Multiferroic Magnetoelectric Coupling Research Articles

Magnetoelectric coupling in multiferroics probed by optical second harmonic generation

Magnetoelectric coupling, as a fundamental physical nature and with the potential to add functionality to devices while also reducing energy consumption, has been challenging to be probed in freestanding membranes or two-dimensional materials due to their instability and fragility. In this paper, we report a magnetoelectric coupling probed by optical second harmonic generation with external magnetic field, and show the manipulation of the ferroelectric and antiferromagnetic orders by the magnetic and thermal fields in BiFeO3 films epitaxially grown on the substrates and in the freestanding ones. Here we define an optical magnetoelectric-coupling constant, denoting the ability of controlling light-induced nonlinear polarization by the magnetic field, and found the magnetoelectric-coupling was suppressed by strain releasing but remain robust against thermal fluctuation for freestanding BiFeO3.

A review on current status and mechanisms of room-temperature magnetoelectric coupling in multiferroics for device applications

A review on current status and mechanisms of room-temperature magnetoelectric coupling in multiferroics for device applications

Magneto-electric coupled ordered PMN-PT/NiFe2O4 composite nanostructures

In this work, a well-ordered array of multiferroic magnetoelectric (ME) dot-like nanostructures of Pb(Mg1/3Nb2/3)O3]0.65–[PbTiO3]0.35 (PMN-PT)/NiFe2O4 is explored for high density and low power consuming memory devices. Ordered arrays of ferromagnetic NiFe2O4 nanodots underneath a ferroelectric PMN-PT layer were fabricated using silicon nitride based stencil masks and pulsed laser deposition techniques. The piezo-response and magnetic force microscopy (PFM) measurements reveal coexistence of magnetic and ferroelectric domains in PMN-PT/NiFe2O4 films at room temperature. The ferroelectric polarization can be switched with the electrically biased PFM tip. The ME coupling is evident in the PMN-PT/NiFe2O4 films, which is attributed to the transfer of the elastic strain from PMN-PT to NiFe2O4. The PMN-PT/NiFe2O4 nanodot films exhibit enhanced ME coupling coefficient (α) as compared to continuous bilayer PMN-PT/NiFe2O4 films, owing to the superior strain transfer efficiency in nanodot heterostructures. The nanodot films demonstrate electric-field controlled nonvolatile switching of α, which can be used to store binary information in memory devices, holding all the advantages of ferroelectric random access memory but overcoming the major disadvantage of destructive reading of polarization. The results reveal a versatile approach for fabrication of well-ordered nanodot arrays for low power consuming, high-density ME device applications.

Thermodynamics of metamagnetoelectric effect in multiferroics

Abstract Magnetoelectric coupling in multiferroics gives rise to various properties such as metamagnetoelectric effect since external fields act. In this study, a general thermodynamic framework is developed to investigate metamagnetoelectric effects in multiferroic materials. The model used is a quasi-two dimensional frustrated spin chain controlled by a static electric field in y-direction and magnetic field in z-direction. The effects of metamagnetoelectric transitions on entropy, heat capacity and on the linear magnetoelectric coupling factor are assessed using Fermi-Dirac statistics of quantum gases and the Landau theory. The entropy behavior is shown to be similar to that of the magnetic susceptibility. In fact, while the magnetic susceptibility characterizes the variations of magnetization and accordingly emphasizes the ferroic transition points of this order, the intrinsic physics of these transition points highlights a muddle occurring due to a rearrangement of magnetic moments in the system, and this is accurately described in terms of entropy. The transition effects due to this rearrangement described in terms of entropy at the corresponding critical points show different loop to that of the heat capacity. The opposite loop showed by the heat capacity compared to the entropy is its weakening at the exact transition point in spite of its strengthening during the transition process. It is also recorded only a few ranges of the electric field which allows the effect provided. The temperature dependence of the magnetoelectric coupling highlights how it is continuously weakened by the increase of temperature, leading to a second order transition from the metamagnetoelectric state.

Complex strain evolution of polar and magnetic order in multiferroic BiFeO3 thin films

Electric-field control of magnetism requires deterministic control of the magnetic order and understanding of the magnetoelectric coupling in multiferroics like BiFeO3 and EuTiO3. Despite this critical need, there are few studies on the strain evolution of magnetic order in BiFeO3 films. Here, in (110)-oriented BiFeO3 films, we reveal that while the polarization structure remains relatively unaffected, strain can continuously tune the orientation of the antiferromagnetic-spin axis across a wide angular space, resulting in an unexpected deviation of the classical perpendicular relationship between the antiferromagnetic axis and the polarization. Calculations suggest that this evolution arises from a competition between the Dzyaloshinskii–Moriya interaction and single-ion anisotropy wherein the former dominates at small strains and the two are comparable at large strains. Finally, strong coupling between the BiFeO3 and the ferromagnet Co0.9Fe0.1 exists such that the magnetic anisotropy of the ferromagnet can be effectively controlled by engineering the orientation of the antiferromagnetic-spin axis.

