositely directed, giving a net polarization of 0.05 C/m 2 which is comparable to the state-of-the-art rare-earth nickelets and manganite perovskites. The direction of structural distortion, governed by the polar mode irrep $Γ^−_5$, depends sensitively on the type of magnetic ordering in the Fe-octahedral layer. Consequently, both the ionic and electronic polarization directions are influenced by magnetic ordering, suggesting the potential for multiferroic behavior with strong magnetoelectric coupling in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Terahertz spin-orbital excitations in the paramagnetic state of multiferroic Sr 2 FeSi 2 O 7

Here, we studied the novel multiferroic material Sr 2 FeSi 2 O 7 and found three absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal-field split 3d 6 Fe 2+ ground-state term in this material. Consideration of the compressed tetrahedral environment of the Fe 2+ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic excitations in the bulk multiferroic two-dimensional triangular lattice antiferromagnet ( Lu , Sc ) FeO 3

Noncollinear two-dimensional triangular lattice antiferromagnets (2D TLAFs) are currently an area of very active research due to their unique magnetic properties, which lead to nontrivial quantum effects that experimentally manifest themselves in the spin excitation spectra. Recent examples of such insulating 2D TLAFs include (Y, Lu) MnO 3 , LiCrO 2 , and CuCrO 2 . Hexagonal LuFeO 3 is a recently synthesized 2D TLAF which exhibits properties of an ideal multiferroic material, partially because of the high spin (S = 5/2) and strong magnetic superexchange interactions. We report the full range of spin dynamics in a bulk single crystal of (Lu 0.6 Sc 0.4 )FeO 3 (Sc doping to stabilize the hexagonal structure) measured via time-of-flight inelastic neutron scattering. Modeling with linear spin-wave theory yields a nearest-neighbor exchange coupling of J = 4.0(2) meV (density functional theory calculations for h – LuFeO 3 predicted a value of 6.31 meV) and anisotropy values of K D = 0.17(1) meV (easy plane) and K A = – 0.05(1) meV (local easy axis). Furthermore, it is observed that the magnon bandwidth of the spin-wave spectra is twice as large for h – (Lu,Sc)FeO 3 as it is for h – LuMnO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Revisiting spin cycloids in multiferroic BiFeO 3

We revisit the inverse spin current model that has been previously used to explain the existence of magnetic cycloids in bulk multiferroic BiFeO 3 . Using a first-principles-based effective Hamiltonian method, and in combination with Monte Carlo simulations, we predict a magnetic phase diagram as a function of first- and second-nearest-neighbor interaction strength in the spin current model and show that, in contrast with previous understanding, both first and second nearest neighbors have to be taken into account to be in accordance with experimental findings, including the existence of type-1 and type-2 cycloids with, respectively, [1¯10] and [11¯2] propagation directions, and the cycloid-to-antiferromagnetic transition under magnetic field. Other previously unknown magnetic arrangements are found in this phase diagram. Here, the microscopic origins of all its magnetic phases are further explained in terms of the coexistence of single solutions of the spin current model having different weights (in magnitude and even sign).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetic field induced antiferromagnetic cone structure in multiferroic BiFeO 3

Neutron diffraction measurements were performed under high magnetic fields up to 17 T in a multiferroic BiFeO 3 single crystal, in which an intermediate magnetic (IM) phase has been found between the cycloid and canted antiferromagnetic phases [S. Kawachi et al., Phys. Rev. Mater. 1, 024408 (2017)]. We clearly found that the incommensurate magnetic peaks, which split perpendicular to the magnetic field in the cycloid phase, rotate by 90 deg to align parallel to the field in the IM phase. The magnetic structure in the IM phase can be best described by an antiferromagnetic cone (AF cone) structure. The transition from the cycloid to AF cone is of first order and the direction of the magnetic wave vector and the easy plane of the cycloidal component are rotated by 90 deg without changing the cycloidal modulation period, whereas the transition from the AF cone to canted antiferromagnetic phase is gradual and the cone angle becomes smaller gradually without changing the modulation period. Interestingly, the cycloidal component as well as the cone angle in the IM phase shows a large hysteresis between the field increasing and decreasing processes. Finally, this result, combined with the magnetostriction with a large hysteresis previously reported in the IM phase, suggests a strong magnetoelastic coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Sawtooth lattice multiferroic BeCr 2 O 4 : Noncollinear magnetic structure and multiple magnetic transitions

