Magnetoelectric Multiferroics Research Articles

Electrically switchable Rashba-type Dzyaloshinskii-Moriya interaction and skyrmion in two-dimensional magnetoelectric multiferroics

Realizing topological magnetism and its electric control are intensive topics in spintronics due to their promising applications in information storage and logic technologies. Here, we unveil that both can be achieved in two-dimensional (2D) magnetoelectric multiferroics. Using first-principles calculations, we show that a strong Dzyaloshinskii-Moriya interaction (DMI), which is the key ingredient for the formation of exotic chiral magnetism, could be induced in 2D multiferroics with vertical electric polarization via the Rashba effect. We verify that such a significant DMI can promote the stabilization of sub-10-nm skyrmions in 2D multiferroics with perpendicular magnetic anisotropy such as a CrN monolayer. In addition, the presence of both magnetization and electric polarization in 2D multiferroics provides us with the unique opportunity for the effective electric control of both the strength and chirality of the DMI and thereby the topological magnetism. As an example, we introduce four multiferroic skyrmions with different chiralities and polarities that can be manipulated by an external field.

New approach for designing bulk multiferroic composites made of two perovskite oxides with enhanced direct magnetoelectric coupling

The scarcity of single-phase magnetoelectric multiferroics has motivated active research towards the design of composites where polarization and magnetization are coupled through elastic strain at their interfaces. Up to now, most of bulk multiferroic composites are made of immiscible constituents that thus suffer from weak interface connectivity between them. Here, we propose an approach for designing particulate composites with constituents of perovskite ABO3 structure that exhibit solid solubility, by acting on the lowering of the sintering temperature. We show that a BiFe0.95Co0.05O3–PbZr0.52Ti0.48O3 multiferroic composite with improved magnetoelectric coupling can be formed before a solid solution is reached. A Bi-rich interface promotes elastic strain transfer between the two constituents resulting in twice stronger direct magnetoelectric coefficient compared to pure BiFe0.95Co0.05O3. These findings provide an easy and effective strategy for producing upgraded and original multiferroic composites made of constituents with the perovskite structure, known to host a wide variety of functionalities.

Progress in designing novel single-phase room temperature multiferroics

Magnetoelectric multiferroics, because of their coupled electric and magnetic ordering in a single phase, are eminent for applications in logic and nonvolatile memory devices. Recent advances in computational modeling, synthesis, and characterization techniques are spurring significant advances in the study of these materials. During last several years, our group at University of Puerto Rico has been involved in synthesizing novel single-phase room temperature multiferroic materials that included not only solid solutions of lead zirconate titanate with lead iron tungstate, lead iron niobate, and lead iron tantalate, but also recently discovered palladium substituted room temperature multiferroics, namely PbPd0.3Ti0.7O3 and Pb(Zr0.2Ti0.8)Pd0.3O3. In this review, we have reported thin films of the above materials that were synthesized using pulsed laser deposition technique on various substrates at Speclab, University of Puerto Rico. We describe in detail thin film fabrication, structural diffraction analyses (XRD, TEM), dielectric, ferroelectric, piezo-response force microscopy, and magnetization results on recently grown Pd-substituted lead titanate and lead zirconate titanate thin films in an elaborate manner. The films were phase pure and stabilized in c-axis oriented tetragonal phase (P4mm) with surface roughness RQ of 2–4 nm. Temperature-dependent dielectric studies on metal-dielectric-metal heterostructure capacitors indicate that ferroelectric to paraelectric phase transition is above 500 K. A proof of ferroelectricity was confirmed from strong domain switching responses using piezo-response force microscopy. These films showed ferromagnetic ordering below 395 K. The origin of magnetic ordering in these materials was attributed to the existence of Pd2+ and Pd4+ mixed oxidation states of palladium dispersed in the polar matrices. We conclude with the above findings that these multiferroics may have potential applications.in nonvolatile memory and other multifunctional devices.

