Multiferroic Structure Research Articles

Correlation of dielectric, electrical and magnetic properties near the magnetic phase transition temperature of cobalt zinc ferrite.

Multiferroic composite structures, i.e., composites of magnetostrictive and piezoelectric materials, can be envisioned to achieve the goal of strong room-temperature ME coupling for real practical device applications. Magnetic materials with high magnetostriction, high Néel temperature (TN), high resistivity and large magnetization are required to observe high ME coupling in composite structures. In continuation of our investigations on suitable magnetic candidates for multiferroic composite structures, we have studied the crystal structure, dielectric, transport, and magnetic properties of Co0.65Zn0.35Fe2O4 (CZFO). Rietveld refinement of X-ray diffraction patterns confirms the phase purity with a cubic crystal structure with the (Fd3[combining macron]m) space group; however, we have found a surprisingly large magneto-dielectric anomaly at the Néel temperature, unexpected for a cubic structure. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photoelectron spectroscopy (XPS), which supports the catonic ordering-mediated large dielectric response. Large dielectric permittivity dispersion with a broad anomaly is observed in the vicinity of the magnetic phase transition temperature (TN) of CZFO suggesting a strong correlation between dielectric and magnetic properties. The evidence of strong spin-polaron coupling has been established from temperature dependent dielectric, ac conductivity and magnetization studies. The ferrimagnetic-paramagnetic phase transition of CZFO has been found at ∼640 K, which is well above room temperature. CZFO exhibits low loss tangent, a high dielectric constant, large magnetization with soft magnetic behavior and magnetodielectric coupling above room temperature, elucidating the possible potential candidates for multiferroic composite structures as well as for multifunctional and spintronics device applications.

Effects of complex modulus and residual stress on the vibration induced resonant fracture behavior of a multiferroic cylindrical structure

Multiferroic devices often serve under harmonic vibration. Resonance induced cracking is a common failure of them. The present work studies the resonant fracture of a multiferroic cylindrical structure by the combined methods of variable separation, infinite series, singular integral equation and Lobatto–Chebyshev collocation. It is found that complex modulus and residual stress are two important factors governing the resonant fracture behavior. Generally, the loss factors only affect the resonant amplitude of the fracture parameter, but the residual stress affects both the resonant amplitude and frequency. The obtained conclusions can provide guidelines for the anti-fracture optimal designs of multiferroic devices.

Theoretical Investigation of Dual Tuning of Solitonic Processes in Multiferroic Structures

. The solitonic wave processes in a multiferroic structure based on ferroelectric and ferrite layers are studied. The influence of external electric and magnetic fields on frequency and wave-number ranges, where bright and dark solitons can exist, are analysed. The investigation was carried out with the nonlinear Schrodinger equation. Results show that an increase of the electric field shifts the boundary between bright and dark solitons to long-wave region. An increase in magnetic field results in the opposite effect.

Dual-tunable ferrite-ferroelectric dynamic magnonic crystal

A dynamic magnonic crystal based on a planar multiferroic structure consisting of a ferrite and a ferroelectric layers has been designed and theoretically investigated for the first time. Spatially periodic electric field applied to the ferroelectric layer turns the regular multiferroic waveguide into periodic one. An electrodynamic model of the dynamic magnonic crystal and theoretical analysis of its performance is presented.

Relaxor properties of barium titanate crystals grown by Remeika method

Barium titanate (BaTiO3, BT) crystals have been grown by the Remeika method using both the regular KF and mixed KF-NaF (0.6-0.4) solvents. Typical acute angle “butterfly wing” BT crystals have been obtained, and they were characterized using x-ray diffraction, scanning electron microscopy (including energy dispersive spectroscopy), conventional dielectric and acoustic emission methods. A typical wing has a triangular plate shape which is up to 0.5mm thick with a 10–15mm2 area. The plate has a (001) habit and an atomically smooth outer surface. Both K+ and F− solvent ions are incorporated as dopants into the crystal lattice during growth substituting for Ba2+ and O2− ions respectively. The dopants' distribution is found to be inhomogeneous, their content being almost an order of magnitude higher (up to 2mol%) at out surface of the plate relatively to the bulk. A few μm thick surface layer is formed where a multidomain ferroelectric net is confined between two≤1µm thick dopant-rich surfaces. The layer as a whole possess relaxor ferroelectric properties, which is apparent from the appearance of additional broad maxima, Tm, in the temperature dependence of the dielectric permittivity around the ferroelectric phase transition. Intense acoustic emission responses detected at temperatures corresponding to the Tm values allow to observe the Tm shift to lower temperatures at higher frequencies, or dispersion, typical for relaxor ferroelectrics. The outer surface of the BT wing can thus serve as a relaxor thin film for various electronic application, such as capacitors, or as a substrate for BT-based multiferroic structure. Crystals grown from KF-NaF fluxes contain sodium atoms as an additional impurity, but the crystal yield is much smaller, and while the ferroelectric transition peak is diffuse it does not show any sign of dispersion typical for relaxor behavior.

