Magnetoelectric Multiferroic Materials Research Articles

Magneto-optic and magneto-electric effects in poly(vinylidene fluoride) - Zn0.2Co0.8Fe2O4 nanocomposite organic flexible film

Abstract The inorganic-organic multiferroic magnetoelectric materials possess extensive research interest for an application point of view considering the flexible magnetoelectric devices. The ferroelectric polymer based poly(vinylidene fluoride) - Zn 0.2 Co 0.8 Fe 2 O 4 magnetoelectric flexible having self-standing nanocomposite film has been prepared through the low temperature chemical process and followed by the solution casting technique. The structural characterizations have been carried out by X-ray diffraction technique. The surface morphology of this film is analysed using atomic force microscopic topography. The electrical properties of the flexible film have been observed with an induced effect of confirmable visible light, where the magnetic field produces a significant influence. The combined effect of the visible light and the magnetic flux is controlling the electrical properties of the film. This film is showing the resistive switching effect with time and the radiation of light energy at room temperature. The magnetoelectric coupling coefficient of the flexible film has been measured as a function of an applied magnetic field at different experimental configurations at room temperature. The attempted research work provides an information about the visible light and magnetic field induced effects of the magnetoelectric material. However, it may be extended for an application purpose to develop the multiferroic magnetoelectric based devices.

Investigation of electrical transport and magnetoelectric coupling of codoped ferrite–PbZr0.58Ti0.42O3 multiferroic nanocomposites

The multiferroic magnetoelectric materials have gained intensive research interest in the recent years due to their prospective applications. In this perspective, the thermally tunable complex impedance, dielectric behavior and room-temperature magnetoelectric coupling of xCo[Formula: see text]Ni[Formula: see text]Fe2O4–(1 - x)PbZr[Formula: see text]Ti[Formula: see text]O3 (x = 0.2, 0.3 and 0.5) nanocomposites have been investigated. A series of samples have been prepared by chemical pyrophoric reaction process. The structural characterization confirms the coexistence of two different types of phases, there is no phase segregation. The temperature-controlled complex impedance analysis reveals that grain boundaries and grain of the nanocomposites are playing a dominating role. The existence of Maxwell–Wagner interfacial polarization of the nanocomposites causes a high dielectric constant at low frequency. The calculated AC conductivity values with frequency at different temperatures follow the Jonscher’s power-law. A small polaronic hopping contributes largely to the conduction process of the decorated composite. The magnetostriction properties lead to the AC and DC magnetic field-dependent magnetoelectric coupling of the nanocomposites. The magnetoelectric coupling coefficient depends on the concentration of the piezomagnetic phase of the composites.

Advances in magnetoelectric multiferroics.

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications.

Microstructures, magnetic, and dielectric properties of Ba‐doped BiFeO3 nanoparticles synthesized via molten salt route

AbstractAs the paradigm of magnetoelectric multiferroic materials, BiFeO3 (BFO) has potential applications in spintronics, memory devices, sensors, and actuators. However, its large leakage current and small magnetism at room temperature restrict its practical applications. It is demonstrated that the substitutions of Bi by alkali earth elements at A‐site of BFO can significantly reduce the leakage current and enhance the remanent magnetization of BFO. In this work, Ba‐doped BFO nanoparticles Bi1‐xBaxFeO3 (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via molten salt route. X‐ray diffraction patterns revealed that with increasing the Ba‐doped content the formation of the impurity phase was depressed and the rhombohedral distortions of these nanoparticles were suppressed, as confirmed by Raman spectra. X‐ray photoelectron spectroscopy measurements reveal that the Fe element in the nanoparticles exists in the dual valence states of Fe3+ and Fe2+, and two kinds of oxygen atoms (lattice oxygen atoms and the adsorbed oxygen atoms) exist in the nanoparticles. With increasing the Ba‐doped content, the content ratios of Fe3+ to Fe2+ ions were generally increased, whereas the oxygen vacancy concentrations were decreased. The average particle sizes of the Ba‐doped BFO nanoparticles were decreased as compared with that of nondoped BFO nanoparticles. In contrast, the room temperature magnetization of the Ba‐doped BFO nanoparticles was greatly enhanced by Ba‐substitution, as confirmed by the M‐H loops. At room temperature, the remanent magnetization and coercive field of the Bi0.8Ba0.2FeO3 nanoparticles were 0.51 emu/g and 1130 Oe, respectively. Furthermore, the leakage current density was reduced by one order of magnitude at x = 0.2 and the dielectric properties are also improved by Ba‐substitution. The improvements on the remanent magnetization, leakage current density as well as dielectric properties of the Ba‐doped BFO nanoparticles make them promising candidates for spintronics and dielectric energy storages.

