Boosting Room‐Temperature Magneto‐Ionics in a Non‐Magnetic Oxide Semiconductor

Voltage control of magnetism through electric field‐induced oxygen motion (magneto‐ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto‐ionic rates and cyclability continue to be key challenges to turn magneto‐ionics into real applications. Here, it is demonstrated that room‐temperature magneto‐ionic effects in electrolyte‐gated paramagnetic Co₃O₄ films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric‐double‐layer transistor‐like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general.     

Electric Field‐Induced Oxidation of Ferromagnetic/Ferroelectric Interfaces

Composite multiferroics are a new class of material where magneto‐electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto‐electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface's chemistry and magnetism in Fe/BaTiO₃ and Fe/LiNbO₃ systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto‐electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.     

Self‐Assembled Room Temperature Multiferroic BiFeO3‐LiFe5O8 Nanocomposites

Multiferroic materials have driven significant research interest due to their promising technological potential. Developing new room‐temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel α‐LiFe₅O₈ (LFO) and a ferroelectric perovskite BiFeO₃ (BFO) is presented. It is observed that lithium (Li)‐doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of Li_xBi_1?xFeO₃ ceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping‐induced phase separation and local ferroic properties in both the BFO‐LFO composite ceramics and self‐assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO‐LFO composites are supported by first principles calculations. These findings shed light on Li's role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites for future energy, sensing, and memory applications.     

Hybrid Improper Ferroelectricity in Multiferroic Superlattices: Finite‐Temperature Properties and Electric‐Field‐Driven Switching of Polarization and Magnetization

The so‐called hybrid improper ferroelectricity (HIF) mechanism allows to create an electrical polarization by assembling two nonpolar materials within a superlattice. It may also lead to the control of the magnetization by an electric field when these two nonpolar materials are magnetic in nature, which is promising for the design of novel magneto‐electric devices. However, several issues of fundamental and technological importance are presently unknown in these hybrid improper ferroelectrics. Examples include the behaviors of its polarization and dielectric response with temperature, and the paths to switch both the polarization and magnetization under electric fields. Here, an effective Hamiltonian scheme is used to study the multiferroic properties of the model superlattice (BiFeO₃)₁/(NdFeO₃)₁. Along with the development of a novel Landau‐type potential, this approach allows to answer and understand all the aforementioned issues at both microscopic and macroscopic levels. In particular, the polarization and dielectric response are both found to adopt temperature dependences, close to the phase transition, that agree with the behavior expected for first‐order improper ferroelectrics. And most importantly, a five‐state path resulting in the switching of polarization and magnetization under an electric field, via the reversal of antiphase octahedral tiltings, is also identified.     

A Large Magnetoresistance Effect in p–n Junction Devices by the Space‐Charge Effect

The finding of an extremely large magnetoresistance effect on silicon based p–n junction with vertical geometry over a wide range of temperatures and magnetic fields is reported. A 2500% magnetoresistance ratio of the Si p–n junction is observed at room temperature with a magnetic field of 5 T and the applied bias voltage of only 6 V, while a magnetoresistance ratio of 25 000% is achieved at 100 K. The current‐voltage (I–V) behaviors under various external magnetic fields obey an exponential relationship, and the magnetoresistance effect is significantly enhanced by both contributions of the electric field inhomogeneity and carrier concentrations variation. Theoretical analysis using classical p–n junction transport equation is adapted to describe the I–V curves of the p–n junction at different magnetic fields and reveals that the large magnetoresistance effect origins from a change of space‐charge region in the p–n junction induced by external magnetic field. The results indicate that the conventional p–n junction is proposed to be used as a multifunctional material based on the interplay between electronic and magnetic response, which is significant for future magneto‐electronics in the semiconductor industry.     

Universal Behavior and Electric‐Field‐Induced Structural Transition in Rare‐Earth‐Substituted BiFeO3

The discovery of a universal behavior in rare‐earth (RE)‐substituted perovskite BiFeO₃ is reported. The structural transition from the ferroelectric rhombohedral phase to an orthorhombic phase exhibiting a double‐polarization hysteresis loop and substantially enhanced electromechanical properties is found to occur independent of the RE dopant species. The structural transition can be universally achieved by controlling the average ionic radius of the A‐site cation. Using calculations based on first principles, the energy landscape of BiFeO₃ is explored, and it is proposed that the origin of the double hysteresis loop and the concomitant enhancement in the piezoelectric coefficient is an electric‐field‐induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase.     

