Large Coercive Field Research Articles

Improving the polarization switching properties of wurtzite AlN by introducing IV–IV/AlN superlattice and P-doping

The large coercive electric field is the main obstacle for the application of wurtzite ferroelectrics in the memory devices. To explore the effective methods to reduce the coercive electric field of wurtzite ferroelectrics, in this work, after screening a series of possible candidates, (MC)1/(AlN)n (M = Ti, Zr) superlattices and anion P-doping are both chosen to regulate the switching energy barrier of wurtzite AlN based on the density functional theory calculations. It is found that the polarization switching energy barriers of wurtzite AlN gradually decrease by increasing the ratio of the MC (M = Ti, Zr) layer. When the ratio of the MC layer is up to 33%, the polarization switching energy barriers of (TiC)1/(AlN)n and (ZrC)1/(AlN)n are decreased by 68% and 55%, respectively, compared with that of pure wurtzite AlN. The anionic P-doping in AlN results in a 48% lower energy barrier. Also, the ferroelectric polarization in the designed superlattices and P-doped AlN is well maintained compared to pure AlN. Thus, the results show that the (MC)1/(AlN)n (M = Ti, Zr) superlattices and anion P-doping approaches are effective in improving the polarization switching properties of wurtzite AlN.

Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator.

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states (TSS) with ultrahigh efficiency. Here, efficient SOT switching of a hard MTI, V-doped (Bi,Sb)2Te3 (VBST), with a large coercive field that can prevent the influence of an external magnetic field, is demonstrated. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems, which makes the Hall channel a good readout and eliminates the need to fabricate complicated magnetic tunnel junction (MTJ) structures. The SOT switching current density can be reduced to 2.8×105Acm-2, indicating its high efficiency. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to (1.56±0.12)×10-6TA-1cm2 and the interfacial charge-to-spin conversion efficiency to 3.9±0.3 nm-1. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.

Ultrafast Switching of Sliding Polarization and Dynamical Magnetic Field in van der Waals Bilayers Induced by Light.

Sliding ferroelectricity is a unique type of polarity recently observed in van der Waals bilayers with a suitable stacking. However, electric-field control of sliding ferroelectricity is hard and could induce large coercive electric fields and serious leakage currents that corrode the ferroelectricity and electronic properties, which are essential for modern two-dimensional electronics and optoelectronics. Here, we proposed laser-pulse deterministic control of sliding polarization in bilayer hexagonal boron nitride by first principles and molecular dynamics simulation with machine-learned force fields. The laser pulses excite shear modes that exhibit certain directional movements of lateral sliding between bilayers. The vibration of excited modes under laser pulses is predicted to overcome the energy barrier and achieve the switching of sliding polarization. Furthermore, it is found that three possible sliding transitions-between AB (BA) and BA (AB) stacking-can lead to the occurrence of dynamical magnetic fields along three different directions. Remarkably, the magnetic fields are generated by the simple linear motion of nonmagnetic species, without any need for more exotic (circular, spiral) pathways. Such predictions of deterministic control of sliding polarization and multistates of dynamical magnetic field thus expand the potential applications of sliding ferroelectricity in memory and electronic devices.

Phase and defect structures of MnO2‐doped 75BiFeO3–25BaTiO3 ceramics and their effects on electrical properties

AbstractBismuth ferrite (BiFeO3)‐based high‐temperature piezoelectric ceramics have high Curie temperatures and excellent thermal stability; however, their applications are limited by inadequate piezoelectric performance due to large coercive fields and high leakage conduction. This study investigates the impact of MnO2 doping on the phase transition, defect structure, domain morphology, as well as the dielectric, ferroelectric, and electrical conduction behaviors of 75BiFeO3–25BaTiO3 ceramics sintered under an oxygen atmosphere. A structural transformation from distorted rhombohedral to pseudo‐cubic symmetry was observed with increasing MnO2 content, accompanied by domain fragmentation. MnO2 doping at an appropriate level inhibited defects such as and . Regarding electrical properties, the reduction in rhombohedral distortion and enhancement in the pseudo‐cubic phase after MnO2 doping resulted in decreased remanent polarization and electrostrain, which could potentially hinder piezoelectric response. However, appropriate MnO2 doping effectively reduced leakage conduction, significantly enhancing the poling efficiency of the ceramics. The maximum d33 value of 107 pC/N and kp of 0.36 were achieved at an MnO2 doping content of 0.1 wt.%. Excessive MnO2 addition may generate defect dipoles, as indicated by increased coercive field (Ec) and dielectric loss. The observed increase in high‐temperature resistivity with MnO2 content is primarily attributed to reduced grain size and increased concentration of mobile defects. This study provides valuable insights for further research on the application of 75BiFeO3–25BaTiO3 systems in high‐temperature piezoelectric devices.