Noncollinear magnetic structures and multiple magnetic phase transitions in a sawtooth lattice antiferromagnet consisting of Cr 3+ are experimentally identified in this work, thereby proposing the scenario of magnetism-driven ferroelectricity in a sawtooth lattice. The title compound, BeCr 2 O 4 , displays three magnetic phase transitions at low temperatures—at T N1 ≈ 7.5 K, at T N2 ≈ 25 K, and at T N3 ≈ 26 K—revealed through magnetic susceptibility, specific heat, and neutron diffraction in this work. These magnetic phase transitions are found to be influenced by externally applied magnetic fields. Isothermal magnetization curves at low temperatures below the magnetic transitions indicate the antiferromagnetic nature of BeCr 2 O 4 with two spin-flop-like transitions occurring at H c1 ≈ 29 kOe and H c2 ≈ 47 kOe. Our high-resolution x-ray and neutron diffraction studies, performed on single crystal and powder samples, unambiguously determined the crystal structure as orthorhombic Pbnm. By performing the magnetic superspace group analysis of the neutron diffraction data at low temperatures, the magnetic structure in the temperature range T N3,N2 < T < T N1 is determined to be the polar magnetic space group P21nm. 1' (00g) 0s0s with a cycloidal magnetic propagation vector k 1 = (0, 0, 0.090 (1)) . The magnetic structure in the newly identified phase below T N1 is determined as P21/b.1' [b] (00 g) 00s with the magnetic propagation vector k 2 = (0, 0, 0.908 (1)). Finally, the cycloidal spin structure determined in our work is usually associated with electric polarization, thereby making BeCr 2 O 4 a promising multiferroic belonging to the sparsely populated family of sawtooth lattice antiferromagnets.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Epitaxial Stabilization of Single-Crystal Multiferroic YCrO 3 Thin Films

We report on the growth of stoichiometric, single-crystal YCrO 3 epitaxial thin films on (001) SrTiO 3 substrates using pulsed laser deposition. X-ray diffraction and atomic force microscopy reveal that the films grew in a layer-by-layer fashion with excellent crystallinity and atomically smooth surfaces. Magnetization measurements demonstrate that the material is ferromagnetic below 144 K. The temperature dependence of dielectric permittivity shows a characteristic relaxor-ferroelectric behavior at T C = 375–408 K. A dielectric anomaly at the magnetic transition temperature indicates a close correlation between magnetic and electric order parameters in these multiferroic YCrO 3 films. These findings provide guidance to synthesize rare-earth, chromite-based multifunctional heterostructures and build a foundation for future studies on the understanding of magnetoelectric effects in similar material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Photocurrent Spectroscopy of Dark Magnetic Excitons in 2D Multiferroic NiI 2

Abstract Two‐dimensional (2D) antiferromagnetic (AFM) semiconductors are promising components of opto‐spintronic devices due to terahertz operation frequencies and minimal interactions with stray fields. However, the lack of net magnetization significantly limits the number of experimental techniques available to study the relationship between magnetic order and semiconducting properties. Here, they demonstrate conditions under which photocurrent spectroscopy can be employed to study many‐body magnetic excitons in the 2D AFM semiconductor NiI 2 . The use of photocurrent spectroscopy enables the detection of optically dark magnetic excitons down to bilayer thickness, revealing a high degree of linear polarization that is coupled to the underlying helical AFM order of NiI 2 . In addition to probing the coupling between magnetic order and dark excitons, this work provides strong evidence for the multiferroicity of NiI 2 down to bilayer thickness, thus demonstrating the utility of photocurrent spectroscopy for revealing subtle opto‐spintronic phenomena in the atomically thin limit.

