Magnetoelectric Properties Research Articles

Multiferroic order parameters in rhombic antiferromagnets RCrO3

Currently, active research is aimed at perovskite—based oxides, including rare earth orthochromites, which exhibit magnetoelectric properties owed to intrinsic magnetic interactions in external electric and magnetic fields. Due to a variety of structural instabilities and couplings in these materials, understanding the underlying magnetoelectric mechanisms is a challenge. In this paper, we explore magnetoelectric properties of the rare earth orthochromites in the framework of symmetry analysis. Our calculations show the presence in RCrO3 of electric dipole moments localized in the vicinity of Cr3+ ions. The electric dipole moments, appearing due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase, are arranged in an antiferroelectric mode. We have demonstrated the presence of electric dipole moments in the unit cell of RCrO3, localized in the vicinity of Cr3+ ions. The inversion symmetry breaks due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase, the electric dipoles become arranged in an antiferroelectric mode. We have introduced the basic distortive order parameters in consistence with the symmetry of RCrO3: the polar order parameters ( D , Q 2, Q 3, P ) and the axial order parameter Ω b and classified them according to the irreducible representations of the RCrO3 symmetry group (D 2h 16). We have determined the symmetry—allowed couplings between distortive, ferroelectric and magnetic orderings and found possible exchange-coupled magnetic and ferroelectric structures. The presented analysis makes it possible to explain experimentally observed polarization reversal and the concomitant reorientation of spins in a series of RCrO3 compounds and to predict the possible scenarios of phase transitions in RCrO3.

Plausible multiferroic and magneto-electric behaviour of polycrystalline Bi0.85Gd0.05La0.1FeO3 at room temperature

The structural, magnetic and magneto-electric (ME) properties of polycrystalline BiFeO3 and Bi0.9–xGdxLa0.1FeO3 (x = 0.00 and 0.05) samples have been investigated. X-ray diffraction patterns confirm the formation of single phases in all the samples with ‘R3c’ space group. Magnetic anisotropy and ferroelectric hysteresis behaviour are observed in Bi0.85Gd0.05La0.1FeO3 through magnetization and polarization measurements respectively. In addition, the same compound shows weak ferromagnetism at room temperature with a significantly enhanced remnant magnetization of ~ 23.6emu.mol−1, concomitant with an exchange bias. ME measurements on the compound show a non-linear dependence of magnetic and electric vectors.

Van der Waals electride: Toward intrinsic two-dimensional ferromagnetism of spin-polarized anionic electrons

Discoveries of two-dimensional (2D) magnetism originated from confined atomic layers in van der Waals (vdW) crystals provide an interesting arena for elucidating its fundamentals and enrich magneto-electric and quantum properties. However, a material that exhibits intrinsic 2D magnetism of interstitial electrons occupying layered space, as a root system of magnetic vdW crystals, remains obscure. In this work, 2D ferromagnetic vdW electride, [RECl]2+·2e− (RE = Y and La) is reported with perfectly isolated ferromagnetic 2D blocks encompassing quasi-atomic electron layers. The ferromagnetism of the vdW electride with Curie temperature of 100 K originates from the spin-polarized quasi-atomic electrons with a substantial moment up to ∼0.91 Bohr magneton, which behave as magnetic elements in paramagnetic lattice framework. Invariable ferromagnetism at the monolayer limit strongly supports the 2D ferromagnetism of quasi-atomic electrons. These findings expand the variety of 2D magnetic crystals, providing a promising platform to study the emergent magnetism of low-dimensional electron phases.