Ferroelectric-driven tunable magnetism in ultrathin platinum films

Electric control of magnetism in magnetoelectric (ME) multiferroics is expected to have a significant impact on a wide range of technological applications. Here, we predict the modulation of magnetism in ultrathin platinum films due to the ferroelectric polarization of the ${\mathrm{BaTiO}}_{3}$ substrate, which along with biaxial strain changes the density of states at the Fermi energy. We demonstrate that both the magnitude and direction of the magnetization depend strongly on the polarization direction and/or strain. This leads to an unprecedented ME effect involving a giant change of magnetocrystalline anisotropy under polarization switching due to the large spin-orbit coupling of Pt. These findings pave the way of an alternative strategy for the design of nonvolatile and ultralow power spintronics and magnetic memory storage devices.

Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices

Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ → Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.

Making EuO multiferroic by epitaxial strain engineering

Multiferroics are materials exhibiting the coexistence of ferroelectricity and ideally ferromagnetism. Unfortunately, most known magnetoelectric multiferroics combine ferroelectricity with antiferromagnetism or with weak ferromagnetism. Here, following previous theoretical predictions, we provide clear experimental indications that ferroelectricity can be induced by epitaxial tensile strain in the ferromagnetic simple binary oxide EuO. We investigate the ferroelectric phase transition using infrared reflectance spectroscopy, finding that the frequency of the soft optical phonon reduces with increasing tensile strain and decreasing temperature. We observe such a soft mode anomaly at 100 K in (EuO)2/(BaO)2 superlattices grown epitaxially on (LaAlO3)0.29-(SrAl1/2Ta1/2O3)0.71 substrates, which is a typical signature for a displacive ferroelectric phase transition. The EuO in this superlattice is nominally subjected to 6.4% biaxial tensile strain, i.e., 50% more than believed needed from previously published calculations. We interpret our results with new first-principles density functional calculations using a hybrid functional, which provides a better quantitative agreement with experiment than the previously used local-density approximation and generalized gradient approximation functionals.

Large Magnetoelectric Coupling in the Thin Film of Multiferroic CuO.

We report observation of large magnetoelectric coupling in an epitaxial thin film of multiferroic CuO grown on the (100)MgO substrate by the pulsed laser deposition technique. The film is characterized by X-ray diffraction, transmission electron microscopy, and Raman spectrometry. The crystallographic structure of the film turns out to be monoclinic (space group C2/c) with [111]CuO||[100]MgO “out-of-plane” epitaxy and “in-plane” domain structure. The lattice misfit strain is found to vary within ±1–3%. The dc resistivity, magnetization, dielectric spectroscopy, and remanent ferroeletric polarization have been measured across 80–300 K. The dielectric constant is found to decrease by >20% under a moderate magnetic field of ∼18 kOe while the remanent ferroelectric polarization, emerging at the onset of magnetic transition (TN ∼ 175 K), decreases by nearly 50% under ∼18 kOe field. These results could assume importance as the strain-free bulk CuO does not exhibit magnetoelectric coupling within such magnetic field regime. The strain-induced large magnetoelectric coupling in the CuO thin film would generate new possibility of further strain tuning to observe room-temperature magnetoelectric multiferroicity suitable for scores of applications such as memories, sensors, energy-harvesting devices, generators, amplifiers, and so forth.

Engineering room-temperature multiferroicity in Bi and Fe codoped BaTiO3

Fe doping into BaTiO3 stabilizes the paraelectric hexagonal phase in place of the ferroelectric tetragonal one. We show that simultaneous doping of Bi along with Fe into BaTiO3 effectively enhances the magnetoelectric (ME) multiferroic response (both ferromagnetism and ferroelectricity) at room temperature, through careful tuning of Fe valency along with the controlled recovery of the ferroelectric-tetragonal phase. We also report a systematic increase in large dielectric constant values as well as reduction in loss tangent values with relatively moderate temperature variation of the dielectric constant around room temperature with increasing Bi doping content in Ba1−xBixTi0.90Fe0.10O3 (0 ≤ x ≤ 0.10), which makes the higher Bi–Fe codoped sample (x = 0.08) promising for use as a room-temperature high-κ dielectric material. Interestingly, the x = 0.08 (Bi–Fe codoped) sample is not only found to be ferroelectrically (∼20 times) and ferromagnetically (∼6 times) stronger than x = 0 (only Fe-doped) at room temperature, but also observed to be better insulating (larger bandgap) with indirect signatures of larger ME coupling as indicated from anomalous reduction of the magnetic coercive field with decreasing temperature. Thus, room-temperature ME multiferroicity has been engineered in Bi and Fe codoped BTO (BaTiO3) compounds.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity.

