Multiferroic Oxide Research Articles

Role of H impurity as compensating center in BiFeO3 by first-principle calculations

The effect of unintentional doping by a light element like hydrogen atom in an important multiferroic perovskite oxide, BiFeO3 has been reported. Hydrogen is the lightest element and ubiquitous in the environment and thus is quite likely to get incorporated in material during fabrication stages. Unintentional dopant like H in a material may result in modification of the electronic properties. Formation energy calculations predict that H 1+ impurity is more stable in BFO grown under oxygen-rich conditions, whereas, H1− is only stable in BFO with n-type doping. H 1− is observed to induce doubly ionizable isolated occupied states, whereas, in the case of neutral H atom these states are singly occupied. Based on first principle calculations, H atom is observed to induce isolated impurity states within the forbidden gap which acts as compensating centers for acceptor and donor type impurities.

Room temperature magnetoelectric coupling in Pb(Fe1/2Nb1/2)O3 by counterbalancing imbalanced spin moments through spin canting

In this study, ferroelectric Pb(Fe1/2Nb1/2)O3 with an antiferromagnetic polymorph at and below 150 K was converted into a room-temperature magnetoelectrically active multiferroic with soft ferromagnetism by disrupting the existing antiparallel spin alignment of Fe ions through the heavy replacement of Fe by Ni. To maximize the induced soft-ferromagnetic properties and the consequent nonlinear magnetoelectric coupling, the substitution level of Ni should be controlled such that the individual Ni ions are separated from one another to avoid mutual spin cancellation. The induced magnetoelectric coupling was found to originate from the collective contribution of oppositely canted pairs of spins in two nearby Fe3+ ions, which counterbalances the relatively smaller spin moment of the in-between Ni ions. The non-material specific nature of this strategy implies that it can be used in the development of new room-temperature multiferroic perovskite oxides.

Ferroelectricity and Ferromagnetism Achieved via Adjusting Dimensionality in BiFeO3/BiMnO3 Superlattices.

Integrating characteristics of materials through constructing artificial superlattices (SLs) has raised extensive attention in multifunctional materials. Here, we report the synthesis of BiFeO3/BiMnO3 SLs with considerable ferroelectric polarizations and tunable magnetic moments. The polarization of BiFeO3/BiMnO3 SLs presents a decent value of 12 μC/cm2, even as the dimensionality of BiFeO3 layers per period is reduced to about five-unit cells when keeping the BiMnO3 layers same. Moreover, it is found that the tunable magnetic moments of SLs are linked intimately to the dimensionality of BiFeO3 layers. Our simulations demonstrate that the superexchange interaction of Fe-O-Mn tends to be antiferromagnetic (AFM) with a lower magnetic domain formation energy rather than ferromagnetic (FM). Therefore, as the dimensionality of BiFeO3 per period is reduced, the AFM superexchange interaction between BiFeO3 and BiMnO3 in the SLs becomes weak, promoting a robust magnetization. This interlayer modulation effect in SLs presents an alluring way to accurately control the multiple order parameters in a multiferroic oxide system.

Regulation of Photovoltaic Response in ZSO-Based Multiferroic BFCO/BFCNT Heterojunction Photoelectrodes via Magnetization and Polarization

Multiferroic devices have attracted renewed attention in applications of photovoltaic devices for their efficient carrier separation driven by internal polarization, magnetization, and above-bandgap generated photovoltages. In this work, Zn2SnO4-based multiferroic Bi6Fe1.6Co0.2Ni0.2Ti3O18/Bi2FeCrO6 (BFCNT/BFCO) heterojunction photoelectrodes were fabricated. Structural and optical analyses showed that the bandgap of the spinel Zn2SnO4 is ∼3.1 eV while those of Aurivillius-type BFCNT and double-perovskite BFCO are 1.62 and 1.74 eV, respectively. Under the simulated AM 1.5G illumination, the as-prepared photoelectrodes delivered a photoconversion efficiency (η) of 3.40% with a short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) of 10.3 mA·cm-2, 0.66 V, and 50.4%, respectively. Analyses of adjustment of an applied electric and magnetic field on photovoltaic properties indicated that both magnetization and polarization of multiferroics can effectively tune the built-in electric field and the transport of charge carriers, providing a new idea for the design of future high-performance multiferroic oxide photovoltaic devices.