Structural, magnetic, magneto-dielectric and magneto-electric properties of (1-x) Ba0.85Ca0.15Ti0.90Zr0.10O3 – (x) CoFe2O4 lead-free multiferroic composites sintered at higher temperature

Here in this article synthesis of lead-free magnetoelectric composites (1-x) Ba0.85Ca0.15 Ti0.9Zr0.1)O3 - xCoFe2O3 (BCTZ-CFO, x = 0.4, 0.5, and 0.6) is reported. Structural, magnetic, dielectric, magnetodielectric and magnetoelectric properties of lead-free BCTZ-CFO composites sintered at higher sintering temperature have been systematically investigated. The piezoelectric (ferroelectric) phase has been synthesized using the sol–gel route, while Metallo-organic decomposition (MOD) route was employed to prepare piezomagnetic (ferrite) phase. The structural and microstructural properties were studied using X-Ray diffraction and Scanning electron Microscope respectively acknowledged the presence of two distinct phases as ferrite and ferroelectric in composite samples. The dense microstructure and larger average grain size were observed for samples with higher ferrite phase content. Long-range magnetic order was observed under magnetic study for all-composites. The magnetic properties enhanced for composites prepared with higher ferrite phase content. Dielectric properties were studied at room temperature and enhanced dielectric constant has been observed for samples with higher ferroelectric phase content, while minimum dielectric loss was observed for sample with 50/50 ferrite/ferroelectric phase composition. The magnetodielectric response has been also investigated for all ME composites. Leakage current characterization has been also discussed at and above the room temperature. The magneto-electric coupling response was measured of all composites. The higher value of magneto-electric coupling coefficient has been observed for 0.5BCTZ-0.5CFO composite due to maximum possible ferrite-ferroelectric hetero-structure phase contact area.

Highly efficient absorber with enhanced magnetoelectric properties based on Y, Gd, and Pr doped NMZ nanoferrites

NMZ ferrites doped with rare earth having spinel composition Ni0.5Mn0.3Zn0.2X0.02Fe1.98O4, whereas X = Y, Pr and Gd were synthesized by sol gel auto-ignition method. XRD, FESEM, VSM and VNA were used to determine the phase, microstructural, magnetic and electromagnetic properties of rare earths doped NMZ ferrite. XRD analysis confirms the single phase of the rare earths doped NMZ ferrite. FESEM images shows Pr doped NMZ ferrite has less agglomerations and more porous structure as compared to NMZ and other rare earths doped NMZ ferrites. Magnetic analysis shows the enhancement in saturation magnetization and remanence with the doping of Y, Pr and Gd in NMZ ferrite. Dielectric properties were determined. Gd doped NMZ ferrite depicted higher ac conductivity, dielectric constant and other electromagnetic and related dielectric properties as compared to Pr-NMZ, Y-NMZ and NMZ ferrite. Similarly, Gd- NMZ spinel ferrite depicted maximum M'' to other doping contents (Y, Pr) and pure NMZ ferrite. The high saturation Q-factor of Pure NMZ spinel ferrite NPs, Y- NMZ, Pr-NMZ and Gd-NMZ were observed at 1.57 GHz, 1.58 GHz, 3.65 and 4.01 GHz frequency regions, respectively. NMZ and RE doped NMZ ferrite absorbers with thin substrate as absorber were simulated using FDTD method. The maximum value of absorptivity for Gd-NMZ at 2.4 GHz were noticed from the simulated absorbers. The absorptivity was also increased with the different incident angles. Therefore, Gd doped NMZ are more preferable for absorption devices in high frequency region.

Magnetoelectric properties of micromagnetic structural inhomogeneities of ferrite garnet films with uniaxial magnetic anisotropy

The method of optical polarimetry was used by us to study the magnetoelectric properties of micromagnetic structural inhomogeneities of epitaxial films of ferrite garnets. The effect of sensitivity of the magnetic structure of vertical Bloch lines and domain walls of the films to an influence of external low-frequency electric field was revealed.