Enhanced multiferroic and magnetoelectric properties of Ni0.92(Cu0.05Co0.03)Fe2O4/Ba1-xCaxZr0.10Ti0.90O3 lead-free composite films

The search for lead-free high performance magnetoelectric multiferroic materials at room temperature continues to be stimulated from the perspective of designing environmentally friendly multifunctional devices. In this context, lead-free composite thin films consisting of Ba1-xCaxZr0.10Ti0.90O3 (BCZT, x = 0, 0.10 and 0.15) as the ferroelectric/piezoelectric phase and Ni0.92(Cu0.05Co0.03)Fe2O4 (NCCFO) as the ferromagnetic/magnetostrictive component have been grown on Pt/TiO2/SiO2/Si substrates via chemical solution spin-coating method. The phase and structural evolutions of two distinct pure phases of as grown NCCFO/BCZT composite films were confirmed using X-ray diffractrometry (XRD) and field emission scanning electron microscopy (FE-SEM). The multiferroic, magneto-dielectric and magnetoelectric properties of the NCCFO/BCZT composite films have been investigated. Moreover, significant enhancement of multiferroic properties and magnetoelectric effect was observed in these composite films when compared to those reported recently for other lead-free BCZT based multiferroic composites. The magnetic studies demonstrate that the NCCFO/Ba0.90Ca0.10Zr0.10Ti0.90O3 (NCCFO/BCZT10) composite film exhibits a large saturation and remnant magnetization of Ms ∼ 346.2 emu/cm3 and Mr ∼ 73.5 emu/cm3 respectively having a coercive field of Hc ∼ 73.5 Oe. The NCCFO/BCZT10 composite film also exhibits good ferroelectric properties with saturation polarization Ps ∼26.7 μC/cm2, remnant polarization Pr ∼8.9 μC/cm2 and coercive field Ec ∼82.7 kV/cm. The observed magneto-dielectric effect is of two orders higher than those reported for other lead-free multiferroic composites. A maximum magnetoelectric voltage co-efficient of αME ∼22.4 mVcm−1Oe−1 is obtained for the NCCFO/BCZT10 composite film. The observed results suggest that the NCCFO/BCZT composite films are potential lead-free multiferroic systems at room temperature.

Ferromagnetism in the multiferroic alloy systems BiFeO3-BaTiO3 and BiFeO3-SrTiO3: Intrinsic or extrinsic?

Among the different bulk alloys of the room temperature multiferroic compound BiFeO3, alloying with ATiO3 (A = Ba, Sr, Pb) is interesting as they have been reported to induce ferromagnetism under certain conditions. While this makes them interesting as potential multiferroic magnetoelectric materials, there is a lack of clarity on the origin of ferromagnetism in these systems. We have performed a detailed magneto-structural analysis on two alloys, namely, BiFeO3-BaTiO3 and BiFeO3-SrTiO3, and found the systems' propensity for the formation of ferrimagnetic hexaferrite (BaFe12O19 and SrFe12O19) phases. Though insignificantly small to the extent of being undetectable by x-ray powder diffraction, we prove that the ferromagnetic character of the specimen is entirely due to spontaneous precipitation of the hexaferrite grains. While our results suggest that care should be exercised before attributing ferromagnetism in such alloy systems as intrinsic to the perovskite phase, the propensity for the spontaneous precipitation of the hexaferrite phase in these multiferroic alloy systems is encouraging as it provides an opportunity for designing self-grown ferroelectric-ferromagnetic composites with good magnetoelectric coupling.

Tuning the Néel Temperature of Hexagonal Ferrites by Structural Distortion.

To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature (T_{N}) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T_{N} in hexagonal ferrites and manganites, because the K_{3} structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between T_{N} and the tolerance factor and a power-law relation between T_{N} and the K_{3} distortion amplitude. Following the analysis, a record-high T_{N} (185K) among hexagonal ferrites was predicted in hexagonal ScFeO_{3} and experimentally verified in epitaxially stabilized films. These results add to the paradigm of spin-lattice coupling in antiferromagnetic oxides and suggests further tunability of hexagonal ferrites if more lattice distortion can be achieved.