Atmosphere‐Induced Reversible Resistivity Changes in Ca/Y‐Doped Bismuth Iron Garnet Thin Films

Bismuth iron garnet Bi₃Fe₅O₁₂ (BIG) is a multifunctional insulating oxide exhibiting remarkably the largest known Faraday rotation and linear magnetoelectric coupling. Enhancing the electrical conductivity in BIG while preserving its magnetic properties would further widen its range of potential applications in oxitronic devices. Here, a site‐selective codoping strategy in which Ca²⁺ and Y³⁺ substitute for Bi³⁺ is applied. The resulting p‐ and n‐type doped BIG films combine state‐of‐the‐art magneto‐optical properties and semiconducting behaviors above room temperature with rather low resistivity: 40 Ω cm at 450 K is achieved in an n‐type Y‐doped BIG; this is ten orders of magnitude lower than that of Y₃Fe₅O₁₂. High‐resolution electron spectromicroscopy unveils the complete dopant solubility and the charge compensation mechanisms at the local scale in p‐ and n‐type systems. Oxygen vacancies as intrinsic donors play a key role in the conduction mechanisms of these doped BIG films. On the other hand, a self‐compensation of Ca²⁺ with oxygen vacancies tends to limit the conduction in p‐type Ca/Y‐doped BIG. These results highlight the possibility of integrating n‐type and p‐type doped BIG films in spintronic structures as well as their potential use in gas sensing applications.     

Magnetic‐Field‐Induced Phase‐Selective Synthesis of Ferrosulfide Microrods by a Hydrothermal Process: Microstructure Control and Magnetic Properties

Microrods of the ferrosulfide minerals greigite (Fe₃S₄) and marcasite (FeS₂) are selectively synthesized by an in situ magnetic‐field‐assisted hydrothermal route. Each complex microrod is composed of fine building blocks with different shapes. The unique magnetic properties of the microrods and electrical performance of a single microrod are studied. The results demonstrate that the magnetic properties of the ferrosulfide minerals are strongly related to their corresponding microstructures. The value of the low‐temperature transition increases as the greigite component in the product decreases. The combination of small‐molecule sulfur precursors and an applied magnetic field makes possible the selective synthesis of ferrosulfide minerals with different phases and distinct microstructures, underlining the fact that the magnetic field can be a useful tool as well as an independent parameter for the phase‐selective synthesis and self‐assembly of inorganic building blocks in solution chemistry.     

Local Built‐In Electric Field Enabled in Carbon‐Doped Co3O4 Nanocrystals for Superior Lithium‐Ion Storage

In this work, a novel concept of introducing a local built‐in electric field to facilitate lithium‐ion transport and storage within interstitial carbon (C‐) doped nanoarchitectured Co₃O₄ electrodes for greatly improved lithium‐ion storage properties is demonstrated. The imbalanced charge distribution emerging from the C‐dopant can induce a local electric field, to greatly facilitate charge transfer. Via the mechanism of “surface locking” effect and in situ topotactic conversion, unique sub‐10 nm nanocrystal‐assembled Co₃O₄ hollow nanotubes (HNTs) are formed, exhibiting excellent structural stability. The resulting C‐doped Co₃O₄ HNT‐based electrodes demonstrate an excellent reversible capacity ≈950 mA h g^?1 after 300 cycles at 0.5 A g^?1 and superior rate performance with ≈853 mA h g^?1 at 10 A g^?1.     

In Situ Reversible Ionic Control for Nonvolatile Magnetic Phases in a Donor/Acceptor Metal‐Organic Framework

Reversible magnetic control by electrical means, which is highly desired from the viewpoint of fundamentals and technological applications such as data storage devices, has been a challenging topic. In this study, the authors demonstrate in situ magnetic phase switching between the ferrimagnetic and paramagnetic states of an electron‐donor/‐acceptor metal‐organic framework (D/A‐MOF) using band‐filling control mediated by the Li⁺‐ion migration that accompanies redox reactions, i.e., “magneto‐ionic control”. By taking advantage of the rechargeability of lithium‐ion battery systems, in which Li⁺‐ions and electrons are simultaneously inserted into/extracted from a cathode material, the reversible control of nonvolatile magnetic phases in a D/A‐MOF has been achieved. This result demonstrates that the combination of a redox‐active MOF with porous flexibility and ion‐migration capability enables the creation of new pathways toward magneto‐electric coupling devices in the field of ionics.     