A phenomenological approach to study the magnetoelectric (ME) response of lead free magnetostrictive NiFe2O4–Piezoelectric BaZr0.2Ti0.8O3 particulate composites

Lead-free magnetoelectric (ME) composites comprises NiFe2O4 (NFO) and BaZr0.2Ti0.8O3 (BZT) were synthesized by using conventional double sintering solid-state reaction technique and it's structural, electric, dielectric, magnetic and ME properties were studied. The spectra of X-ray diffraction displayed the formation of parent phase and presence of two phases within the composites (ferrite-ferroelectric). DC resistivity as a function of temperature has been studied, it indicates the semiconducting behavior. Frequency dependent dielectric spectra of the composites indicates small polaron hopping model of conduction mechanism. Magnetic measurements at room temperature shows the composites are soft magnetic and such materials reveals M-H hysteresis type behavior. The composite with x = 0.3 showed the highest value of ME voltage coefficient ~3.82 mV/cmOe. Such ME composite materials furnish fabulous chances as prospective lead-free systems for multifunctional device applications.

Magnetoelectric boundary simulated by a Chern\u2013Simons-like model

In this work we study some physical phenomena that emerge in the vicinity of a magnetoelectric boundary. For simplicity, we restrict to the case of a planar boundary described by a coupling between the gauge field with a planar external Chern–Simons-like potential. The results are obtained exactly. We compute the correction undergone by the photon propagator due to the presence of the Chern–Simons coupling and we investigate the interaction between a stationary point-like charge and the magnetoelectric boundary. In the limit of a perfect mirror, where the coupling constant between the field and the potential diverges, we recover the image method. For a non perfect mirror, we show that we have an attenuated image charge and, in addition, an image magnetic monopole whose field strength does not exhibit the presence of the undesirable and artificial divergences introduced by Dirac strings. We also study the interaction between the plate and a quantum particle with spin. In this case we have a kind of charge-magnetic dipole interaction due to the magnetoelectric properties of the plate.

Transport properties in spinel ferrite/graphene oxide nanocomposites for electromagnetic shielding

Herein, (Co0.5Ni0.5)Cr0.3Fe1.7O4/graphene oxide nanocomposites were fabricated by ultrasonication technique, using pure spinel ferrite and graphene oxide synthesized by sol-gel method and modified Hummers' method, respectively. The effect of graphene incorporation with ferrite nanoparticles was studied by X-ray diffraction (XRD), electrical and dielectric measurements. XRD analysis revealed the spinel phase for the ferrite sample and confirmed the formation of graphene oxide. The crystallite size was found in the range of 37−43 nm and the porosity increased with the increase in the concentration of graphene oxide in the composites. The DC electrical resistivity of spinel ferrite was found equal to 3.83×109 Ω.cm and it substantially decreased with the increase in the percentage of graphene oxide at room temperature. The real and imaginary part of relative permittivity followed the Maxwell-Wagner type of interfacial polarization. AC conductivity confirmed the conduction by hopping mechanism and increased on increasing the GO content. The coupling of magnetic ferrite with graphene oxide tunes the magneto-electrical properties for potential applications at high frequencies.

Sea urchin-like Ni encapsulated with BaTiO3 to form multiferroic core-shell structures for room temperature magnetoelectric sensors

Since core-shell structures are known to have a high surface area to volume ratio, they are preferred for next-generation memory device applications that require effective strain transfer at the ferroic interface. The present investigation reports the encapsulation of sea urchin-like Ni nanoparticles using BaTiO3 nanoparticles to realize room temperature ME coupled Ni@BaTiO3 system. The magnetic, electric and magnetoelectric properties of Ni nanoparticles partially coated with BaTiO3 provide enough evidence for the existence of ME coupling between Ni and BaTiO3 at their interface. Additionally, we have observed an exchange bias shift upon field cooling, which confirms the presence of the Ni-NiO-BaTiO3 interface in the Ni@BaTiO3 core-shell. The obtained magnetoelectric coupling coefficient of Ni@BaTiO3 composite is 225 µV cm−1 Oe−1. The present study reveals that chemically derived sea urchin-like Ni@BaTiO3 core-shell architecture can be a promising way to design strongly coupled multiferroic memory devices and sensors.