Lebedev, Dmitry↗
DOE OSTI2026

On the Ordering Mechanism of Cu + in 2D van der Waals Multiferroic CuCrP 2 S 6

CuCrP 2 S 6 is a van der Waals multiferroic where the tunable Cu + sublattice underpins its exceptional ferroelectric and electronic switching properties. Yet, the microscopic mechanism governing Cu + ordering has remained elusive. Here, we combine single-crystal X-ray and neutron diffraction with pair distribution function analysis to uncover a temperature-driven evolution of Cu + ordering, giving rise to an incommensurate quasi-antipolar phase between the paraelectric and antiferroelectric states. The modulation originates from correlated Cu + occupancy redistribution coupled to breathing distortion of surrounding S 3 triangles, establishing a symmetry-adapted lattice distortion mode. Diffuse scattering persisting over 35 K above the transition confirms that the structural instability follows an order-disorder mechanism. The spontaneous off-centering of Cu + positions CuCrP 2 S 6 as a model platform for correlated order-disorder phenomena in 2D layered ferroics, and provides design principles for next-generation memory and logic devices.

ferroelectrics↗
DOE OSTI2015

Octonary resistance states in La 0.7 Sr 0.3 MnO 3 /BaTiO 3 /La 0.7 Sr 0.3 MnO 3 multiferroic tunnel junctions

General drawbacks of current electronic/spintronic devices are high power consumption and low density storage. A multiferroic tunnel junction (MFTJ), employing a ferroelectric barrier layer sandwiched between two ferromagnetic layers, presents four resistance states in a single device and therefore provides an alternative way to achieve high density memories. Here, an MFTJ device with eight nonvolatile resistance states by further integrating the design of noncollinear magnetization alignments between the ferromagnetic layers is demonstrated. Through the angle-resolved tunneling magnetoresistance investigations on La 0.7 Sr 0.3 MnO 3 /BaTiO 3 /La 0.7 Sr 0.3 MnO 3 junctions, it is found that, besides collinear parallel/antiparallel magnetic configurations, the MFTJ shows at least two other stable noncollinear (45° and 90°) magnetic configurations. As a result, combining the tunneling electroresistance effect caused by the ferroelectricity reversal of the BaTiO 3 barrier, an octonary memory device is obtained, representing potential applications in high density nonvolatile storage in the future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

AFe{sub 2}O{sub 4}/(Pb{sub 0.80}Sr{sub 0.20})TiO{sub 3} (A = Mn, Ni and Co): a New Room-Temperature Magnetoelectric Multiferroic Bi-layered Composite Films

The room temperature and magnetic field-dependent dielectric, impedance and magnetoelectric (ME) coupling effect of polycrystalline AFe{sub 2}O{sub 4}/(Pb{sub 0.80}Sr{sub 0.20})TiO{sub 3} (A = Mn, Ni and Co) bi-layered composite films have been investigated. The structural and microstructural analyses using the X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) reveal the presence of homogenous growth of both tetragonal and spinel phases without any extra phase and diffusion in the AFO/PST20 bi-layered composite films. Our results show that all composite films exhibit ferroelectric as well as considerable magnetic, indicating magnetoelectric coupling effect. Our results show that the dielectric and impedance properties of AFO/PST20 bi-layered composite films can be manipulated by the magnetic field at room temperature, also indicating the existence of magnetoelectric coupling. The impedance (Z{sup ′} and Z{sup ″}) Nyquist plots show distinct electrical responses with the magnetic field. The maximum magnetoelectric coefficient (α) is found to be α{sub ME}∼ 239 and ∼ 195 mV/cm/Oe for the MFO/PST20 and CFO/PST20 bi-layered composite films, respectively. The above results show that the AFO/PST20 bi-layered composite films are room-temperature multiferroic material that can be potentially used in magnetoelectric devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Weak Ferroelectricity and Leakage Current Behavior of Multiferroic CoCr{sub 2}O{sub 4} Nanomaterials