Boosting Photocurrent via Heating BiFeO3 Materials for Enhanced Self‐Powered UV Photodetectors

BiFeO₃ (BFO) is a potentially important Pb‐free ferroelectric with a narrow bandgap and is expected to become a novel photodetector. The photocurrent in BFO₃ strongly depends on the temperature but only a few studies have investigated in detail the relationships between photocurrent and temperature. Here, the temperature‐dependent photocurrent and the corresponding photosensing properties of a Ag/BFO/indiumtin oxide (ITO) photodetector based on an optimized planar‐structured electrode configuration are investigated. The photocurrent and responsivity of the BFO₃‐based photodetector can first be increased and then be decreased with increasing temperature. The largest photocurrent and responsivity can reach 51.5 µA and 6.56 × 10^?4 A W^?1 at 66.1 °C, which is enhanced 126.3% as compared with that at room temperature. This may be caused by the temperature‐modulated bandgap and barrier height in Ag/BFO/ITO device. This study clarifies the relationship between photosensing performance and the operating temperature of BFO‐based photodetector and will push forward the application of ferroelectric materials in photoelectric field.     

Room‐Temperature AFM Electric‐Field‐Induced Topotactic Transformation between Perovskite and Brownmillerite SrFeOx with Sub‐Micrometer Spatial Resolution

Reversible structural transformations between perovskite (PV) ABO_3?δ and brownmillerite (BM) ABO_2.5 (A = Ca²⁺, Sr²⁺; B = Fe^4+/3, Co^4+/3+) oxides can be induced by topotactic oxygen exchange at moderate temperatures under reducing/oxidizing conditions. The combination of a large oxide‐ion conductivity and a small free energy difference between the 4+/3+ oxidation states of many 3d transition metal ions enables these topotactic transformations. Herein, it is demonstrated that the electric field produced by a voltage‐biased atomic force microscopy tip can induce such transformation between PV SrFeO_3?δ and BM SrFeO_2.5 at room temperature and with sub‐micrometer spatial resolution. Interestingly, the structural transformation is kept after the electric field is removed, allowing a nonvolatile control of the local chemical, electrical, optical, and magnetic properties. Thus, the results presented in this paper open the door for the fabrication of stable ionic‐based devices through the electric field patterning of different crystallographic phases.     

Electric‐Field‐Assisted Charge Generation and Separation Process in Transition Metal Oxide‐Based Interconnectors for Tandem Organic Light‐Emitting Diodes

The charge generation and separation process in transition metal oxide (TMO)‐based interconnectors for tandem organic light‐emitting diodes (OLEDs) is explored using data on electrical and spectral emission properties, interface energetics, and capacitance characteristics. The TMO‐based interconnector is composed of MoO₃ and cesium azide (CsN₃)‐doped 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) layers, where CsN₃ is employed to replace the reactive metals as an n‐dopant due to its air stability and low deposition temperature. Experimental evidences identify that spontaneous electron transfer occurs in a vacuum‐deposited MoO₃ layer from various defect states to the conduction band via thermal diffusion. The external electric‐field induces the charge separation through tunneling of generated electrons and holes from MoO₃ into the neighboring CsN₃‐doped BPhen and hole‐transporting layers, respectively. Moreover, the impacts of constituent materials on the functional effectiveness of TMO‐based interconnectors and their influences on carrier recombination processes for light emission have also been addressed.     