Predicting and optimizing coupling effect in magnetoelectric multi-phase composites based on machine learning algorithm

In this paper, we present a machine learning (ML) method to search for geometric patterns of magnetoelectric multi-phase composites with optimal magnetoelectric coupling properties. The 2D finite element method is used to calculate the coupling coefficients and build the database for the training and testing of ML algorithms. Both the convolution neural network (CNN) and artificial neural network (ANN) algorithms are used as ML algorithms in this work. By investigating the effects of network parameters, such as training data density, iteration number and batch size, we construct the networks with proper parameters and good prediction accuracy. We present the predicted geometric patterns by two methods, CNN and ANN, and compare them with the FEM patterns. Two types of magnetoelectric composites, two -volume fractions of magnetostrictive phase and two system sizes are considered to investigate and improve the prediction efficiency. The presented results show that by using the ML methods, it can well predict the coupling effect and rank optimal patterns. The results also prove the feasibility that by using the ML method, we can accurately predict the coupling performance with very limited data. We aim to predict the optimal patterns instead of the best pattern. Therefore, this work demonstrates the feasibility and offers a new perspective way for the design and optimization of magnetoelectric multi-phase composites by using the ML algorithm.

Structural, magnetic and magnetoelectric investigations on CoFe2O4 prepared via various wet chemical synthesis route : A Comparative Study

Magnetostrictive materials are potential candidates for many applications such as sensors, actuators, transducers, and other magnetoelectric applications. Cobalt Ferrite (CoFe2O4) has proven to be favourable in comparison to commonly used magnetostrictive materials due to its high magnetostriction coefficient and it’s preparation being cost effective. This work deals with the preparation of CoFe2O4 (CFO) using four different wet chemical synthesis techniques in the form of autocombustion, co-precipitation, sol–gel and hydrothermal methods. Same conditions of heat treatment have been maintained for all the methods which facilitates a fair comparison of the performance of the samples prepared by various methods. The prepared CFO samples were subjected to subsequent characterization of their structural, magnetic, magnetostrictive and magnetoelectric properties. Static and dynamic magnetoelectric measurements were carried out on trilayered composites comprising of CFO synthesized by hydrothermal and sol–gel methods. Effect of in-plane and transverse compressive stress on the magnetoelectric response of CFO/PZT/CFO trilayered structure have been studied. All the experimental characterisations indicated the sample synthesized by hydrothermal method to yield better properties. The results thus reveal the importance of synthesis route towards attaining an improved magnetic, magnetostrictive and magnetoelectric properties of CFO.

Nickel‐Decorated Silver Nanowires for Polymer‐Based Magnetoelectric Composite

Polymer‐based magnetoelectric composites are processed with an intrinsic piezoelectric poly(vinylidenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) matrix and high‐aspect‐ratio magnetic nickel–decorated silver nanowires. These core/ shell wires are obtained by the polyol reduction of Ni2+ ions on high‐aspect‐ratio silver nanowires. Scanning electron microscopy and transmission electron microscopy analyses show a homogeneous and well‐crystallized nickel layer. Magnetic properties of the intrinsic particles and their composites are measured and show a strong enhancement of the coercive field compared to spherical Ni nanopowders. They also show the existence of strong magnetic anisotropy in the composites. The composites processed with a small amount of metallic particles (0.3 vol%) allow the maintaining of the P(VDF‐TrFE) crystalline phase and the ability of a macroscopic polarization. The latter is conducted under a 80 MV m−1 field, leading to a significant d33 (3.2 pC N−1). Thus, the magnetoelectric properties are analyzed and an important coupling coefficient αME equal to 1.8 V m−1 Oe−1 is measured.