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

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

Recognition of exchange striction as the origin of magnetoelectric coupling in multiferroics

The magnetoelectric coupling, a phenomenon inducing magnetic (electric) polarization by application of an external electric (magnetic) field and first conjectured by Curie in 1894, is observed in most of the multiferroics and used for many applications in various fields such as data storage or sensing. However, its microscopic origin is a long-standing controversy in the scientific community. An intense revival of interest developed in the beginning of the 21st century due to the emergence of multiferroic frustrated magnets in which the ferroelectricity is magnetically induced and which present an inherent strong magnetoelectric coupling. The Dzyaloshinskii-Moriya interaction (DMI) well accounts for such ferroelectricity in systems with a noncollinear magnetic order such as the ${\mathrm{RMnO}}_{3}$ manganites. The DMI effect is, however, inadequate for systems presenting ferroelectricity induced by quasicollinear spin arrangements such as the prominent ${\mathrm{RMn}}_{2}{\mathrm{O}}_{5}$ manganites. Among different microscopic mechanisms proposed to resolve this incompatibility, the exchange-striction model stands as the most invoked candidate. In this scenario, the polar atomic displacements originate from the release of a frustration caused by the magnetic order. Despite its theoretical description 15 years ago, this mechanism had yet to be unambiguously validated experimentally. The breakthrough finally comes from ${\mathrm{SmMn}}_{2}{\mathrm{O}}_{5}$ presenting a unique magnetic order revealed by powder neutron diffraction. The unique orientation of its magnetic moment establishes the missing element that definitely validates the exchange striction as the effective mechanism for the spin-induced ferroelectricity in this series. More generally, this is a proof of concept that validates this model on actual systems, facilitating the development of a new generation of multiferroics with unrivaled magnetoelectric properties.

Multiferroic approach for Cr,Mn,Fe,Co,Ni,Cu substituted BaTiO3 nanoparticles

Multiferroic magnetoelectric (ME) at room temperature is significant for new design nano-scale spintronic devices. We have given a comparative study to report multiferroicity in BaTM0.01Ti0.99O3 [TM = Cr,Mn,Fe,Co,Ni,Cu (1 mol% each) substituted BaTiO3 (BTO)] nanoparticles. The TM ions influenced both nano-size and lattice distortion of Ti–O6 octahedra to the BTO. X ray diffraction study indicates that the dopant TM could influence lattice constants, distortion, tetragonal splitting of diffraction peaks (002/200) as well as peak shifting of diffraction angle in the BTO lattice. This can induce lattice strain which responsible to oxygen defects formation to mediate ferromagnetism. Also, the lattice strain effect could responsible to reduce the depolarization field of ferroelectricity and provide piezoelectric and magnetostrictive strains to enhance ME coupling. The size of BTO nanoparticles is varied in 13–51 nm with TM doping. The room temperature magnetic measurement indicates antiferromagnetic exchange interactions in BTO lattice with TM ions. The zero-field cooling and field cooling magnetic measurement at 500 Oe indicates antiferromagnetic to ferromagnetic transition. It also confirms that the substitution of Cr, Fe and Co into BTO could induce strong antiferromagnetic behavior. However, the substitutions of Mn, Ni and Cu have weak antiferromagnetic character. The temperature dependent dielectric measurements indicates polarization enhancement that influenced with both nano-size as well TM ions and exhibits ferroelectric phase transition with relaxor-like characteristics. Dynamic ME coupling is investigated, and the longitudinal ME voltage coefficient, αME is equivalent to linear ME coupling coefficient, is also calculated.

Magnetoelectric Coupling in Well-Ordered Epitaxial BiFeO3/CoFe2O4/SrRuO3 Heterostructured Nanodot Array.

Multiferroic magnetoelectric (ME) composites exhibit sizable ME coupling at room temperature, promising applications in a wide range of novel devices. For high density integrated devices, it is indispensable to achieve a well-ordered nanostructured array with reasonable ME coupling. For this purpose, we explored the well-ordered array of isolated epitaxial BiFeO3/CoFe2O4/SrRuO3 heterostructured nanodots fabricated by nanoporous anodic alumina (AAO) template method. The arrayed heterostructured nanodots demonstrate well-established epitaxial structures and coexistence of piezoelectric and ferromagnetic properties, as revealed by transmission electron microscopy (TEM) and peizoeresponse/magnetic force microscopy (PFM/MFM). It was found that the heterostructured nanodots yield apparent ME coupling, likely due to the effective transfer of interface couplings along with the substantial release of substrate clamping. A noticeable change in piezoelectric response of the nanodots can be triggered by magnetic field, indicating a substantial enhancement of ME coupling. Moreover, an electric field induced magnetization switching in these nanodots can be observed, showing a large reverse ME effect. These results offer good opportunities of the nanodots for applications in high-density ME devices, e.g., high density recording (>100 Gbit/in.(2)) or logic devices.