A microwave interferometer based on a ferrite–ferroelectric layered structure

A microwave interferometer using spin-electromagnetic waves was developed for the first time. Theoretical and experimental investigation of its characteristics was performed. The interferometer is a bridge circuit with a phase shifter in one of its arms, which is based on a planar multiferroic structure consisting of ferrite and ferroelectric layers. The device can be regarded as a prototype of spin-electromagnetic logic.

Fracture analysis on a layered multiferroic smart structure containing non-periodic interfacial cracks under magnetic/electric and mechanical loadings

ABSTRACTThe problems of periodic cracks have been widely solved in existing literature of fracture mechanics. Although the assumption of periodicity facilitates the theoretical derivation, practical multiple cracks in most cases are not periodically distributed in materials and structures. The present article performs fracture analysis on a piezoelectric/piezomagnetic/piezoelectric (PE/PM/PE) tri-layered smart structure containing nonperiodic interfacial cracks under magnetic/electric and mechanical loadings. The methods of dislocation simulation, Green's function and Cauchy singular integral equations are employed to solve the problem. Stress intensity factors (SIFs) are numerically calculated, and parametric studies are conducted to demonstrate the variation laws of SIFs versus geometrical parameters. The obtained conclusions may provide references for the optimal design of PE/PM/PE smart structures.

Voltage-impulse-induced nonvolatile tunable magnetoelectric inductor based on multiferroic bilayer structure

In this study, we developed a voltage-impulse-induced nonvolatile tunable magnetoelectric inductor, which used an amorphous magnetic ribbon/lead zirconate titanate (PZT) multiferroic bilayer structure as a magnetic core. The PZT substrate, which contained defect dipoles through acceptor doping, was used in the bilayer structure to obtain an asymmetric strain–E “butterfly” curve. Different and stable voltage-impulse-induced in-plane residual stain states could be obtained by applying specific voltage impulse excitation modes. These residual strain states induced a nonvolatile inductance variation in the inductor through strain-mediated magnetoelectric coupling. This method provided a promising approach to realize nonvolatile tunable inductors for miniaturized circuits and systems.

Artificial multiferroic structures based on barium-strontium titanate

Artificial extrinsic and intrinsic multiferroic structures were developed in the form of yttrium-iron garnet/barium-strontium titanate bi-layers and manganese-doped barium-strontium titanate films with a relatively high doping rate in the range of 10–20 mol%. For the experimental investigations, the planar capacitors were formed on the surfaces of the structures and the capacitance–voltage characteristics were measured. Structural characterization of the samples under study was based on X-ray diffraction analysis and film–substrate interfaces were investigated by X-ray photoelectron spectroscopy. The relative dielectric permittivity, dielectric loss tangent, and electric field tunability of the samples were measured in a temperature range from 77 to 300 К. The additional 6 and 3 % tuning of the dielectric permittivity by external magnetic field of 2000 Oe was demonstrated for the extrinsic and intrinsic structures, respectively.

The influence of a metal on transverse characteristics of hybrid waves in a layered ferrite–ferroelectric structure

We present electrodynamic characteristics of a layered multiferroic structure of metal–dielectric–ferrite–dielectric–ferroelectric–dielectric–metal type calculated by the finite elements method. Properties of transverse modes of hybrid waves (dispersion characteristics, electric field distribution at different frequencies, and damping rate) as functions of the distance between the ferroelectric layer and the metal screen are analyzed for the first time.

Heteroepitaxial Pb0.9Sr0.1TiO3/Bi0.9La0.1FeO3/Pb0.9Sr0.1TiO3 multiferroic structure: an effective way to improve the electrical, ferroelectric and magnetic performance

A ferroelectric/multiferroic structure composed of (Pb0.9Sr0.1)TiO3 (PST) and (Bi0.9La0.1)FeO3 (BLF) was heteroepitaxially deposited on a LaNiO3 buffered (001) SrTiO3 substrate by pulse laser deposition. Interestingly, the PST layer was acting as a buffering layer between BLF and electrode to suppress the migration of free carries in the heterostructure. A well-grown crystalline dense structure was observed from the surface morphology of PST/BLF/PST heterostructure. Moreover, the PST/BLF/PST trilayer exhibited improved ferroelectric and dielectric behaviors, together with two orders of magnitude lower leakage current and higher dielectric constant. Our result indicate that the multiferroic PST/BLF/PST heterostructure may have a promising application for the future high-density memory.

Engineering charge ordering into multiferroicity

Multiferroic materials have attracted great interests but are rare in nature. In many transitional metal oxides, charge ordering and magnetic ordering coexist, so that a method of engineering charge-ordered materials into ferroelectric materials would lead to a large class of multiferroic materials. We propose a strategy for designing new ferroelectric or even multiferroic materials by inserting a spacing layer into each two layers of charge-ordered materials and artificially making a superlattice. One example of the model demonstrated here is the perovskite (LaFeO$_3$)$_2$/LaTiO$_3$ (111) superlattice, in which the LaTiO$_3$ layer acts as the donor and the spacing layer, and the LaFeO$_3$ layer is half doped and performs charge ordering. The collaboration of the charge ordering and the spacing layer breaks the space inversion symmetry, resulting in a large ferroelectric polarization. As the charge ordering also leads to a ferrimagnetic structure, the (LaFeO$_3$)$_2$/LaTiO$_3$ is multiferroic. It is expected that this work can encourage the designing and experimentally implementation of a large class of multiferroic structures with novel properties.