Electromechanical properties and domain evolution of a tetragonal Er3+-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 ferroelectric single crystal

Herein, a tetragonal Er3+-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal is comprehensively investigated. The actual composition of the studied crystal is Pb(Mg1/3Nb2/3)O3–36PbTiO3 doped with 0.17-mol% Er3+ (PMN–36PT: 0.17-mol% Er3+). The crystal exhibits a high shear piezoelectric constant of ∼936 pC/N, approximately three times the longitudinal piezoelectric constant when poled along [001]c, as well as a higher Curie temperature of ∼170 °C and large coercive field of ∼4.58 kV/cm. At room temperature, the crystal presents a mass of domain walls along the <110>c direction and extinction is observed four times within a 360° rotation, showing typical domain features of the tetragonal phase. Under a direct current–electric field along the [010]c direction, the extinction rule remains unchanged; however, changes in the domain configuration can be observed. In addition, the orientation dependence of the piezoelectric constant, dielectric coefficient, elastic compliance constant, and electromechanical coupling factor is calculated using the coordinate transformation method. The findings of this study are of considerable importance for the fundamental studies and device design of tetragonal ferroelectric crystals.

Perspectives on nitride ferroelectric semiconductors: Challenges and opportunities

The recent demonstration of ferroelectricity in nitride materials has enabled a broad spectrum of applications across electronics, optoelectronics, photovoltaics, photonics, ferroelectrics, acoustoelectronics, and quantum devices and systems. Ensuring that nitride ferroelectrics meet the rigorous demands of modern microelectronics and photonics necessitates addressing significant challenges, such as large coercive fields, low endurance, poor stability, high leakage current, and high optical loss. In this perspective, we review the latest advancements, highlight the distinctive properties and notable achievements of nitride ferroelectrics, and delve into their origins, material synthesis, operational mechanisms and devices. Moreover, we highlight the principal obstacles faced by nitride ferroelectrics in practical applications. Finally, we discuss potential solutions, future research directions, and the prospects for further advancements in this rapidly evolving domain.

Ferromagnetism on an atom-thick & extended 2D metal-organic coordination network

Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in the absence of external magnetic fields. Reducing this fundamental property into strictly two-dimensions was proposed in metal-organic coordination networks, but thus far has eluded experimental realization. In this work, we demonstrate that extended, cooperative ferromagnetism is feasible in an atomically thin two-dimensional metal-organic coordination network, despite only ≈ 5% of the monolayer being composed of Fe atoms. The resulting ferromagnetic state exhibits an out-of-plane easy-axis square-like hysteresis loop with large coercive fields over 2 Tesla, significant magnetic anisotropy, and persists up to TC ≈ 35 K. These properties are driven by exchange interactions mainly mediated by the molecular linkers. Our findings resolve a two decade search for ferromagnetism in two-dimensional metal-organic coordination networks.

The H/F substitution strategy can achieve large spontaneous polarization in 1D hybrid perovskite ferroelectrics.

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics exhibit polarization reversibility and have a wide range of applications in the fields of smart switches, memorizers, sensors, etc. However, the inherent limitations of small spontaneous polarization (P s) and large coercive field (E c) in ferroelectrics have impeded their broader utilization in electronics and data storage. Molecular ferroelectrics, as a powerful supplement to inorganic ferroelectrics, have shown great potential in the new generation of flexible wearable electronic devices. The important research responsibility is to greatly improve progressiveness and overcome the above limitations. Here, a novel one-dimensional (1D) HOIP ferroelectric, (3-F-BTAB)PbBr3 (3-F-BTAB = 3-fluorobenzyltrimethylammonium), was successfully synthesized by employing the H/F substitution strategy to modify parent compound (BTAB)PbBr3 (BTAB = benzyltrimethylammonium), which undergoes a ferroelectric phase transition with Aizu notation 2/mF2 at 420 K. Notably, (3-F-BTAB)PbBr3 demonstrates exceptional ferroelectric properties with a large P s of 7.18 μC cm-2 and a low E c of 1.78 kV cm-1. As far as we know, (3-F-BTAB)PbBr3 features the largest P s among those reported for 1D lead-based HOIP ferroelectrics. This work enriches the 1D lead-based ferroelectric family and provides guidance for applying ferroelectrics in low-voltage polar memories.

Field induced metamagnetic transition and mixed charge transfer Mott-Hubbard character in nanocrystalline FeCo2O4 spinel oxide: A combined study using experimental and first principles techniques