Current‐Induced Restructuring and Chemical Modification of N‐Doped Multi‐walled Carbon Nanotubes

Multi‐walled carbon nanotubes (MWCNTs) have long been anticipated as candidates for electrical components in an increasingly miniaturized electronics industry due to their inherent electrical properties. It is possible to manipulate and control these properties by introducing dopants such as N, B, and P. Although some current‐induced structural changes in MWCNTs have been observed, no systematic study has been carried out to explore the correlation of changes in the internal structure with the electronic behavior of doped‐MWCNTs in terms of the current densities present. In situ transmission electron microscopy (TEM) investigations are presented here of individual, N‐doped MWNCT (N‐MWCNTs) using the in situ TEM/scanning tunneling microscopy (TEM/STM) Nanofactory© holder. It is observed for the first time that N‐MWCNTs not only undergo current‐induced structural transformation; i.e., from the typical bamboo structure of N‐MWCNTs to the stacked cones, but also—and most importantly—the complete removal of the dopant causes a significant change in the electronic behavior. This has serious implications for the use of doped CNTs as electronic components, especially since tremendous efforts are being made to synthesize CNTs with controlled dopant concentrations.     

Built‐In Electric Fields Dramatically Induce Enhancement of Osseointegration

Rapid and effective osseointegration is a great challenge in clinical practice. Endogenous electronegative potentials spontaneously appear on bone defect sites and mediate healing. Thus, bone healing can potentially be stimulated using physiologically relevant electrical signals in implants. However, it is difficult to directly introduce physiologically relevant electric fields in bone tissue. In this study, built‐in electric fields are established between electropositive ferroelectric BiFeO₃ (BFO) nanofilms and electronegative bone defect walls to trigger implant osseointegration and biological healing. Epitaxial growth technique is used to organize the crystal panel at an atomic scale, and ferroelectric polarization of BFO nanofilms matching the amplitude and direction of endogenous electric potentials on bone defect walls is achieved. In the presence of built‐in electric fields, implants with BFO nanofilms with downward polarization (BFO+) show rapid and superior osseointegration in the rat femur. The mechanism of this phenotypic osteogenic behavior is further studied by protein adsorption and stem cell behavior in different time points. BFO+ promotes protein adsorption and mesenchymal stem cell (MSC) attachment, spreading, and osteogenic differentiation. Custom‐designed PCR array examination shows sequentially initiated Ca²⁺ signaling, cell adhesion and spreading, and PI3K‐AKT signaling in MSCs. The results of this study provide a novel strategy for the development of implant surface modification technology.     

Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN‐PT Single‐Crystal‐Based Field Effect Transistors via Electric Charge Mediation

Multiferroic heterostructures composed of complex oxide thin films and ferroelectric single crystals have aroused considerable interest due to the electrically switchable strain and charge elements of oxide films by the polarization reversal of ferroelectrics. Previous studies have demonstrated that the electric‐field‐control of physical properties of such heterostructures is exclusively due to the ferroelectric domain switching‐induced lattice strain effects. Here, the first successful integration of the hexagonal ZnO:Mn dilute magnetic semiconductor thin films with high performance (111)‐oriented perovskite Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) single crystals is reported, and unprecedented charge‐mediated electric‐field control of both electronic transport and ferromagnetism at room temperature for PMN‐PT single crystal‐based oxide heterostructures is realized. A significant carrier concentration‐tunability of resistance and magnetization by ≈400% and ≈257% is achieved at room temperature. The electric‐field controlled bistable resistance and ferromagnetism switching at room temperature via interfacial electric charge presents a potential strategy for designing prototype devices for information storage. The results also disclose that the relative importance of the strain effect and interfacial charge effect in oxide film/ferroelectric crystal heterostructures can be tuned by appropriately adjusting the charge carrier density of oxide films.     

Bifurcated Polarization Rotation in Bismuth‐Based Piezoelectrics

ABO₃ perovskite‐type solid solutions display a large variety of structural and physical properties, which can be tuned by chemical composition or external parameters such as temperature, pressure, strain, electric, or magnetic fields. Some solid solutions show remarkably enhanced physical properties including colossal magnetoresistance or giant piezoelectricity. It has been recognized that structural distortions, competing on the local level, are key to understanding and tuning these remarkable properties, yet, it remains a challenge to experimentally observe such local structural details. Here, from neutron pair‐distribution analysis, a temperature‐dependent 3D atomic‐level model of the lead‐free piezoelectric perovskite Na_0.5Bi_0.5TiO₃ (NBT) is reported. The statistical analysis of this model shows how local distortions compete, how this competition develops with temperature, and, in particular, how different polar displacements of Bi³⁺ cations coexist as a bifurcated polarization, highlighting the interest of Bi‐based materials in the search for new lead‐free piezoelectrics.     

