Multiferroic Devices Research Articles

Modified structural, optical, magnetic and ferroelectric properties in (1-x)BaTiO3-xBaCo0.5Nb0.5O3-δ ceramics

Lead-free perovskite ceramics with the formula of (1-x)BaTiO3-xBaCo0.5Nb0.5O3-δ ((1-x)BT-xBCN), exhibit structural phase transition, modified band-gap (Eg), ferromagnetic and ferroelectric orders at room-temperature (RT). All ceramics have no impurity phase but the structural transition from tetragonal to cubic phase is observed with the increase of x. A possible mechanism is proposed to explain the variation of crystal structure. Raman scattering analysis not only proves the existence of doping-induced phase transition but also indicates the changed bonding vibration modes between adjacent atoms. The appearance of two band-gaps in doping samples, the low Eg is about 1.3 eV, obviously smaller than that of 3.2 eV for pure BaTiO3 (BT), which is attributed to the electronic orbital of Co 3d within the gap of BT. Moreover, the RT ferromagnetic behavior is obtained at x = 0.05, which can be interpreted via the F center exchange model. These results show the potential applications for perovskite oxides on solar energy conversion apparatus and multiferroic devices.

Probing local order in multiferroics by transmission electron microscopy

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

Electric-field-controlled magnetization switching in multiferroic heterostructures containing interactive magnetic nanoislands

Switching the magnetization with an electric field in multiferroic heterostructures is desirable since it provides a possibility to achieve information storage with lower power consumption than traditional spintronic devices by using an electric current to drive magnetization rotation. Most of the existing studies, however, have been on multiferroic heterostructures containing either a single magnetic island or spatially well-separated multiple magnetic islands, where the effect of long-range magnetostatic interactions among magnetic islands (i.e. so-called cross-talking) can be ignored. Here we employ phase-field simulations to study the effect of magnetic interactions among islands on the electric-field-controlled magnetization switching in multiferroic heterostructures. As an example, we consider two interactive Co40Fe40B20(CoFeB) magnetic nanoislands grown on a PMN-PT ((0 0 1)-oriented Pb(Mg1/3Nb2/3)0.7Ti0.3O3) ferroelectric layer. We find that the distance between two neighboring nanoislands has to exceed a critical value to achieve an independent 180° magnetization switching in each nanoisland. The present work provides guidance for further experimental studies on the electric field control of magnetization and design of novel multiferroic devices.

Multiferroic Metal–PbNb0.12Ti0.88O3−δ Films on Nb-Doped STO

Ferroelectricity-the switchable intrinsic electric polarization-has not yet been attained in a metal experimentally and is in fact generally deemed irreconcilable with free carriers, although polar metal has been achieved recently. Multiferroic metal has never even been proposed though multiferroics have been widely investigated. Here we report a room-temperature coexistence of multiferroicity and metallic behavior in PbNb0.12Ti0.88O3-delta films. The oxygen-vacancy-induced electrons become delocalized and ameliorate the ferromagnetic properties of these films, whereas they fail to vanish the polar displacements nor the individual dipole in each unit cell. This concurrent appearance of multiferroicity and metallicity is also confirmed by our first-principles calculation performed on 12.5% Nb-doped PbTiO3 with oxygen vacancies. These findings break a path to multiferroic metallic materials and offer a potential application for multiferroic spintronic devices.