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN2O4} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ0H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm2 at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Magnetodielectric effect in novel multiferroic (1-x) (K0.49Na0.47Li0.04)/ (Ta0.1Sb0.06Nb0.84)O3 /xNi0.1Mn0.2Co0.7Fe2O4 nanocomposites

Abstract A novel multiferroic (1-x)(K0.49Na0.47Li0.04)(Ta0.1Sb0.06Nb0.84)O3/xNi0.1Mn0.2Co0.7Fe2O4 (x = 0, 10, 20, 30 and 100 wt%) nanocomposite was prepared. According to the XRD data, ferroelectric (K0.49Na0.47Li0.04)(Ta0.1Sb0.06Nb0.84)O3 (abbreviated as KNN-LST) matrix and ferromagnetic Ni0.1Mn0.2Co0.7Fe2O4 (abbreviated as NMCFO) reinforcement are recognized without the formation of impurity phase such as K3LiNb6O17. The optimum dielectric properties ( e r ∼ 700 and tan δ ∼ 0.08) and ferroelectric behavior (Pr ∼ 9 μC/cm2 and Ps ∼11 μC/cm2) are obtained for the x = 10 wt% due to the minimum weight fraction of non-piezoelectric ferrite phase. With increasing x up to 30 wt%, ferroelectric properties of multiferroic nanocomposites gradually weaken. From x = 10 to x = 30, the values of Ms for composite samples increase from 7 to 20 emu/g. At the same time, Hc diminishes from 360 to 280 Oe. The maximum magnetodielectric constant (5%) is achieved by the multiferroic magnetoelectric (MFME) nanocomposite (x = 30 wt%) at the applied magnetic field of 6 kOe.

Antiferromagnetic and dielectric behavior in polycrystalline GdFe0.5Cr0.5O3 thin film

Single phase materials with both spontaneous electric polarization and magnetization are rare, despite remarkable efforts in developing magnetoelectric multiferroics. In this work, a single-phase polycrystalline GdFe0.5Cr0.5O3 (GFCO) thin film was spin-coated onto a platinized silicon substrate. X-ray diffraction data suggest that the film exhibits an orthorhombic perovskite structure with a Pbnm space group. No other impurity phases were detected. Magnetization measurements reveal the Néel temperature of the GFCO film to be ∼220 K and illustrate a weak ferromagnetic component at 5 K, which could be due to spin canting. Frequency dependent ferroelectric–paraelectric transition was observed around 480 K, indicating the diffuse relaxor-like behavior. The electric field dependent polarization measurements show a lossy behavior below 200 K. The electric field dependent dielectric constant (tunability) measured at 1 MHz in a wide temperature range reveals that the tunability maximizes near the observed dielectric maxima, which further confirms the ferroelectric to paraelectric transition in the present film.

Homogenization method for microscopic characterization of the composite magnetoelectric multiferroics

Tuning of magnetization or electrical polarization using external fields other than their corresponding conjugate fields (i.e., magnetic field for the former or electric field for the latter response) attracts renewed interest due to its potential for applications. The magnetoelectric effect in multiferroic 1–3 composite composed of alternating magnetic and ferroelectric layers operating in linear regime consequent to external biasing fields is simulated and analysed theoretically. Two-scale homogenization procedure to arrive at the equilibrium overall physical properties of magnetoelectric multiferroic composite is formulated using variational analysis. This procedure is extended to quantify the underlying local (microscopic) electric, magnetic and elastic fields and thereby compute local distribution of stresses and strains, electrical and magnetic potentials, the electric and magnetic fields as well as the equivalent von Mises stresses. The computational model is implemented by modifying the software POSTMAT (material postprocessing). Computed local stress/strain profiles and the von Mises stresses consequent to biasing electrical and magnetic fields provide insightful information related to the magnetostriction and the ensuing electrical and magnetic polarization. Average polarization and magnetization against magnetic and electric fields respectively are computed and found to be in reasonable limits of the experimental results on similar composite systems. The homogenization model covers multiferroics and its composites regardless of crystallographic symmetry (with the caveat of assuming an ideal and semi-coherent interface connecting the constituent phases) and offer computational efficiency besides unveiling the nature of the underlying microscopic field characteristics.