Electrical Transport and Magnetic Properties of Metal/Metal Oxide/Metal Junctions Based on Anodized Metal Oxides

In this paper, we describe magnetoelectric properties of metal/metal-oxide/metal junctions based on anodized metal oxides. Specifically, we use Ti and Fe metallic layers separated by the porous metal-oxides of iron or titanium formed by the anodization method. Thus, we prepare double junctions with at least one ferromagnetic layer and measure magnetoresistance, as well as their current-voltage and magnetic characteristics. We find that magnetoresistance depends on that junction composition and discuss the nature of differential resistance calculated from I-V characteristics. Our findings show that a top metallic layer and the interface between this layer and anodized oxide, where strong interatomic diffusion is expected, have the strongest influence on this observed behavior.

Magnetoelectric polarizability of a multiferroic antiferromagnetic quantum spin system: The influence of both the site-dependent magnetic and electric fields

In the present issue, the magnetoelectric effect in a multiferroic antiferromagnetic spin system with the Dzyaloshinskii Moriya interaction is investigated. The Dzyaloshinskii Moriya interaction is assimilated to a local electric polarization coupled to an electric field which is a site-dependent field as well as the applied external magnetic field. By using the spin-wave theory as a diagonalization method and via the partition function, the physical properties of the system such as magnetoelectric polarizability, magnetization, and electric polarization are obtained. The numerical results reveal that the site-dependent magnetic and electric fields play a remarkable role in the quantum phase transition phenomena that occur in our system. In fact, the magnetoelectric properties response of the system due to the change of the temperature or the external fields reveals the formation of the intermediate states tuneable by the site-dependent parameters. Furthermore, the electric polarization and the magnetoelectric polarizability response of the system due to the variation of the magnetic field show that the magnetoelectric effect can be control by varying the magnetic or/and the electric site-dependent parameter. Overall, the cumulative influence of the site-dependent magnetic and electric fields is a good strategy to detect, elucidate, and control the unconventional phases transition that undergoes a multiferroic antiferromagnetic quantum spin system. Moreover, the results obtained in this work are very interesting for the technological application point of view because there could provide a new prescription for the construction of multifunctional spintronic devices.

Multipole orders and Bragg diffraction patterns for the chain ferrate Na2FeSe2

Fundamental block and staggered orders of magnetic Fe multipoles in ${\mathrm{Na}}_{2}\mathrm{Fe}{\mathrm{Se}}_{2}$ are classified by their symmetry and magnetoelectric properties. Our minimal structure models incorporate ferromagnetic or antiferromagnetic coupling between chains, and they are not compatible with axial ferromagnetism. The ferrate salt is valued in studies of highly correlated electrons as the only iron selenide known to possess chainlike structural units hosting ferrous cations. Axial and polar (Dirac) multipoles are compulsory in the electronic structure since Fe ions exhibit enantiomorphic symmetry in the parent ${\mathrm{K}}_{2}\mathrm{Zn}{\mathrm{O}}_{2}$-type compound. Calculated Bragg diffraction patterns for neutrons and x rays prove that the techniques can unlock individual contributions from both multipole types.

Anisotropic spin-driven ferroelectricity and magnetoelectric effect in a Y-type hexaferrite

We report a systematic study on the magnetic, dielectric, and magnetoelectric (ME) properties of the Y-type hexaferrite (Ba0.4Sr1.6)Zn2(Fe11.3Al0.7)O22 single crystal. The phase diagrams were established by the observed magnetic anomalies under magnetic field and temperature scans for the in-plane and out-of-plane cases. Intrinsic large anisotropy in the magnetodielectric and converse ME effects was revealed. The spin-driven electric polarizations induced in the ab-plane and out-of-plane were found to be closely related to the transverse conical and alternating longitudinal conical spin structures, respectively. The amplitudes of varied magnetization ΔM are about ∼0.128 μB/f.u and ∼0.0178 μB/f.u. in the E oscillating between ±1 MV/m at 100 K, corresponding to the converse ME effect coefficient of ∼3500 ps/m and ∼480 ps/m for the in-plane and out-of-plane cases, respectively. All these results demonstrate the essential and unique spin-order-induced anisotropic ferroelectricity and ME properties in these Y-type hexaferrites.