The Study of Microstructure, Dielectric and Multiferroic Properties of (1 − x) Co0.8Cu0.2Fe2O4-xBa0.6Sr0.4TiO3 Composites

Magnetoelectric multiferroic materials have attracted great attention due to their intriguing properties and potential applications. In this paper, (1 − x) Co0.8Cu0.2Fe2O4-xBa0.6Sr0.4TiO3 (x = 0, 0.4, 0.45, 0.475, 0.5, 0.525, 0.55, 1) composites were prepared by the conventional solid-state method combining with a sol–gel process. The effect of x on the microstructure, dielectric and multiferroic properties has been systematically investigated and discussed. The results confirm pure bi-phase Co0.8Cu0.2Fe2O4 and Ba0.6Sr0.4TiO3 composites except for slight impurity phases, which are indexed as BaCuO2/BaFe12O19. All the samples show uniform and relatively smooth surfaces with the mean grain size of ∼ 1.5 μm which decreases slightly with the increase of x, demonstrating the moderating effect of the ferroelectric phase on the grain size. The dielectric constant shows intense dependence on the components and the highest value is obtained when x = 0.525. The ferroelectric loops demonstrate that the coercive field and remnant polarization of the sample x = 0.525 are higher than other samples except for the pure Ba0.6Sr0.4TiO3 specimen at room temperature. The Co0.8Cu0.2Fe2O4 shows better magnetic properties and the performance of magnetization decreases with the decrease of Ba0.6Sr0.4TiO3; meanwhile, the impurity phase and inhomogeneous structure have certain influences on the magnetic properties.

Origin of Ferrimagnetism and Ferroelectricity in Room-Temperature Multiferroic ϵ−Fe2O3

In devices, being able to control magnetism with electric rather than magnetic fields is very appealing, and magnetoelectric multiferroic materials are key to this functionality. \ensuremath{\epsilon}-FeO is a room-temperature multiferroic featuring both strong magnetization and strong ferroelectricity simultaneously, but the origin of this multiferroicity is unclear. This work shows that the compound's ferrimagnetism arises from strong spin frustration, and that a low-energy transition state is responsible for switchable ferroelectricity. At a higher level, these results also suggest how to search for other high-performance multiferroics, for use in next-generation data storage.

Structural distortion, ferroelectricity and ferromagnetism in Pb(Ti1−xFex)O3

Recently interest in finding magnetoelectric coupling in Fe substituted PbTiO3 is observed. Synthesis of single phase PbTi(1−x)Fe(x)O3 is a challenge which we report for 0 ≤ x ≤ 0.5. PbTiO3 is a strong ferroelectric. Fe substitution at the Ti site may lead to a potential magnetoelectric multiferroic material maintaining the non-centrosymmetric lattice distortion due to elongation of Ti-O octahedral in PbTiO3 intact. The synthesis chemistry is a determining factor on the pure phase. Valence state, size and coordination of substituting Fe is a determining factor of how the lattice distortion will get modified deciding several changes including oxygen deficiency, etc., and results in variations of electronic/magnetic properties including ferroelectricity and magnetism. We investigate the structural and electronic properties and correlate discussing briefly the introduced magnetic properties and subsequent structural changes linked to ferroelectricity.

Comprehension of the Electric Polarization as a Function of Low Temperature

Polarization response to warming plays an increasingly important role in a number of ferroelectric memory devices. This paper reports on the theoretical explanation of the relationship between polarization and temperature. According to the Fermi–Dirac distribution, the basic property of electric polarization response to temperature in magnetoelectric multiferroic materials is theoretically analyzed. The polarization in magnetoelectric multiferroic materials can be calculated by low temperature using a phenomenological theory suggested in this paper. Simulation results revealed that the numerically calculated results are in good agreement with experimental results of some inhomogeneous multiferroic materials. Numerical simulations have been performed to investigate the influences of both electric and magnetic fields on the polarization in magnetoelectric multiferroic materials. Furthermore, polarization behavior of magnetoelectric multiferroic materials can be predicted by low temperature, electric field and magnetic induction using only one function. The calculations offer an insight into the understanding of the effects of heating and magnetoelectric field on electrical properties of multiferroic materials and offer a potential to use similar methods to analyze electrical properties of other memory devices.