Polymer–Nanocrystal Nanocomposites: Device Concepts in Capacitors and Multiferroics

Nanotechnology offers a variety of pathways to novel and low cost device fabrication for a future that will undoubtedly contain literally billions of ubiquitous electronic devices. Nanocomposite metal−insulator−metal (MIM) capacitors are of interest due to the possibility of reducing the number of discrete components in printed circuit boards, and alternatively using embedded or directly printed capacitors for nanopackaging in conjunction with integrated circuits. Flexible substrates also become candidates when employing low processing temperatures. In addition to compatibility with nanomanufacturing, the prospect of designing a nanocomposite dielectric, by combining colloidal nanoparticle fillers and polymer hosts, lends itself very well to the idea of tunability of the mechanical and electrical properties. In exploring combinations of complex oxide materials and polymers, the field goes beyond simple capacitors, towards multiferroic devices in which the potential overlap of magnetic permeability with permittivity can produce enactable devices for magnetodielectrics and magnetoelectrics. Our work in solution processing of inorganic oxide dielectrics and multiferroics, using a modified sol-gel approach allows for the preparation of a variety of formulations that can be treated as inks for deposition as layers and/or for the design of novel nanocomposite films. Starting with high dielectric materials, we discuss nanocomposite capacitance based energy storage, and progress to materials development for multiferroic nanocomposites.

Mechanical-Resonance-Enhanced Thin-Film Magnetoelectric Heterostructures for Magnetometers, Mechanical Antennas, Tunable RF Inductors, and Filters.

The strong strain-mediated magnetoelectric (ME) coupling found in thin-film ME heterostructures has attracted an ever-increasing interest and enables realization of a great number of integrated multiferroic devices, such as magnetometers, mechanical antennas, RF tunable inductors and filters. This paper first reviews the thin-film characterization techniques for both piezoelectric and magnetostrictive thin films, which are crucial in determining the strength of the ME coupling. After that, the most recent progress on various integrated multiferroic devices based on thin-film ME heterostructures are presented. In particular, rapid development of thin-film ME magnetometers has been seen over the past few years. These ultra-sensitive magnetometers exhibit extremely low limit of detection (sub-pT/Hz1/2) for low-frequency AC magnetic fields, making them potential candidates for applications of medical diagnostics. Other devices reviewed in this paper include acoustically actuated nanomechanical ME antennas with miniaturized size by 1–2 orders compared to the conventional antenna; integrated RF tunable inductors with a wide operation frequency range; integrated RF tunable bandpass filter with dual H- and E-field tunability. All these integrated multiferroic devices are compact, lightweight, power-efficient, and potentially integrable with current complementary metal oxide semiconductor (CMOS) technology, showing great promise for applications in future biomedical, wireless communication, and reconfigurable electronic systems.

Optically tunable magnetoelectric properties of inorganic-organic multiferroic flexible film

Magnetoelectric composites are promising materials for their physical properties such as spontaneous electrical polarization or magnetization with the application of magnetic or electric field. In addition, Spinel ferrite based magnetoelectric composite materials absorb a range of light energy, which offers a lot of physical phenomena behind the light-dependent magnetoelectric properties of the composite. The piezoelectric organic polymer based magnetoelectric composite of Zn0.2Co0.8Fe2O4-poly(vinylidene fluoride) flexible film has been fabricated by a chemical pyrophoric reaction process followed by a solution casting process. Structural and surface morphologies of the composite film have been studied. The visible light and applied magnetic field dependent AC electrical properties of the composite flexible film have been investigated at room temperature which reveals that the maximum change in impedance is found to be ∼10% and ∼20%, respectively. However, with an influence of both the light and the magnetic field, the impedance of the composite film is largely changed attributing that the excited electron may get a low resistive path through the grain and grain boundaries where the maximum change in impedance is found to be ∼30%. This result further is confirmed from the change of grain and grain boundary resistance and capacitance through the fitting of Nyquist plots. The present work provides information about the optically and magnetically influenced physical properties of the composite flexible film, which will be expanded for multiferroic magnetoelectric device applications.