Multiferroic CoCr{sub 2}O{sub 4} is synthesized by low-temperature sol-gel auto combustion technique. High energy synchrotron x-ray diffraction pattern confirms the single-phase cubic structure with space group Fd3m. Average crystallite size (17.91 nm) and negative micro-strain (9.86 × 10{sup −4}) are estimated by Williamson–Hall (W-H) plot. FTIR confirms the formation of spinel metal oxide-based cobalt chromites. The strong decrease in ε′ and tan δ at higher frequency can be interpreted by Maxwell-Wagner-type interfacial polarization. Weak ferroelectricity is mainly attributed to the partial reversal of polarization. J-E characteristic reveals the Ohmic (I–V) and Child’s square law (I–V{sup 2}) behavior observed in CoCr{sub 2}O{sub 4} nanomaterials with slope value ~ 1.04 and ~ 1.6, respectively. The conduction process for the leakage current density is interpreted using the space-charge limited current (SCLC) mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Extended temperature regions of multiferroicity in nanoscale CuO

We have measured the magnetic susceptibility and heat capacity of CuO nanoparticles (16 nm) from 2 K to 400 K using a Quantum Design Physical Properties Measurement System (PPMS). The magnetization curves, acquired at various field strengths from 0 Oe to 50 kOe, are similar to literature data of bulk and nanoscale CuO showing a minimum at about 150 K, an upturn as temperature approaches 0 K, and a broad maximum at high temperatures extending beyond 400 K. The heat capacity data between 200 K and 400 K show several broad peaks. The number of these peaks and the temperatures at which they occur differ significantly from the magnetic transitions known to exist in bulk CuO. To further investigate these transitions in nanoscale CuO, we performed temperature dependent x-ray diffraction (XRD) at temperatures from 90 K to 700 K from which lattice parameters as a function of temperature were derived using a Rietveld refinement. Although no phase transitions were observed in these data, changes in the slopes of the lattice parameters are apparent at the transition temperatures observed in the heat capacity and susceptibility measurements. The various transitions in the heat capacity data are attributed to competing ferromagnetic and antiferromagnetic interactions caused by structural properties that are unique to nanophase CuO, and the temperature range of multiferroicity in nanoscale CuO is shown to extend to temperatures higher than those observed for bulk CuO. The heat capacity and temperature dependent XRD measurements are the first of their kind to be reported for CuO nanoparticles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

63,65 Cu NMR study of the magnetically ordered state of the multiferroic CuFeO 2

In this work, field-swept 63,65 Cu NMR spectra under magnetic fields up to 8.3 T at a constant NMR frequency and temperatures T < 12 K on a single crystalline sample of multiferroic CuFeO 2 were measured and analyzed. When the magnetic field is applied along the c axis, a nearly zero internal magnetic field at the Cu site in magnetic ordered state was observed. This is explained by the perfect cancellation of the internal fields produced by the 6 nearest neighbor Fe 3+ ( S = 5/2) ions, revealing the magnetic structure to be a collinear four-sublattice structure. On the other hand, when the magnetic field is applied along the ab plane, we observed a finite internal field at the Cu sites, which is due to the canting of the Fe moments. Strong change in the NMR signal intensity is observed around 7–8 T, corresponding to the magnetic phase transition from the collinear magnetic to ferroelectric incommensurate states. The ratio of the two magnetic phases significantly depends on the history of the change in the external magnetic field and the temperature of the sample. The details of history dependence of the ratio were discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of Al{sup 3+} substitution on the structural, magnetic, and electric properties in multiferroic Bi{sub 2}Fe{sub 4}O{sub 9} ceramics