Predicting room-temperature multiferroic phase of NaLaFeMO6 (M = Ru, Re, Os) with strong magnetoelectric coupling

Discovering multiferroic materials is one of the most important questions for facilitating devices based on the electric field control of magnetism. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the “bicolor” layering of P21 double ferrimagnetic perovskites, e.g., NaLaFeMO6 (M = Ru, Re, Os). Using density functional theory calculations, we elucidated the origin of the ferroelectricity and show that it is a general consequence of the layering of any bicolor P21 double ferrimagnetic perovskites. Our calculations showed that the ferroelectric polarization is up to several μC/cm2 and the net magnetic moments are 3, 2, and 3 μB for NaLaFeRuO6, NaLaFeReO6, and NaLaFeOsO6, respectively in these proposed materials. More importantly, we highlighted the existence of a low-energy ferroelectric switching path, along which the magnetization is also likely reversed, evidencing the strong magnetoelectric coupling existing in this system. In addition, the critical temperature of polarization and magnetization are both well above room temperature. It is expected that this work can encourage the designing and experimental implementation of a large class of strong magneto-electric multiferroic materials with large magnetization and electric polarization above room temperature.

Magnetoelectric effects via pentalinear interactions

Magnetoelectric multiferroic materials, particularly with the perovskite structure, are receiving a lot of attention because of their inherent coupling between electrical polarization and magnetic ordering. However, very few types of direct coupling between polarization and magnetization are known, and it is unclear whether they can be useful to the design of spintronic devices exploiting the control of magnetization by electric fields. For instance, the typical biquadratic coupling only allows changing the magnitude of the magnetization by an electric field, but it does not permit an electric-field-induced switching of the magnetization. Similarly, the so-called Lifshitz invariants allow an electric-field control of complicated magnetic orderings, but not of the magnetization. Here, we report on original direct couplings between polarization and magnetization in epitaxial perovskite films, via the use of first-principles methods and the development of an original Landau-type phenomenological theory. Our results feature pentalinear interactions involving the ferromagnetic and antiferromagnetic vectors as well as the polar distortions and oxygen octahedral tilting, and permit a number of striking effects. Examples include a continuous electric-field control of the magnetization magnitude and sign, and the discrete switching of the magnetization magnitude. Thus, the high-order, pentalinear couplings demonstrated in this work may open paths towards specific magnetoelectric effects, as well as spintronic and magnonic devices.

Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2 K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.

Designing switchable polarization and magnetization at room temperature in an oxide

Ferroelectric and ferromagnetic materials exhibit long-range order of atomic-scale electric or magnetic dipoles that can be switched by applying an appropriate electric or magnetic field, respectively. Both switching phenomena form the basis of non-volatile random access memory, but in the ferroelectric case, this involves destructive electrical reading and in the magnetic case, a high writing energy is required. In principle, low-power and high-density information storage that combines fast electrical writing and magnetic reading can be realized with magnetoelectric multiferroic materials. These materials not only simultaneously display ferroelectricity and ferromagnetism, but also enable magnetic moments to be induced by an external electric field, or electric polarization by a magnetic field. However, synthesizing bulk materials with both long-range orders at room temperature in a single crystalline structure is challenging because conventional ferroelectricity requires closed-shell d(0) or s(2) cations, whereas ferromagnetic order requires open-shell d(n) configurations with unpaired electrons. These opposing requirements pose considerable difficulties for atomic-scale design strategies such as magnetic ion substitution into ferroelectrics. One material that exhibits both ferroelectric and magnetic order is BiFeO3, but its cycloidal magnetic structure precludes bulk magnetization and linear magnetoelectric coupling. A solid solution of a ferroelectric and a spin-glass perovskite combines switchable polarization with glassy magnetization, although it lacks long-range magnetic order. Crystal engineering of a layered perovskite has recently resulted in room-temperature polar ferromagnets, but the electrical polarization has not been switchable. Here we combine ferroelectricity and ferromagnetism at room temperature in a bulk perovskite oxide, by constructing a percolating network of magnetic ions with strong superexchange interactions within a structural scaffold exhibiting polar lattice symmetries at a morphotropic phase boundary (the compositional boundary between two polar phases with different polarization directions, exemplified by the PbZrO3-PbTiO3 system) that both enhances polarization switching and permits canting of the ordered magnetic moments. We expect this strategy to allow the generation of a range of tunable multiferroic materials.