МАГНИТОЭЛЕКТРИЧЕСКИЕ ЭФФЕКТЫ В МУЛЬТИФЕРРОИКАХ

Research of magnetoelectric effects and multiferroic materials, in which these effects are manifested, is among key areas in modern magnetism. This is associated with promising aspects of applying the results to spintronics, orbitronics, and new generation systems for information storage and processing. Despite a great many of already known materials which, in varying degrees, have magnetoelectric properties, the question regarding physical mechanisms and nature of magnetoelectric effects still remains open. Single-phase multiferroics with their magnetic and segnetoelectric properties implemented in a single crystalline phase are of especially great interest for studying. The most known "traditional multiferroics", such as bismuth ferrite, manganites, rare earthearth orthoferrites and orthochromites, fall into the class of perovskite multiferroics, i.e. the crystalline structure of ABO3 perovskites serve as their pre-phase. However, the difference between crystallographic distortions that results in the formation of various crystalline structures, for example, bismuth ferrite and rare-earth orthoferrites/orthochromites, leads to an essential difference in their physical properties, including magnetoelectric ones. Whereas bismuth ferrite characterized by the R3c symmetry space group is a segnetoelectric (i.e. it has spontaneous segnetoelectric polarization), the presence of segnetoelectric polarization in orthoferrites/orthochromites with the Pbnm symmetry space group is impossible from the symmetry standpoint. However, recent experimental and theoretical research works show that under certain conditions magnetoelectric properties are found in both classes of the said multiferroics. This paper is an overview by its nature and discusses magnetoelectric effects in various classes of single-phase multiferroics with the distorted perovskite structure: proper multiferroics exemplified by bismuth ferrite and improper multiferroics exemplified by rare-earth orthoferrites/orthochromites. Consideration is given to basic principles of the symmetry approach used to study magnetoelectric effects in multiferroics; calculations and analysis of magnetoelectric effects in rare-earth orthoferrites/orthochromites are performed through the methods of group-theoretical analysis.

Magnetic field-induced polarization reversal in Y-type hexaferrites Ba0.7 Sr1.3CoZnFe11AlO22 single crystals

Y-type hexaferrites with non-collinear conical magnetic structures, which induce electric polarization (P), are prospective single-phase multiferroics with broad applications, such as magnetic sensors, energy convertors, and memory devices, among others. Y-type hexaferrites have attracted considerable attention. In this study, we investigated the magnetoelectric properties of Ba0.7Sr1.3CoZnFe11AlO22 single crystals with Y-type hexaferrite structures by replacing half of Zn2+ ions with Co2+ ions. A transition from a screw order to the ferromagnetic is observed at TN2 = 356 K in Ba0.7Sr1.3CoZnFe11AlO22 single crystals. The electric polarization (P) and magnetoelectric coupling coefficient(α) reach a maximum of 130 μC/m2 and 16667 ps/m at T = 30 K and E = 880 kV/m, respectively. Most importantly, the in-plane Hab induced P can be fully reversed by applying in-plane Hab and it maintains a non-zero value even when the applied magnetic field is zero. Since the P does not decay and repeats many times under a fluctuating low magnetic field, it is important for potential applications in new types of non-volatile memory devices.

Strain relaxation dynamics of multiferroic orthorhombic manganites

Resonant ultrasound spectroscopy has been used to characterise strain coupling and relaxation behavior associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic to colinear-sinusoidal incommensurate antiferromagnetic transition at ∼41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ∼150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, E a, of ≳0.04 eV from a loss peak at ∼80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ∼10−6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded E a ⩾ ∼0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ∼0.1–1‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic–orthorhombic transition due to the combined Jahn–Teller/octahedral tilting transition in the vicinity of 1500 K are 2–3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.