Structural, magnetic, and electric properties have been investigated in polycrystalline Bi{sub 2}(Fe{sub 1−x}Al{sub x}){sub 4}O{sub 9} (0≤x≤0.25) ceramics synthesized by a modified Pechini method. Structural analysis reveals that Al{sup 3+} doped Bi{sub 2}Fe{sub 4}O{sub 9} crystallizes in orthorhombic structure with Pbnm space group. Surface morphology of the end products is examined by scanning electron microscopy and the grain size has a tendency to decrease with increase in Al{sup 3+} doping level. Compared with pure Bi{sub 2}Fe{sub 4}O{sub 9}, room temperature coexistent multiferroic-like behavior is observed in Al{sup 3+} doped Bi{sub 2}Fe{sub 4}O{sub 9}. By analyzing magnetic properties, the Néel temperature monotonously shifts to low temperatures from ~260 K (x=0) to ~35 K (x=0.25). Moreover, the spin dynamic measured by the shift in ac magnetic susceptibility as a function of frequency provides a possibility of spin-glass-like behavior, which is further confirmed by fitting the critical slowing down power law and memory effect. - Graphical abstract: Compared with pure Bi{sub 2}Fe{sub 4}O{sub 9}, room temperature weak ferromagnetic property and enhanced ferroelectric-like behavior can be achieved simultaneously with proper Al{sup 3+} doping. - Highlights: • Bi{sub 2}(Fe{sub 1−x}Al{sub x}){sub 4}O{sub 9} (0≤x≤0.25) ceramics are fabricated via a Pechini method. • Weak ferromagnetic and ferroelectric behaviors can be achieved simultaneously. • Spin-glass-like behavior is detected with proper Al{sup 3+} doping. • The memory and aging effects are observed with proper Al{sup 3+} doping.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Room temperature multiferroic properties of (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film

Graphical abstract: - Highlights: • Chemical solution deposition of (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}–NiFe{sub 2}O{sub 4} double layered thin film. • Studies on structural, electrical and multiferroic properties. • NiFe{sub 2}O{sub 4} acts as both resistive buffer layer and magnetic source. - Abstract: (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film was prepared on a Pt(111)/Ti/SiO{sub 2}/Si(100) substrate by a chemical solution deposition method. X-ray diffraction and Raman scattering spectroscopy studies confirmed the formation of the distorted rhombohedral perovskite and the inverse spinel cubic structures for the (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film. The (Bi{sub 0.95}La{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3}/NiFe{sub 2}O{sub 4} double layered thin film exhibited well saturated ferromagnetic (2 M{sub r} of 18.1 emu/cm{sup 3} and 2H{sub c} of 0.32 kOe at 20 kOe) and ferroelectric (2P{sub r} of 60 μC/cm{sup 2} and 2E{sub c} of 813 kV/cm at 866 kV/cm) hysteresis loops with low order of leakage current density (4.5 × 10{sup −6} A/cm{sup 2} at an applied electric field of 100 kV/cm), which suggest the ferroelectric and ferromagnetic multi-layers applications in real devices.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thin films sputtered from Ba{sub 2}NdFeNb{sub 4}O{sub 15} multiferroic targets on BaFe{sub 12}O{sub 19} coated substrates

Highlights: • Synthesis of Ba{sub 2}NdFeNb{sub 4}O{sub 15}/BaFe{sub 12}O{sub 19} (BaM) heterostructures by RF magnetron sputtering. • Growth of TTB layer were retained regardless of the underlayer (Pt bottom electrode or BaM). • Dielectric and magnetic properties were obtained from the Pt/TTB/BaM/Pt stacks. - Abstract: Ba{sub 2}NdFeNb{sub 4}O{sub 15} tetragonal tungsten bronze (TTB)/BaFe{sub 12}O{sub 19} (BaM) hexaferrite bilayers have been grown by RF magnetron sputtering on Pt/TiO{sub 2}/SiO{sub 2}/Si (PtS) substrates. The BaM layer is textured along (0 0 1) while the TTB layer is multioriented regardless of the PtS or BaM/PtS substrate. Dielectric properties of TTB films are similar to those of bulk, i.e., ε ∼ 150 and a magnetic hysteresis loop is obtained from TTB/BaM bilayers, thanks to the BaM component. This demonstrates the possibility of transferring to 2 dimensional structures the composite multiferroic system TTB/BaM previously identified in 3 dimensional bulk ceramics.

36 MATERIALS SCIENCE↗