Thermally mediated mechanism to enhance magnetoelectric coupling in multiferroics.

The main roadblock on the way to practical realization of magnetoelectric devices is the lack of multiferroics with strong magnetoelectric coupling. We propose an unusual route to dramatically enhance this coupling through a thermally mediated mechanism. Such a thermally mediated magnetoelectric effect is quantified by an isentropic rather than isothermal magnetoelectric response and is computed here from first principles. A robust enhancement of the magnetoelectric coupling is predicted for both naturally occurring and heterostructured materials.

Multiferroic properties of o-LuMnO3 controlled by b-axis strain.

Strain is a leading candidate for controlling magnetoelectric coupling in multiferroics. Here, we use x-ray diffraction to study the coupling between magnetic order and structural distortion in epitaxial films of the orthorhombic (o-) perovskite LuMnO(3). An antiferromagnetic spin canting in the E-type magnetic structure is shown to be related to the ferroelectrically induced structural distortion and to a change in the magnetic propagation vector. By comparing films of different orientations and thicknesses, these quantities are found to be controlled by b-axis strain. It is shown that compressive strain destabilizes the commensurate E-type structure and reduces its accompanying ferroelectric distortion.

Electric polarization of magnetic textures: New horizons of micromagnetism

A common scenario of magnetoelectric coupling in multiferroics is the electric polarization induced by spatially modulated spin structures. It is shown in this paper that the same mechanism works in magnetic dielectrics with inhomogeneous magnetization distribution: the domain walls and magnetic vortexes can be the sources of electric polarization. The electric field driven magnetic domain wall motion is observed in iron garnet films. The electric field induced nucleation of vortex state of magnetic nanodots is theoretically predicted and numerically simulated. From the practical point of view the electric field control of micromagnetic structures suggests a low-power approach for spintronics and magnonics.

Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO 3–0.3BaTiO 3 nanocrystals

The structural, microstructural, polarization, magnetization, dielectric constant, and relaxor characteristics of 0.7BiFeO 3–0.3BaTiO 3 (BF–BT) nanocrystals have been studied. BF–BT nanocrystals were prepared by a chemical route using polyvinyl alcohol as surfactant. The phase structure is confirmed by X-ray diffraction and average particle size by transmission and scanning electron microscopy. The magnetoelectric coupling is studied by polarization hysteresis loops under the influence of applied magnetic field and the phase transition anomaly. The diffuse phase transition is studied by modified Curie–Weiss law and relaxor characteristics by Vogel–Fulcher relation.

Multiferroic magnetoelectric composite nanostructures

Multiferroics are attracting increasing interest and provoking much research activity driven by the profound physics of these materials, the coexistence and coupling of ferroelectric and magnetic orders, and the potential applications in novel multifunctional devices such as sensors, transducers, memories and spintronics. Multiferroic magnetoelectric (ME) composite systems, such as ferromagnetic–ferroelectric heterostructures, which offer a novel route for integrating ferroelectric and ferromagnetism, have been widely studied in recent years. In these ME composite systems, ME coupling is strain-mediated, that is, the strain induced in one component, either by magnetostriction in the ferromagnetic or by the piezoelectric effect in the ferroelectric, is transferred to the other component, altering the polarization or magnetization. This article reviews the magnetic-field control of electric polarization and its converse effect, electric-field control of magnetization, in multiferroic ME composite nanostructures. The review focuses on three kinds of ME nanostructures: vertical heterostructures, horizontal heterostructures and particulate nanocomposite films. Theoretical approaches, such as Green’s function methods, the phase field model and first-principles methods, have been used to simulate and predict the ME coupling effect in such nanostructures. Herein we briefly describe the potential applications of the ME nanostructured composites using representative examples, and outline the challenges and promising future for this field.

Three-sublattice mean-field approach for magnetoelectric coupling in multiferroics

A three-sublattice mean-field approximation (MFA) to ferroelectromagnets with coexisting antiferromagnetic order and ferroelectric order is proposed to understand the magnetic transitions induced by magnetoelectric coupling between the two types of orders. The temperature dependence of magnetization, magnetic susceptibility and magnetoelectric susceptibility under different coupling strengths are calculated, indicating the intrinsic weak ferromagnetic transition at low temperature due to the magnetoelectric coupling. Comparing with the predictions given by earlier MFA, the present MFA predictions are in better accord with the Monte Carlo simulations.