Artificial multiferroic heterostructures for an electric control of magnetic properties

The control of magnetism by electric fields is an important goal for future low-power spintronics devices. This partly explains the intensified recent interest for magnetoelectric multiferroic materials and heterostructures. The lack of ferro- or ferrimagnetic–ferroelectric materials with large magnetoelectric coupling between the two orders has spurred intensive research on artificial multiferroics combining ferroelectric or piezoelectric materials and ferromagnets. In this paper we review synthetically the potential of thin-film-based heterostructures in which a magnetic film is in contact with a ferroelectric or piezoelectric one to obtain an electric control of magnetic properties. This electric control either results from a strain-induced magnetoelectric coupling, a charge-driven one, or from the modulation of an interfacial exchange-bias interaction. Contrôler électriquement les propriétés magnétiques des matériaux et réaliser ainsi de nouveaux composants faiblement consommateurs en énergie est un des enjeux de la future électronique de spin. Ceci explique l'intérêt considérable porté aux matériaux et architectures multiferroïques. Malgré des recherches intenses, le graal d'un composé à la fois ferroélectrique et ferro- ou ferrimagnétique à température ambiante avec un fort couplage magnétoélectrique entre ces deux propriétés n'a pas encore été trouvé. Pour pallier ce manque, de nombreux travaux ont porté sur les multiferroïques artificiels, hétérostructures combinant un matériau ferroélectrique ou piézoélectrique et un composé ferromagnétique. Cet article constitue une revue succincte du potentiel de ces hétérostructures en géométrie planaire pour obtenir un contrôle électrique des propriétés magnétiques. Un tel contrôle peut être obtenu par un effet magnétoélectrique indirect basé sur un couplage d'origine élastique, de façon directe par une modulation des charges dans le ferromagnétique induite par la ferroélectricité ou encore en exploitant le couplage d'échange à l'interface entre un multiferroïque et un matériau magnétique.

Prospects for nanostructured multiferroic composite materials

Magnetoelectric multiferroic materials have the potential to transform a range of applications, including tunable microelectronics and multiphase memories. However, the coexistence of ferromagnetism and ferroelectricity in single-phase materials is quite rare, driving the development of composite multiferroic materials. In these composites, the coupling arises from strain transfer across a shared interface between the ferroelectric and ferromagnetic phase. This viewpoint article highlights recent work as well as future challenges in the synthesis and characterization of nanostructured multiferroic materials.

Multiferroicity in manganite/titanate superlattices determined by oxygen pressure-mediated cation defects

Increasing demand for spintronic devices, such as high-density memory elements, has generated interest in magnetoelectric coupling and multiferroic materials. In heteroepitaxial structures, magnetoelectric coupling occurs only near the strained interfaces, which is why the interface-rich multiferroic multilayer/superlattice is viewed as one of the most efficient ways to enhance the magnetoelectric coupling coefficient. However, both ferroelectric and ferromagnetic properties are difficult to be maintained when materials are shrunk to ultrathin layers, forming interfacial dead layers and limiting the application of these materials in atomic-scale devices. In this work, we demonstrate that the largely suppressed multiferroic properties of the La0.8Sr0.2MnO3 (16 unit cells)/BaTiO3 (12 unit cells) superlattice correlate with cation defects including both pure edge dislocations and planar defects. This conclusion is reached by combining atomic-resolution electron microscopy, piezoelectric force microscopy, and low-temperature magnetism measurements. Furthermore, it is shown that the density of the observed cation defects can be largely reduced by improving the oxygen off-stoichiometry through increasing oxygen pressure during growth, resulting in robust multiferroic properties. Only by eliminating oxygen vacancies during growth can the ferroic dead layers be further reduced. This work therefore opens the pathway for the integration of ferromagnetic and ferroelectric materials into magnetoelectric devices at diminished length scales.

Observation of Skyrmions in a Multiferroic Material

A magnetic skyrmion is a topologically stable particle-like object that appears as a vortex-like spin texture at the nanometer scale in a chiral-lattice magnet. Skyrmions have been observed in metallic materials, where they are controllable by electric currents. Here, we report the experimental discovery of magnetoelectric skyrmions in an insulating chiral-lattice magnet Cu(2)OSeO(3) through Lorentz transmission electron microscopy and magnetic susceptibility measurements. We find that the skyrmion can magnetically induce electric polarization. The observed magnetoelectric coupling may potentially enable the manipulation of the skyrmion by an external electric field without losses due to joule heating.

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO3 (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO3 has a weak ferromagnetic ground state below 356 K—this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO3.