Structural, electrical and magneto-electric properties of chemically synthesized Bi/PbTiO3-modified cobalt titanate

In the present work, the dielectric CoTiO3 and the magneto-electric multiferroics BiCo0.5Ti0.5O3 and 0.5BiCo0.5Ti0.5O3-0.5PbTiO3 were developed by a chemical modification route. The structural, micro-structural, dielectric, impedance, conductivity, ferroelectric and magneto-electric properties of the studied compounds were investigated. Structural analysis of the materials showed that rhombohedral structure of CoTiO3 transforms into orthorhombic structure on addition of bismuth in it (i.e. in BiCo0.5Ti0.5O3) and tetragonal structure on further addition of lead titanate (i.e. in 0.5BiCo0.5Ti0.5O3-0.5PbTiO3). From the dielectric analysis and P-E hysteresis measurements, it is observed that CoTiO3 is non-ferroelectric, whereas the compounds formed after modifications with bismuth and lead titanate not only exhibit ferroelectricity, but also magneto-electric multiferroicity.

Insights into the displacement field in magnetoelectric composites

The performance of strain-mediated magnetoelectric composite multiferroics hinges on the interface condition in both direct and converse coupling paradigms. The objective of this article is to report experimentally validated computational models of composite cylinder structure consisting of an outer piezoelectric cylinder mechanically attached to an inner magnetostrictive layer. Three contact conditions were computationally investigated, including bonding, no separation, and standard definitions from the used finite element package. The simulations were used to extract the harmonic, modal, and transient responses of the composite cylinder structure, which were compared to experimental work. Under the influence of an AC electric field, the in-plane and out-of-plane displacement maps were simultaneously measured using a noncontact interferometric technique. Results from the harmonic analysis were used to tune the material properties and boundary conditions used in all subsequent simulations, whereas the resonance frequency was in excellent agreement with the experiment. The modal analysis was validated by comparing a subset of the experimental and computational vibrational modes. Finally, the transient analysis was found to be in reasonable agreement with the experimental results with a focus on the response at the excitation frequency. The validated analysis framework can be used in the development of sensors and actuators based on composite multiferroics cylinder structures.

Wireless energy transfer based on strain-mediated composite multiferroics

A subclass of magnetoelectric composite multiferroics can bi-directionally couple AC electric fields with AC magnetic fields using mechanical strain as a mediator. Serendipitously, AC electric fields, AC magnetic fields, and vibrations, are the mediums used in capacitance-, induction-, and acoustic-based wireless energy transfer (WET), respectively. As a result, strain-mediated composite multiferroics are uniquely positioned to transform all these modes of WET. This paper analytically and experimentally reports the ability of composite multiferroic hollow cylinders to wirelessly transfer energy to and from laminated multiferroic plates by the use of AC magnetic flux as an energy carrier. In all, the composite cylinders in conjunction with laminated plates are successfully demonstrated as a novel technology of bi-directional magnetoelectric-based WET. We report a peak extracted power of ∼100 μW, which is enough to wirelessly power a multitude of small electronic devices.

Analyses of structure, electronic and multiferroic properties of Bi1-xNdxFeO3 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25) system

Magnetoelectric multiferroics are of considerable interest due to the fascinating science and potential applications. In this study, multiferroic Bi1-xNdxFeO3 (0 ≤ x ≤ 0.25) samples have been prepared by a two-stage solid-state reaction method and correlation between various properties has been studied. It can be seen that Nd3+ substitution destabilized the polar R3c symmetry due to chemical strain effects arising from the size mismatch between Bi3+ and Nd3+ cations which triggered partial structural transformation to triclinic structure (P1symmetry). Raman scattering experiments elucidated slight changes in covalency of Bi–O bond and softening of modes corresponding to oxygen motion suggests a significant destabilization of FeO6 octahedra. The increase in magnetization has been attributed to the suppression of the spiral spin structure and variation in Fe–O–Fe bond angle due to the strain induced in the unit cell by Nd3+ substitution. The step-like feature in magnetization at the low field has been attributed to charge melting in connection with manganites. An anomalous decrease in coercivity with decrease in temperature has been observed for Bi0.75Nd0.25FeO3 sample which can be explained on the basis that the decrease in effective magnetic anisotropy and is an indicator of presence of magnetoelectric coupling. The bandgap increases with increase in Nd3+ content which is attributed to the decrease in interatomic lattice parameters which resulted in increased binding forces of valance electrons.

