Magnetoelectric Multiferroics Research Articles

Creating multiferroic and conductive domain walls in common ferroelastic compounds

Domain walls in ferroelectrics and ferroelastics often present peculiar functional properties, offering an intriguing route toward the design of nano-devices. Here we use first-principles simulations to illustrate an approach for engineering such walls, working with representative ferroelastic perovskites LaGaO3 and CaTiO3 (insulating, non-magnetic, non-polar). We show that a wide range of substitutional dopants can be used to create long-range-ordered structures confined within the walls of these compounds, yielding functional interfaces with tailor-made properties. We thus identify clear-cut strategies to produce metallic walls within an insulating matrix. Further, we find ways to create magnetic walls that also display ferroelectric order (proper or improper), thus providing an original route to obtain magnetoelectric multiferroics. Given the recent developments on the preparation of high-density domain structures in perovskite films, our results suggest a definite path toward new functional nano-materials.

Influences of sintering atmosphere on the magnetic and electrical properties of barium hexaferrites

BaFe12O19 (BaM) hexaferrite has drawn increasing attention due to the high magneto-crystalline anisotropy and chemical stability, being a promising candidate in the field of magnetic microwave devices and magnetoelectric multiferroics. In this work, we studied the differences in microstructure and physical properties of BaM ceramics sintered in different atmospheres of nitrogen and air. It is found that N2 atmosphere does not significantly affect the morphology and magnetic properties of BaM, but dramatically reduces the resistivity from ∼ 1010 to ∼ 105 Ω·cm. By virtue of X-ray photoelectron and impedance analysis, the reduced resistance and increased permittivity value of the N2-sintered BaM were ascribed to the electron hopping mechanism caused by the formation of Fe2+ from Fe3+ ions.

Magnetization-polarization cross-control near room temperature in hexaferrite single crystals

Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully stabilized a simultaneously ferrimagnetic and ferroelectric phase in a Y-type hexaferrite single crystal up to 450 K, and demonstrated the reversal of large non-volatile M by E field close to room temperature. Manipulation of the magnetic domains by E field is directly visualized at room temperature by using magnetic force microscopy. The present achievement provides an important step towards the application of ME multiferroics.

Nanotube array-based barium titanate–cobalt ferrite composite film for affordable magnetoelectric multiferroics

Barium titanate–cobalt ferrite nanocomposite film showing room-temperature magnetoelectric multiferroics was fabricated onlyviaaffordable liquid phase processes.

Pressure effect on spin-driven multiferroicity in a Y-type hexaferrite

We have investigated the influence of pressure on the magnetoelectric multiferroicity of the Ba1.5Sr0.5Mg2Fe12O22 hexaferrite under hydrostatic pressures up to 3.08 GPa.

Hysteresis Loops for Magnetoelectric Multiferroics Using Landau-Khalatnikov Theory

<p>We present a theoretical discussion of the hysteresis in magnetoelectric multiferroics with bi-quadratic magnetoelectric coupling. The calculations were performed by employing Landau-Khalatnikov equation of motion for both the ferroelectric and ferromagnetic phase, then solve it simultaneously. In magnetoelectric, we obtain four types of hysteresis: ferroelectric hysteresis, ferromagnetic hysteresis and two types of cross hysteresis (electric field versus magnetization and magnetic field versus electric polarization). The cross hysteresis has butterfly shape which agree with the result from the previous research. It can also be seen from that hysteresis, that magnetization / electric polarization can not be flipped into the opposite direction using external electric / magnetic field when the magnetoelectric coupling is bi-quadratic type. Overall, the result shows that Landau-Khalatnikov equation is able to approximate hysteresis loops in multiferroics system.</p>

Influence of core size on the multiferroic properties of CoFe2O4@BaTiO3 core shell structured composites

Core shell structured magnetoelectric multiferroics have attracted great attentions due to their improved multiferroic properties and potential applications. In this paper, CoFe2O4 @BaTiO3 core shell structured composites with different core size were prepared by hydrothermal method and the effect of core size on the microstructure and multiferroic properties have been systematically studied. The results indicate that with the decrease of the core size, the dielectric, ferroelectric properties as well as the magnetoelectric coupling effect were increased dramatically. Improved interface area and the perfect distribution of CoFe2O4 particles into BaTiO3 matrix are ascribed to be the result of this enhancement of magnetoelectric properties. Our result may shield light on the improvement of multiferroic properties for composite multiferroic materials.