Resistive Switching Effect of Multiferroic Complex Oxide Solid Solution Thin Films

Resistive switching in multiferroics has attracted increasing attention due to the potential application for next-generation nonvolatile memory and could lead to forms of computing. However, the resistive switching mechanism in solid solution oxides is unclear. In this article, we have successfully fabricated binary 0.72BiTi0.27Fe0.46Mg0.27O3-0.28LaFeO3 (BTFM-LFO) and ternary 0.625BiTi0.27Fe0.46Mg0.27O3-0.25LaFeO3-0.125La2MgTiO6 (BTFM-LFO-LMT) thin films with precise component control by the spin coating method. The solid solution films exhibit obvious ferroelectricity and magnetism at room temperature. Both binary and ternary solid solution films show switchable polarization, weak magnetism, and reversible and repeatable resistive switching effects. Multistage resistive switching behavior was observed. Four resistive switching states were obtained in the binary film; the highest Ion/off is up to 106. The influence of the film composition on the resistive switching effect was discussed. It is considered that the oxide vacancy/valance exchange-induced defects play a dominant role in the resistive switching effect of complex oxide thin films. This work provides an alternative pathway to explore the resistive switching in multiferroic oxides by fabrication of complex solid solution films.

Advances in strain engineering on oxide heteroepitaxy

Advances in strain engineering on oxide heteroepitaxy

Ferroelectric Self-Polarization Controlled Magnetic Stratification and Magnetic Coupling in Ultrathin La0.67Sr0.33MnO3 Films.

Multiferroic oxide heterostructures consisting of ferromagnetic and ferroelectric components hold the promise for nonvolatile magnetic control via ferroelectric polarization, advantageous for the low-dissipation spintronics. Modern understanding of the magnetoelectric coupling in these systems involves structural, orbital, and magnetic reconstructions at interfaces. Previous works have long proposed polarization-dependent interfacial magnetic structures; however, direct evidence is still missing, which requires advanced characterization tools with near-atomic-scale spatial resolutions. Here, extensive polarized neutron reflectometry (PNR) studies have determined the magnetic depth profiles of PbZr0.2Ti0.8O3/La0.67Sr0.33MnO3 (PZT/LSMO) bilayers with opposite self-polarizations. When the LSMO is 2-3 nm thick, the bilayers show two magnetic transitions on cooling. However, temperature-dependent magnetization is different below the lower-temperature transition for opposite polarizations. PNR finds that the LSMO splits into two magnetic sublayers, but the inter-sublayer magnetic couplings are of opposite signs for the two polarizations. Near-edge X-ray absorption spectroscopy further shows contrasts in both the Mn valences and the Mn-O bond anisotropy between the two polarizations. This work completes the puzzle for the magnetoelectric coupling model at the PZT/LSMO interface, showing a synergic interplay among multiple degrees of freedom toward emergent functionalities at complex oxide interfaces.

Unusual anisotropic magnetic orbital moment obtained from x-ray magnetic circular dichroism in a multiferroic oxide system

The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.

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.

Inorganic-Organic Hybrid Molecular Materials: From Multiferroic to Magnetoelectric.

Inorganic-organic hybrid molecular multiferroic and magnetoelectric materials, similar to multiferroic oxide compounds, have recently attracted increasing attention because they exhibit diverse architectures, a flexible framework, fascinating physics, and potential magnetoelectric functionalities in novel multifunctional devices such as energy transformation devices, sensors, and information storage systems. Herein, the classification of multiferroicity and magnetoelectricity is briefly outlined and then the recent advances in the multiferroicity and magnetoelectricity of inorganic-organic hybrid molecular materials, particularly magnetoelectricity and the relevant magnetoelectric mechanisms and their categories are summarized. In addition, a personal perspective and an outlook are provided.