Electric field tuning non-volatile dynamic magnetism in half-metallic alloys Co2FeAl/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

The half-metallic characteristic of Co2FeAl Huesler alloy films in Co2FeAl/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure was controlled by different substrate temperatures during deposition. The evolution of microstructure shows the B2 ordering degree of Co2FeAl thin film increase with the substrate temperature decreasing from 550 °C to 480 °C during deposition, which can be verified by anisotropy magnetoresistance measurement. Moreover, the result measured by rotating-angle ferromagnetic resonance demonstrates B2 ordering degree of Co2FeAl thin film controlled the change of symmetry of magnetic anisotropy. In addition, the loop-like curve of magnetic resonance field/linewidth versus electric field exhibits the non-volatile behavior, which can be attributed to the 109° ferroelectric domain switching of PMN-PT substrate. This result can provide opportunities for magnetization control in multiferroic devices.

Switching on superferromagnetism

Electric-field control of magnetism has emerged as a potential approach for low-power consumption spintronics. Although recent results in multiferroic systems have shown the possibility to manipulate the magnetic state of magnetic materials deposited on ferroelectric substrates by means of electric fields, the resulting magnetic state is multidomain in nature. For real multiferroic devices to become a true single magnetic domain state, lower dimensionality is required. In this work, the authors show, for an assemble of iron nanograins deposited on top of a BaTiO${}_{3}$ substrate, that an electric field-induced strain is capable of switching on a collective long-range ferromagnetic order (superferromagnetism) on an otherwise zero-dimensional superparamagnetic nanoparticle system. The effect, observed slightly above room temperature, holds promise for the implementation of nanoscale multiferroic systems in spin-based storage and logic architectures operating at ambient conditions.

Pas de deux of electricity and magnetism: an interview with Sang-Wook Cheong.

Materials can be ferroelectric, having a spontaneous electric polarization that can be reversed by an external electric field, or they can be ferromagnetic, exhibiting spontaneous magnetization that is switchable by an applied magnetic field. However, until the 1960s, scientists did not expect that these two ferroic properties could co-exist in a single material. Today, materials exhibiting more than one of the primary ferroic properties are called multiferroics. Here, the primary ferroic properties can be ferroelectricity, ferromagnetism, antiferromagnetism, ferroelasticity, ferrotoroidicity or others. Basically, the multiferroic effect originates from the simultaneous breaking of space inversion and time-reversal symmetries. Multiferroics can be imagined as a pas de deux of electricity and magnetism. Recently, National Science Review interviewed Professor Sang-Wook Cheong from Rutgers University, who is one of the pioneering scientists in this field. Cheong talked about the multiferroics field, which has been fast developing since the early 2000s. His introductions and opinions on diverse multiferroic materials and potential multiferroic devices, as well as future research directions, may provide a useful resource for researchers both inside and outside the multiferroic research field.

Epitaxial Growth of Orthorhombic GaFeO₃ Thin Films on SrTiO₃ (111) Substrates by Simple Sol-Gel Method.

A Sol-gel method assisted with spin-coating has been successfully used to grow orthorhombic GaFeO3 epitaxial films on SrTiO3 (111) substrates for the first time. The film with Pna21 crystal structure has been grown along the c-axis. The rocking curve of (004) reflection shows that the Full-Width at Half-Maximum (FWHM) value could be determined to be 0.230°, indicating good single crystallinity and high quality. X-ray Φ scan reveals a three-fold symmetry of the substrate and a six-fold symmetry of the film, respectively. The in-plane domains rotate 60° from each other in the film. Uniform film with dense structure, columnar grains with similar grain size was obtained. The thickness of the film was evaluated to be ~170 nm. The roughness value (RMS) measured by AFM was 4.5 nm, revealing a flat film. The in-plane Magnetization versus Magnetic field (M-H) curve at 5 K performs a typical ferri- or ferromagnetic hysteresis loop with a saturated magnetization (Ms) value of 136 emu/cm3. The Curie temperature could be determined to be 174 K. Compared to Pulsed Laser Deposition (PLD), the sol-gel method can prepare large area films with low cost. These new films show promising applications in multiferroic devices.

Magnon-driven interfacial magnetoelectric coupling in Co/PMN-PT multiferroic heterostructures.