Laminate composite magnetoelectric multiferroics optimized by global derivative-free optimization method

The magnetoelectric multiferroics where magnetism and ferroelectricity coexist in one material have recently attracted renewed interest due to its potential applications in novel functional devices. Natural multiferroic single-phase compounds are rare and an alternative approach to obtain a magnetoelectric (ME) effect is through multilayered composites of a ferroelectric and a ferromagnetic material. An applied electric field creates a piezoelectric strain in the ferroelectric, which produces a corresponding strain in the ferromagnetic material and a subsequent change in magnetization. Various efforts to improve the value of ME coupling coefficient α have been made by modifying preparation techniques of the samples, by the proper choice of materials or different structures and by choosing different thickness of the samples. In this study, we have applied numerical optimization for arriving at the solution for maximum ME coupling coefficient α of a laminar ME composite by making use of the anisotropy of the ferroelectric phase. We have used a global derivative-free optimization method based in directional direct search coupled with specific multistart strategies for setting up the optimization problem. The effective ME couping coefficients αij∼ are computed using the asymptotic homogenization method. Optimal composite microstructure with a range of the constituent ferroelectric single-crystal configurations that enhances the overall α is identified. Optimal composite would have the [001]-axis of the ferroelectric phase oriented out-of-plane of the lamina. Yet the elasticity of the composite is found to be anisotropic at the optimal orientations of the ferroelectric phase. Stress-mediated enhancement of the ME coupling is demonstrated using the analysis of the inplane elastic stiffness of the composite.

Effect of Gd substitution on the structural, magnetic, and magnetocaloric properties of HoCrO3

Rare-earth chromites are a new type of magnetoelectric multiferroics. In this work, a Ho0.33Gd0.67CrO3 powder sample was synthesized via a citrate route, and the structural properties were characterized by X-ray diffraction, scanning electron microscopy, and the Raman technique. The UV-Visible optical absorbance spectra were also measured in the wavelength range of 200–800 nm. The valence state of Cr was found to be purely 3+ according to the X-ray photoelectron spectroscopy. The temperature-dependent dielectric constant and loss tangent data measured between the frequencies of 1 kHz and 1 MHz show no anomalies around the magnetic transition temperature of the material. The dc magnetization measurements show that the ordering temperature of Cr3+ (TNCr) is 155 K for Ho0.33Gd0.67CrO3, which is larger than 140 K for HoCrO3. The positive slope of the Arrott plots from 0 T to 7 T reveals that the antiferromagnetic-paramagnetic phase transition is second-order in nature. At a field of 7 T, the Ho0.33Gd0.67CrO3 sample showed a giant magnetocaloric entropy change, −ΔS, of ∼23.3 J/kg K at 5 K, and a refrigeration capacity of ∼481.2 J/kg, which are much higher than those of pure bulk HoCrO3. This renders this material prospective for magnetic refrigeration in the low temperature (<30 K) range.

High pressure synthesis and physical properties of multiferroic materials with multiply-ordered perovskite structure

Perovskite is one of the most important material systems for magnetoelectric multiferroic study. However, multiferroic is not expected to occur in a cubic perovskite on account of the highly symmetric crystal structure. Besides, magnetoelectric multiferroics with large ferroelectric polarization and strong magnetoelectric coupling have not been found to occur simultaneously in a single-phase multiferroic material discovered so far, challenging to the potential applications of this kind of material. Here we briefly review two multiferroic materials with multiply-ordered perovskite structure synthesized under high pressure and high temperature conditions. In the cubic perovskite LaMn3Cr4O12, we observed spin-induced ferroelectric polarization, providing the first example where ferroelectric takes place in a cubic perovskite material. In another multiply-ordered provskite BiMn3Cr4O12, type-I and type-Ⅱ multiferroic phases successively developed when cooled. It provides a rare example where two different types of multiferroic phases occur subsequently so that both large polarization and strong magnetoelectric effect are achieved in a single-phase material. In addition, since double ferroelectric phases take place in BiMn3Cr4O12, one can obtain four different polarization states by adopting different poling procedures, thus opening up a new way for generating multifunctional spintronics and multistate storage devices.

Modeling of nonlinear magnetoelectric coupling in layered magnetoelectric nanocomposites with surface effect

Multiferroic magnetoelectric (ME) composites are widely discussed for new kinds of smart structures. However, studies on nonlinear ME coupling in layered composites with nano-scale thickness which have unique superiorities are rather limited. It is well known that the physical and mechanical properties of nanostructures are size-dependent. Considering surface effect and nonlinear material characteristics, this work proposes a nonlinear theoretical model for ME nanocomposites with different magnetostrictive materials based on the surface stress model. The results show that surface effect can enhance the ME coupling in the composites with nano-scale thickness. The ME voltage coefficient can be suppressed due to the consideration of flexural strain and imperfect interfacial conditions. The weak-bonding interface makes the optimal volume fraction shift to PZT-rich composites. Moreover, when subjected to combined pre-stress and magnetic loadings, ME nanocomposites exhibit complex magneto-mechanical coupling characteristics. The proposed model provides a method to analyze and evaluate the nonlinear ME coupling characteristics of nanostructures-based devices.

Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3 - NiFe2O4 multiferroics: A relationship between phase-connectivity and interface coupling

Studying multiferroic magnetoelectrics has been a focus field for the last decade and a half, and the exploration of new materials is one of the several aspects of this quest. Here we report on the synthesis and characterization of NiFe2O4-based multiferroic composites which employ (Ba,Ca)(Zr,Ti)O3 as the ferroelectric/piezoelectric component and NiFe2O4 as the magnetostrictive phase. We find that these composites show excellent magnetoelectric properties. Especially the composite with 30 vol% of NiFe2O4 has a converse ME coefficient approximately two times larger than the previously reported one for BaTiO3-CoFe2O4 composites. A relationship between the phase connectivity within these composites and the ME properties was explored by the time of flight secondary ion mass microscopy. We believe that our investigation will be helpful for the design of magnetoelectric materials as components of sensors and memory devices.

Realization of Large Electric Polarization and Strong Magnetoelectric Coupling in BiMn3 Cr4 O12.

Magnetoelectric multiferroics have received much attention in the past decade due to their interesting physics and promising multifunctional performance. For practical applications, simultaneous large ferroelectric polarization and strong magnetoelectric coupling are preferred. However, these two properties have not been found to be compatible in the single-phase multiferroic materials discovered as yet. Here, it is shown that superior multiferroic properties exist in the A-site ordered perovskite BiMn3 Cr4 O12 synthesized under high-pressure and high-temperature conditions. The compound experiences a ferroelectric phase transition ascribed to the 6s2 lone-pair effects of Bi3+ at around 135 K, and a long-range antiferromagnetic order related to the Cr3+ spins around 125 K, leading to the presence of a type-I multiferroic phase with huge electric polarization. On further cooling to 48 K, a type-II multiferroic phase induced by the special spin structure composed of both Mn- and Cr-sublattices emerges, accompanied by considerable magnetoelectric coupling. BiMn3 Cr4 O12 thus provides a rare example of joint multiferroicity, where two different types of multiferroic phases develop subsequently so that both large polarization and significant magnetoelectric effect are achieved in a single-phase multiferroic material.

Room temperature magnetoelectric coupling effect in CuFe2O4-BaTiO3 core-shell and nanocomposites

Novel magnetoelectric (ME) CuFe2O4@BaTiO3 core-shell and (1-x)BaTiO3-xCuFe2O4 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) composites were prepared by two step sol-gel and a sol-gel followed by a solid state reaction respectively. Crystal structure and microstructure of the samples were examined using X-ray diffraction (XRD) and transmission electron microscopic (TEM) techniques. The ferroelectric and magnetic properties of the materials were confirmed by polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) measurements respectively. To determine the coupling between ferroelectric and magnetic orderings, ME coupling studies were performed using a lock-in amplifier setup. The highest value of the ME coupling coefficient (α) was noticed for the CuFe2O4@BaTiO3 core-shell (α = 22.5 mV cm−1 Oe−1) sample. Superior ME coupling behavior in the core-shell material is due to better connectivity between the ferroelectric and magnetic phases. The optical measurements indicate the possibility of easy manipulation of the band gap over a range of energies by mere control of the molar ratio of the phases. The smart architecture enables CuFe2O4@BaTiO3 to be a highly promising material for the design of devices based on ME multiferroics.

Effect of Al3+ modification on cobalt ferrite and its impact on the magnetoelectric effect in BCZT–CFO multiferroic composites

One of the methods to enhance the functional properties of two-phase multiferroic magnetoelectrics is to increase magnetostriction of the ferrite phase. Al3+-modified cobalt ferrite Co(Al0.5Fe1.5)O4 shows better magnetostriction than unmodified cobalt ferrite. It is used in combination with (Ba,Ca)(Zr,Ti)O3 which has very good piezoelectric properties, to form a multiferroic composite. The composite shows good magnetoelectric characteristics, both macroscopically (converse magnetoelectric coefficient of 11 ps/m) and microscopically. Al3+ proves to be the best non-magnetic dopant to enhance magnetostriction in CoFe2O4 and thus the magnetoelectric coefficient.