Terahertz Emission Functionality of High‐Temperature Superconductors and Similar Complex Systems

AbstractDuring the past two decades, terahertz (THz) science and technology have rapidly expanded in all fundamental and applied aspects of physical, chemical, and biological sciences. In physical sciences, these developments have been twofolds: i) the use of THz technology in understanding the complexities of materials and ii) the exploration of new THz functionalities of emerging materials for further advancement of THz technology. Here, the THz emission spectroscopy and the ultrafast functionality of three pillars of electronic and magnetic condensed matter systems that emerged in the following order: high‐temperature superconductors, colossal magnetoresistive manganites, and magnetoelectric multiferroics are reviewed. Emission functionality refers to the emitted THz radiation that carries the information of the underlying functional property of the material such that the overall effect evolves as the THz decoding of the information in a noninvasive manner and on an ultrafast timescale.

Probing local order in multiferroics by transmission electron microscopy

AbstractThe ongoing trend toward miniaturization has led to an increased interest in the magnetoelectric effect, which could yield entirely new device concepts, such as electric field-controlled magnetic data storage. As a result, much work is being devoted to developing new robust room temperature (RT) multiferroic materials that combine ferromagnetism and ferroelectricity. However, the development of new multiferroic devices has proved unexpectedly challenging. Thus, a better understanding of the properties of multiferroic thin films and the relation with their microstructure is required to help drive multiferroic devices toward technological application. This review covers in a concise manneradvancedanalytical imaging methods based on (scanning) transmission electron microscopy which can potentially be used to characterize complex multiferroic materials. It consists of a first broad introduction to the topic followed by a section describing the so-called phase-contrast methods, which can be used to map the polar and magnetic order in magnetoelectric multiferroics at different spatial length scales down to atomic resolution. Section 3 is devoted to electron nanodiffraction methods. These methods allow measuring local strains, identifying crystal defects and determining crystal structures, and thus offer important possibilities for the detailed structural characterization of multiferroics in the ultrathin regime or inserted in multilayers or superlattice architectures. Thereafter, in Section 4, methods are discussed which allow for analyzing local strain, whereas in Section 5 methods are addressed which allow for measuring local polarization effects on a length scale of individual unit cells. Here, it is shown that the ferroelectric polarization can be indirectly determined from the atomic displacements measured in atomic resolution images. Finally, a brief outlook is given on newly established methods to probe the behavior of ferroelectric and magnetic domains and nanostructures during in situ heating/electrical biasing experiments. These in situ methods are just about at the launch of becoming increasingly popular, particularly in the field of magnetoelectric multiferroics, and shall contribute significantly to understanding the relationship between the domain dynamics of multiferroics and the specific microstructure of the films providing important guidance to design new devices and to predict and mitigate failures.

Magneto-electric multiferroics: designing new materials from first-principles calculations

AbstractIn parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century,first-principlessimulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributedfirst-principlescalculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Study of switching phenomenon of weak magnetoelectric coupling in proper multiferroics using Landau theory

The Landau-Khalatnikov theory is used to study the switching phenomenon of weak magnetoelectric (ME) coupling in proper multiferroics. We investigate two different types of ME multiferroics: one with a bare magnetic transition temperature below the Curie temperature of ferroelectricity, and the other type is when the Curie temperature of ferroelectricity is below the bare magnetic transition temperature. We adopt a fourth-order potential for ferromagnetics and sixth-order potential of ferroelectrics in the Landau free energy based on experimental reports. We found that the electric field alone cannot switch the magnetization and that the magnetic field alone cannot switch the polarization in weak ME multiferroics. Some interesting results were found: at a normalized transition temperature, the magnetization drops abruptly. Two switching current peaks appear when magnetic and electric fields are simultaneously applied to the system.