Enhancing magnetoelectric effect in multiferroic composite bilayers via flexoelectricity

We employ the flexoelectricity to enhance the magnetoelectric (ME) (coupling) effect in multiferroic (MF) composites and structures. An analytical model is presented to predict the ME effect in a MF composite bilayer consisting of piezomagnetic and piezoelectric layers. The flexoelectric effect in the piezoelectric layer is taken into account. The static ME effect in the MF composite bilayer with free boundary conditions is investigated. The results indicate that flexoelectricity can dramatically enhance the ME effect in multiferroic composites and structures.

Solitons of electric polarization in multiferroics

Complete analysis of the properties of breathers in spiral multiferroic structures in the sine-Gordon model has been presented. The methods of the excitation and detection of the breathers in the external electric and magnetic fields have been discussed.

Ballistic transfer of spin momentum in multiferroic tunnel junction

The field dependence of critical voltages of switching of magnetic states of a synthetic multiferroic structure is studied based on a bifurcation analysis of Landau–Lifshitz–Gilbert equations with the torques caused by a tunneling spin transport, taking into account the voltage dependence of transferred spin momenta at the variations in the magnitude and direction of electric polarization. The voltage dependences of transferred spin momenta are determined based on the free electron model, taking into account exchange splitting of electron energy subbands in magnetic beaches and the effect of changing the tunnel barrier height at the variations in the polarization magnitude and state.

Brillouin light scattering study of transverse mode coupling in confined yttrium iron garnet/barium strontium titanate multiferroic

Using the space-resolved Brillouin light scattering spectroscopy we study the transformation of dynamic magnetization patterns in a bilayer multiferroic structure. We show that in the comparison with a single yttrium iron garnet (YIG) film magnetization distribution is transformed in the bilayer structure due to the coupling of waves propagating both in an YIG film (magnetic layer) and in a barium strontium titanate slab (ferroelectric layer). We present a simple electrodynamic model using the numerical finite element method to show the transformation of eigenmode spectrum of confined multiferroic. In particular, we demonstrate that the control over the dynamic magnetization and the transformation of spatial profiles of transverse modes in magnetic film of the bilayer structure can be performed by the tuning of the wavevectors of transverse modes. The studied confined multiferroic stripe can be utilized for fabrication of integrated dual tunable functional devices for magnonic applications.

A phenomenological theory for polarization flop in spiral multiferroic TbMnO3

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO 3. The Neel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to ab-spiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum. With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between T flop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy of the system is temperature- and magnetic field-dependent.

Dispersion characteristics of spin-electromagnetic waves in planar multiferroic structures

A method of approximate boundary conditions is used to derive dispersion relations for spin-electromagnetic waves (SEWs) propagating in thin ferrite films and in multiferroic layered structures. A high accuracy of this method is proven. It was shown that the spin-electromagnetic wave propagating in the structure composed of a thin ferrite film, a thin ferroelectric film, and a slot transmission line is formed as a result of hybridization of the surface spin wave in the ferrite film and the electromagnetic wave in the slot-line. The structure demonstrates dual electric and magnetic field tunability of the SEW spectrum. The electric field tunability is provided by the thin ferroelectric film. Its efficiency increases with an increase in the thicknesses of the ferrite and ferroelectric films and with a decrease in the slot-line gap width. The theory is confirmed by experimental data.

Multiferroic materials and magnetoelectric physics: symmetry, entanglement, excitation, and topology

Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism and electricity. The discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this exciting area. The availability of more multiferroic materials and improved magnetoelectric performance are approaching to make the applications within reach. While seminal review articles covering the major progress before 2010 are available, an updated review addressing the new achievements since that time becomes imperative. In this review, following a concise outline of the basic knowledge of multiferroicity and magnetoelectricity, we summarize the important research activities on multiferroics, especially magnetoelectricity and related physics in the last six years. We consider not only single-phase multiferroics but also multiferroic heterostructures. We address the physical mechanisms regarding magnetoelectric coupling so that the backbone of this divergent discipline can be highlighted. A series of issues on lattice symmetry, magnetic ordering, ferroelectricity generation, electromagnon excitations, multiferroic domain structure and domain wall dynamics, and interfacial coupling in multiferroic heterostructures, will be revisited in an updated framework of physics. In addition, several emergent phenomena and related physics, including magnetic skyrmions and generic topological structures associated with magnetoelectricity will be discussed.

Investigation of spin-electromagnetic wave envelope solitons in a multiferroic layered structure

Hybrid spin-electromagnetic wave (SEW) envelope solitons have been studied both experimentally and theoretically. The solitons were formed during auto-generation of SEWs in an active ring resonator for which the role of the nonlinear dispersive waveguide media was played by a multiferroic layered ferrite-ferroelectric structure. It is demonstrated that the dielectric constant of the structure affects the nonlinear properties of SEWs.