Ordered aeschynite-type polar magnets RFeWO6 ( R=Dy , Eu, Tb, and Y): A new family of type-II multiferroics

We report the discovery of magnetoelectric multiferroicity in a family of oxides, $R\mathrm{FeW}{\mathrm{O}}_{6}$ ($R=\mathrm{Dy}$, Eu, Tb, and Y) that crystallize in a polar aeschynite-type structure ($Pna{2}_{1}$) with an ordered arrangement of $\mathrm{F}{\mathrm{e}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions. Magnetization and analysis of neutron-diffraction data of $\mathrm{DyFeW}{\mathrm{O}}_{6}$ reveal a commensurate noncollinear antiferromagnetic ordering of $\mathrm{F}{\mathrm{e}}^{3+}$ spins (${T}_{\mathrm{N}}^{\mathrm{Fe}}\ensuremath{\sim}18\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), which induce Dy spins to order at the same temperature. A sudden change in electric polarization $(\mathrm{\ensuremath{\Delta}}P)$ appears in all the compounds at ${T}_{\mathrm{N}}^{\mathrm{Fe}}=15--18\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The electric polarization is sensitive to an applied magnetic field, and the coupling between different magnetic $R$-ion and Fe-ion moments suppresses the polarization to a different extent. Although the measured polarization in polycrystalline $\mathrm{DyFeW}{\mathrm{O}}_{6}$ at 3.5 K is about $3\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$, a simple calculation of the ionic contribution to polarization with formal charges is $75\phantom{\rule{0.16em}{0ex}}560\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ and it is of the form $\mathbf{P}=({p}_{x},0,{p}_{z})$ where, ${p}_{x}$ comes from magnetic ordering and ${p}_{z}$ is associated with the polar structure in the paramagnetic state. These findings open up an avenue to explore further new polar magnets with rare-earth/transition-metal ions in the ordered aeschynite-type structure.

Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO3-BaTiO3 Superlattices.

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO3-BaTiO3 superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO3-BaTiO3. Surprisingly, the highest ME voltage coefficient of 55 V cm-1 Oe-1 at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO3 layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO3 and BiFeO3. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices.

Polaritons in magnetoelectric multiferroics films: Switching magnon polaritons using an electric field

ABSTRACTWe present a theoretical study of polarions on the magnetoelectric multiferroics with finite thickness (film) geometry. The multiferroics are ferroelectric-antiferromagnet with canted spin structure. Dispersion relation for surface and bulk polaritons are calculated for the case of transverse electric (TE) polarization. Example results are presented using parameters appropriate for BaMnF4. It is found that bulk polaritons posses discreet character due to the film geometry. We also found that there are two branches of surface polaritons which are reciprocal. Example results for attenuated total reflection (ATR) are also presented. Analyzing the ATR reflection, we found that the branches of surface polaritons are able to be switched by applying an electric field opposite to the polarization.

Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain

A current challenge in the field of magnetoelectric multiferroics is to identify systems that allow a controlled tuning of states displaying distinct magnetoelectric responses. Here we show that the multiferroic ground state of the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial strain. Neutron diffraction reveals that in highly strained films the magnetic order changes from the bulk-like incommensurate bc-cycloidal structure to commensurate magnetic order. Concomitant with the modification of the magnetic ground state, optical second-harmonic generation (SHG) and electric measurements show an enormous increase of the ferroelectric polarization, and a change in its direction from along the c- to the a-axis. Our results suggest that the drastic change of multiferroic properties results from a switch of the spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric magnetostriction in epitaxially-strained TbMnO3. These findings experimentally demonstrate that epitaxial strain can be used to control single-phase spin-driven multiferroic states.

Multiferroic Dislocations in Ferroelectric PbTiO3.

Ultrathin multiferroics with coupled ferroelectric and ferromagnetic order parameters hold promise for novel technological paradigms, such as extremely thin magnetoelectric memories. However, these ferroic orders and their functions inevitably disappear below a fundamental size limit of several nanometers. Herein, we propose a novel design strategy for nanoscale multiferroics smaller than the critical size limit by engineering the dislocations in nonmagnetic ferroelectrics, even though these lattice defects are generally believed to be detrimental. First-principles calculations demonstrate that Ti-rich PbTiO3 dislocations exhibit magnetism due to the local nonstoichiometry intrinsic to the core structures. Highly localized spin moments in conjunction with the host ferroelectricity enable these dislocations to function as atomic-scale multiferroic channels with a pronounced magnetoelectric effect that are associated with the antiferromagnetic-ferromagnetic-nonmagnetic phase transitions in response to polarization switching. The present results thus suggest a new field of dislocation (or defect) engineering for the fabrication of ultrathin magnetoelectric multiferroics and ultrahigh density electronic devices.