Inside Front Cover: Green Synthesis and Property Characterization of Single‐Crystalline Perovskite Fluoride Nanorods (Adv. Funct. Mater. 1/2008)

The synthesis of single‐crystalline, cubic perovskite KMnF₃ and NH₄MnF₃ nanorods, and their rare‐earth‐ion‐doped analogues with reproducible shapes and sizes, has been realized using a modified template‐directed approach, report Stanislaus Wong and co‐workers on p. 103. The properties of the nanorods and their as‐doped counterparts suggest their practical incorporation into functional nanometer‐scale devices with applications in a number of fields. The cover shows the crystal structure of perovskite fluorides overlaid on a SEM image of as‐prepared KMnF₃ nanorods with diameters measuring around 50 nm. The generalized green synthesis of single‐crystalline KMnF₃ and NH₄MnF₃ nanorods as well as of their rare‐earth ion doped analogues, possessing reproducible shape and controllable size, has been achieved using a modified template‐directed approach under ambient room‐temperature conditions, with simple inorganic salts as functional precursors. Extensive characterization of the resulting nanorods has been performed using diffraction, electron microscopy, optical spectroscopy, as well as magnetic techniques. We have studied the antiferromagnetism of as‐prepared ternary metal fluoride nanorods as well as the luminescence of their as‐doped counterparts. Our collective data suggest the possibility of the incorporation of these high‐quality, chemically pure materials into functional nanoscale devices with various potential applications that exploit the interesting optomagnetic properties of these systems.     

Morphological Evolution and Magnetic Property of Rare‐Earth‐Doped Hematite Nanoparticles: Promising Contrast Agents for T1‐Weighted Magnetic Resonance Imaging

A series of uniform rare‐earth‐doped hematite (α‐Fe₂O₃) nanoparticles are synthesized by a facile hydrothermal strategy. In a typical case of gadolinium (Gd)‐doped α‐Fe₂O₃, the morphology and chemical composition can be readily tailored by tuning the initial proportion of Gd³⁺/Fe³⁺ sources. As a result, the products are observed to be stretched into more elongated shapes with an increasing dopant ratio. As a benefit of such an elongated morphological feature and Gd³⁺ ions of larger effective magnetic moment than Fe³⁺, the doped product with the highest ratio of Gd³⁺ at 5.7% shows abnormal ferromagnetic features with a remnant magnetization of 0.605 emu g^?1 and a coercivity value of 430 Oe at 4 K. Density of states calculations also reveal the increase of total magnetic moment induced by Gd³⁺ dopant in α‐Fe₂O₃ hosts, as well as possible change of magnetic arrangement. As‐synthesized Gd‐doped α‐Fe₂O₃ nanoparticles are probed as contrast agents for T1‐weighted magnetic resonance imaging, achieving a remarkable enhancement effect for both in vitro and in vivo tests.     

High‐Temperature Performance of Alumina–Zirconia Composite Coatings Containing Amorphous Phases

Amorphous phases are commonly found in nanostructured plasma‐sprayed coatings. Nonetheless, the role of these phases in the resulting coatings’ properties has remained uninvestigated until now. In the present work, pseudo‐eutectic coatings—based on alumina and 8 wt% yttria‐stabilized zirconia (YSZ)—containing amorphous phases are produced using a suspension‐plasma‐spray process. These composite materials are a potential choice for thermal‐barrier coating applications. The role of the amorphous phase on the performance of the coatings is investigated before and after heat treatment. Results show that, although the amorphous phases in untreated coatings reduce the thermal conductivity, they impair the mechanical properties. However, treatment above the crystallization temperature leads to better mechanical properties as well as enhanced high‐temperature stability of the resulting nanostructure. Moreover, the role of alumina as a stabilizer of high‐temperature YSZ phases (tetragonal and cubic) is confirmed and the high‐temperature phase stability of the alumina–YSZ composite is demonstrated. The amorphous phases are found to crystallize into their corresponding high‐temperature stable phases; i. e., α‐alumina and tetragonal zirconia.