Advances in the modification and device integration of multiferroic bismuth ferrite

As a single-phase multiferroic oxide, BiFeO3 (BFO) is suitable for applications such as magnetoelectric memories, sensors and actuators. However, BFO suffers from high leakage current density and small magnetization. And the BFO in film form is apt to react with Pt bottom electrode during the thermal treatment. In addition, its required high processing temperatures can not be compatible well with the interconnect technologies for semiconductor. In this review, we summarize both the improvement in leakage characteristics and enhancement of magnetic properties of BFO. The effects of buffer layers on properties and device integration of BFO thin films are also introduced.

Interplay of Magnetic Properties and Doping in Epitaxial Films of h-REFeO3 Multiferroic Oxides.

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO3 multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (TN ). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO3 . Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.

Artificial laminar oxide multiferroic magnetoelectric thin film structures - Elaboration methods and study by synchrotron radiation techniques

Nanometric laminar two-dimensional artificial multiferroic oxide thin films can be elaborated using spinel ferrites and perovskite ferroelectrics like CoFe2O4 and BaTiO3. Such materials can retain their individual ferromagnetic or ferroelectric properties. In the thin epitaxial film regime a cross coupling of these properties is possible thanks to strain engineering. After introducing the concepts supporting artificial multiferroic laminar structures, the growth of strained BaTiO3 thin films and the growth of subsequent Co-ferrites layers will be detailed. With respect to the relative film thickness, a detailed understanding of the elastic behavior of these films will be proposed based on the characterization using several synchrotron radiation techniques including x-ray specular and off-specular diffraction, x-ray absorption spectroscopy, as well as x-ray magnetic circular dichroism.

Quantum chaos dynamics on the surface of topological insulator attached to spiral multiferroic oxide in a magnetic field

Quantum dynamics of Dirac fermions on the surface of a 3D topological insulator attached to a spiral multiferroic oxide in an applied magnetic field is investigated. We find that the particle dynamics is significantly affected by interaction of its spin with the proximity-induced exchange field provided by a spiral multiferroic oxide, which changes considerably by changing the spin orientations, the strength of exchange field, and spiral wave vector. The strong dependence of dynamics on the initial conditions together with the unpredictable irregular behavior of cyclotron motion suggests that the dynamics is chaotic. The chaos signatures are diagnosed by evaluating the relevant out-of-time-ordered correlator and are analyzed using Poincaré maps. The appearance of chaotic behavior in the dynamics of Dirac fermions is attributed to the interplay of zitterbewegung, cyclotron oscillations, and proximity-induced exchange field oscillations, leading to nonlinear complicated oscillating modes that drive the system dynamics to chaotic regime.

Magnetic and Magnetocaloric Properties of Multiferroic Oxides Gd0.5Y0.5MnO3 and Eu0.5Dy0.5MnO3

In this article, crystal structure, magnetism, heat capacity, magnetocaloric effect (MCE), and electrical resistivity of mixed rare-earth Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> manganites have been studied. Temperature-dependent magnetization M(T) measurements suggest an antiferromagnetic order just below 6 K in Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , while no magnetic transition has been observed down to 5 K in Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Heat capacity data reveal anomalies at 40 and 42 K in Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and these may correspond to Mn magnetic order as previously reported. The magnetization values are 2.8 and 3 μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> /f.u. in 70 kOe field at 5 K for Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , respectively. MCE has been estimated in terms of isothermal magnetic entropy change (ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) at low temperatures. The maximum values of ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> are about -11 and -6.2 J kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively, for Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> samples for a field change of 70 kOe at 8 K. Electrical resistivity of Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Eu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> shows variable range polaron hopping behavior in the temperature range of 210-350 K.

Phase transition enhanced superior elasticity in freestanding single-crystalline multiferroic BiFeO3 membranes.