Magnon-driven interfacial magnetoelectric coupling in Co/PMN-PT multiferroic heterostructures is investigated at room temperature. The electric field controlled ferromagnetic resonance field possesses a loop-like curve, with a large resonance field shift between positive and negative remanent polarizations, which confirms a non-volatile strong magnetoelectric coupling. However, with a non-magnetic Ta layer inserted at the Co/PMN-PT interface, a piezostrain-induced butterfly-like curve of the resonance field versus applied electric field of the Co/Ta/PMN-PT multiferroic heterostructure is observed. Further, the non-volatile behavior of the resonance field changing with the applied electric field can be obtained, consistent with the result of polarization versus applied electric field curve, which can be attributed to the magnon-driven interfacial magnetoelectric coupling, showing a strong correlation of magnetization of Co thin film and the polarization of PMN-PT. The result is promising for the design of future multiferroic devices.

Anisotropic Magnetoelectric Coupling and Cotton-Mouton Effects in the Organic Magnetic Charge-Transfer Complex Pyrene-F4TCNQ.

Magnetoelectric coupling is of high current interest because of its potential applications in multiferroic memory devices. Although magnetoelectric coupling has been widely investigated in inorganic materials, such observations in organic materials are extremely rare. Here, we report our discovery that organic charge-transfer (CT) complex pyrene-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (pyrene-F4TCNQ) can display anisotropic magnetoelectric coupling. Investigation of the crystal structure of pyrene-F4TCNQ complex demonstrates that the magnetoelectric coupling coefficient along the π-π interaction direction is much larger than the value along other directions. Furthermore, magnetoelectric coupling and magnetization can be tuned by changing the fluorine content in complexes. Besides, the Cotton-Mouton effect in pyrene-F4TCNQ is observed, enabling the control of optomagnetic devices. These results can pave the way for a new method for the future development of organic CT complexes and their applications in perpendicular memory devices and energy-transfer-related multiferroics.

The enhanced magnetoelectric effect and piezoelectric properties in the lead-free Bi3.15Nd0.85Ti3O12/La0.7Ca0.3MnO3 nano-multilayers composite thin films

The lead-free multiferroic nano-multilayers composite thin films were fabricated on basis of ferroelectric (FE) Bi3.15Nd0.85Ti3O12 (BNT) and ferromagnetic (FM) La0.7Ca0.3MnO3 (LCMO) parents with 8 nm/6 nm periodicity, respectively. The crystal structure, switching of polarization, magnetization and temperature dependence of magnetoelectric coupling (ME) effect for the multiferroic BNT/LCMO composite thin films were investigated in detail, respectively. Ferroelectric behavior along with an enhanced remnant polarization (2Pr) of 42 μC/cm2, saturated magnetization around 151 emu/cm3, the enhanced piezoelectric coefficient d33 of 263 pm/V and the enhanced magnetoelectric effect voltage coefficient of 72.1 mV/cm*Oe were obtained. The results show that the lead-free multiferroic films exhibit both good ferroelectric, dielectric and magnetic properties, as well as the enhanced ME effects, indicating that the coupling effects of electric and magnetic field exist in the fabricated Bi3.15Nd0.85Ti3O12/La0.7Ca0.3MnO3 (BNT/LCMO) nano-multilayer thin films, and the strong ME effect originating from the magnetic-mechanical-electric interaction and interface coupling were demonstrated. These results can be particularly important for developing nano-composite multiferroic devices.

Integrated magnetoelectric devices: Filters, pico-Tesla magnetometers, and ultracompact acoustic antennas

Abstract

Voltage Control of Two-Magnon Scattering in Multiferroic Layers for Tunable Magnetoelectric Devices

In this paper, multiferroic heterostructures have been prepared by growing MnZn ferrite films on ferroelectric single substrates using pulsed laser deposition technology. An enhanced magnetoelectric coupling has been achieved at the critical two-magnon scattering (TMS) angles, where the ferromagnetic resonance field shifts −653 and 211 Oe. The contribution of TMS effects is much larger than that of the conventional strain effect. This heterostructure provides an alternative route for novel integrated multiferroic materials and devices.