The integration of ferroic oxide thin films into advanced flexible electronics will bring multifunctionality beyond organic and metallic materials. However, it is challenging to achieve high flexibility in single-crystalline ferroic oxides that is considerable to organic or metallic materials. Here, we demonstrate the superior flexibility of freestanding single-crystalline BiFeO3 membranes, which are typical multiferroic materials with multifunctionality. They can endure cyclic 180° folding and have good recoverability, with the maximum bending strain up to 5.42% during in situ bending under scanning electron microscopy, far beyond their bulk counterparts. Such superior elasticity mainly originates from reversible rhombohedral-tetragonal phase transition, as revealed by phase-field simulations. This study suggests a general fundamental mechanism for a variety of ferroic oxides to achieve high flexibility and to work as smart materials in flexible electronics.

Magnetoelectric response in honeycomb antiferromagnet Fe4NbTaO9

In this paper, we have investigated magnetic, dielectric, and pyroelectric properties of polycrystalline Fe4NbTaO9 which is closely related to the recently discovered Fe4Nb2O9 and Fe4Ta2O9 that show magnetoelectric/multiferroic effect. Fe4NbTaO9 samples were synthesized using two synthesis routes i.e. solid-state reaction and arc melting methods. Rietveld analysis of XRD patterns confirmed the proper phase formation with P3-c1 space-group symmetry of these two samples. No impure phase was observed in the arc melted sample. Magnetic measurements have indicated an antiferromagnetic phase transition at TN ≈ 88 K for both synthesized samples. Temperature dependent dielectric curves reveal two broad transitions at TN1 ≅ 88 K and TN2 ≅ 50 K. Further, magnetic field induced electric polarization is evidenced by pyro-electric curves in both samples of Fe4NbTaO9. Herein, all findings demonstrate that partial replacement of Nb5+ by Ta5+ is sufficient to modify the ground state in the Fe4B2O9 (B = Nb, Ta) members. Our results provide new insights into magnetoelectric phase control of the novel oxide multiferroics.

Design of Two-Dimensional Multiferroics with Direct Polarization-Magnetization Coupling.

The recent discovery of two-dimensional (2D) ferromagnetism in van der Waals materials has opened the door to the control of 2D magnetism by means of electric field. Here we demonstrate the magnetization reversal through switching polarization in a designed 2D multiferroic oxide by combining group theory analysis and first-principles calculation. We show that ferroelectricity can be induced by a specific octahedral rotation in a perovskite bilayer. Ferromagnetism can be introduced simultaneously by extending the guideline to the B-site ordered double-perovskite bilayer. We have found two coupling mechanisms between polarization and magnetization that enable the reversal of the in-plane magnetization by ferroelectric switching. Our work provides guidelines for the design of 2D multiferroics with intrinsic magnetoelectric coupling and helps to control the 2D magnetism by electric field.

Density Functional Theory investigation of rhombohedral multiferroic oxides for photocatalytic water splitting and organic photodegradation

In this work, ab initio simulations based on DFT were performed to evaluate the electronic and optical properties of new multiferroic materials obtained from the A and B cations replacement on R3c structures aiming its application on water splitting and photodegradation of emergent pollutants. Among all the materials proposed, we highlight the GeNiO3, SnNiO3, NiGeO3, NiSnO3, NiPbO3, FeTiO3, FeGeO3, FeSnO3, FeVO3, and PbMnO3 materials, which are potential alternatives for this type of application in at least one spin direction. In particular, the best choice for such application is the FeTiO3 because it can act as an oxidizer as a reducer in the molecular mechanisms to produce radicals in water splitting or organic photodegradation. Furthermore, our methodology represents a useful theoretical tool for the investigation of photocatalytic properties of Solid-State Materials since it can predict the conduction process stability and photocatalytic potential offering very representative results.

Wet synthesis endows scalable epitaxy of complex oxide multiferroics.

Wet synthesis endows scalable epitaxy of complex oxide multiferroics.