Integrated ferroics for sensing, power, RF, and µ-wave electronics

Abstract

Ultra-low-field magneto-elastocaloric cooling in a multiferroic composite device

The advent of caloric materials for magnetocaloric, electrocaloric, and elastocaloric cooling is changing the landscape of solid state cooling technologies with potentials for high-efficiency and environmentally friendly residential and commercial cooling and heat-pumping applications. Given that caloric materials are ferroic materials that undergo first (or second) order phase transitions near room temperature, they open up intriguing possibilities for multiferroic devices with hitherto unexplored functionalities coupling their thermal properties with different fields (magnetic, electric, and stress) through composite configurations. Here we demonstrate a magneto-elastocaloric effect with ultra-low magnetic field (0.16 T) in a compact geometry to generate a cooling temperature change as large as 4 K using a magnetostriction/superelastic alloy composite. Such composite systems can be used to circumvent shortcomings of existing technologies such as the need for high-stress actuation mechanism for elastocaloric materials and the high magnetic field requirement of magnetocaloric materials, while enabling new applications such as compact remote cooling devices.

Magnetic domain wall motion in nanoscale multiferroic devices under the combined action of magnetostriction, Rashba effect and dry-friction dissipation

The one-dimensional propagation of magnetic domain walls in a ferromagnetic nanostrip is investigated analytically in the framework of the extended Landau-Lifshitz-Gilbert equation. In particular, this study focuses on the characterization of the domain wall motion in the presence of stresses induced by a piezoelectric actuator, Rashba spin-orbit-torque due to structural inversion asymmetry and dry-friction dissipation accounting for structural disorder into the crystal lattice. By adopting the formalism of travelling waves and using realistic assumptions on the parameters here involved, it has been possible to deduce an explicit analytical expression of the DW velocity in the steady regime. It is also proven that the depinning threshold and the Walker breakdown, representing the boundaries of such a dynamical regime, are both affected by the strength of magnetostriction, Rashba field and dry-friction. Moreover, it is observed that the Rashba effect can also modify the domain wall mobility as well as the direction of propagation. The results here obtained are in qualitative good agreement with recent numerical simulations and experimental observations.

Structural transition, defect complexes and improved ferroelectric behaviors of Bi0.88Sr0.03Gd0.09Fe0.94Mn0.04Co0.02O3/Co1-xMnxFe2O4 bilayer thin films

Bi0.88Sr0.03Gd0.09Fe0.94Mn0.04Co0.02O3/Co1-xMnxFe2O4 (BSGFMC/CMxFO) bilayer thin films were successfully prepared by the sol-gel method. There exists a structural transition from the co-existence of trigonal-R3c:H and trigonal-R3m:R phases to a single trigonal-R3m:R phase in the BSGFMC layer, which is induced by Mn-doping in the CoFe2O4 layer. The amount of oxygen vacancies in the bilayer films is reduced by Mn-doping, leading to a decreased amount of defect complexes. The dielectric dispersion of the BSGFMC/CMxFO bilayer films also disappears with the increase of Mn content. The BSGFMC/CM0.3FO bilayer film exhibits saturated polarization at a lower applied voltage compared with the other samples, while, a large remnant polarization (106 μC cm−2) and a small coercive field (263 kV cm−1) are obtained in this bilayer film. The capacitance-voltage behaviors further confirm its impressive ferroelectric performance. The excellent ferroelectric properties of the BSGFMC/CM0.3FO bilayer film mainly result from the presence of fewer defect complexes and the structural transition in the BSGFMC layer. In addition, the BSGFMC/CM0.3FO bilayer film also displays a giant saturation magnetization with 45.91 emu cm−3. The superior multiferroic properties provide a potentially enable application in novel multiferroic devices.