Unusual magnetic behavior of nanocrystalline FeCo2O4 (FCO) has been observed at different annealing temperatures. FCO nanoparticles have been prepared by co-precipitation method and the prepared sample annealed at different temperatures to vary its crystallite size. The powder x-ray diffraction analysis confirms the presence of pure spinel phase in the samples annealed at 900 ℃ and 1000℃. The increase in particle size with annealing temperature has been confirmed by field emission scanning electron microscopy (FESEM) analysis. High resolution x-ray photoelectron spectroscopy (HRXPS) analysis reveals the presence of mixed oxidation states of Fe+2/ Fe+3 and Co+3/Co+2 in the sample annealed at 900℃. The as-synthesized sample at 80 °C shows strong antiferromagnetic behavior due to their smaller particle size, but all the annealed samples have strong ferromagnetic behavior with large coercive field and remanence due to their larger particle size. We have observed a sharp rise in magnetization near small field values in the virgin curve and also the presence of virgin curve completely outside the main hysteresis loop in the M(H) curves measured at 5 K for the samples annealed at and above 1000 ℃ reveal the characteristic of first order nature of field induced metamagnetic transition. Structural parameters obtained after Rietveld refinement of powder x-ray diffraction pattern for sample annealed at 900 ℃ have been used for investigating the magnetic properties using first principles density functional theory. Our calculations using hybrid exchange-correlation functional, HSE06 captures the correct insulating ferromagnetic ground state and band gap for inverse spinel structure in broad agreement with our experimental results while calculations within GGA+U leads to a wrong half metallic ferromagnetic ground state. The partial density of states obtained for inverse spinel structure with HSE06 functional suggests a mixed charge transfer and Mott-Hubbard character in our system. We have explored the interplay among structural, magnetic and transport properties of FCO using experiment as well as first principles density functional theory.

Bifunctional bimetallic PtNi, PtCu, and NiCu nanoparticles: Electrocatalytic activities for hydrogen evolution reaction and magnetic properties

Pt-based materials are commonly used in sustainable energy technologies and fuel cells due to their catalytic ability. In this study, we synthesized bifunctional bimetallic PtNi, PtCu, and NiCu nanoparticles using the modified polyol method to investigate their catalytic for hydrogen evolution reactions (HER) and magnetic properties. X-ray diffraction and Rietveld refinement analyses revealed that all samples exhibited a face-centered cubic structure with a Fm3¯m space group. The chemical compositions of PtNi, PtCu, and NiCu bimetallic NPs were determined to be Pt28Ni72, Pt25Cu75, and Ni27Cu73, via stoichiometric calculations using the energy-dispersive X-ray spectroscopy data. The nanostructure and size were characterized by scanning electron microscopy, and the average particle size was found to be between 10 and 13 nm for all three catalysts with a narrow size distribution. The Pt28Ni72 catalyst exhibited the highest HER activity, with a minimum overpotential of −33 mV at −10 mA cm−2 and Tafel slopes with 55 mV dec-1 revealing the Volmer– Heyrovsky HER mechanism. The HER performance of the catalysts revealed that the highest HER activity is Pt28Ni72 > Pt25Cu75 > Ni27Cu73. The magnetic results revealed that the PtNi NPs exhibited a ferromagnetic to paramagnetic transition with a large coercive field of 628 Oe at 5 K. In this work, the addition of magnetic Ni in the structure resulted in multifunctional materials such as magnetically reusable bimetallic PtNi, PtCu, and NiCu catalysts with potential biomedical applications.

Exchange bias effect and compensation point in the bulk ferrimagnet Fe2CrSe4

Polycrystalline samples of the Fe2CrSe4 compound have been synthesized and studied by means of x-ray diffraction and magnetization measurements. The as-synthesized Fe2CrSe4 sample shows ferrimagnetic ordering below TN ∼ 300 K with a compensation point Tcomp ∼ 140 K. This compound is found to exhibit a large coercive field (half-width of the hysteresis loop reaches 15 kOe at 2 K) and a significant exchange bias (EB) effect below 50 K (HEB up to 6.6 kOe) after cooling in an applied field and after zero-field cooling. The exchange bias in Fe2CrSe4 is suggested to result from the presence of regions (clusters) with short-range correlations in the ferrimagnetic matrix due to the inhomogeneous distribution of iron and chromium atoms over the lattice. Additional heat treatment and subsequent rapid cooling lead to an increase in magnetic ordering temperature, the disappearance of compensation for sublattice magnetizations and a decrease in the exchange bias, thus showing the possibility of controlling the exchange bias and magnetic characteristics in the system.

Correction to “Reentrant Spin Glass and Large Coercive Field Observed in a Spin Integer Dimerized Honeycomb Lattice”

Correction to “Reentrant Spin Glass and Large Coercive Field Observed in a Spin Integer Dimerized Honeycomb Lattice”

Comprehensive optimization of piezoelectric coefficient and depolarization temperature in Mn-doped Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-BaTiO3 lead-free piezoceramics

Lead-free Bi0.5Na0.5TiO3 (BNT) piezoelectric ceramics have the advantages of large coercive fields and high Curie temperatures. But the improvement of piezoelectric coefficient (d33) is usually accompanied by a huge sacrifice of depolarization temperature (Td). In this work, a well-balanced performance of d33 and Td is achieved in MnO2-doped 0.79(Bi0.5Na0.5TiO3)-0.14(Bi0.5K0.5TiO3)-0.07BaTiO3 ternary ceramics. The incorporation of 0.25 mol% MnO2 enhances the d33 by more than 40%, while Td remains almost unchanged (i.e., d33=181 pC/N, Td=184 °C). X-ray diffraction (XRD) shows that an appropriate fraction of the small axis-ratio ferroelectric phase (pseudo-cubic, Pc) coexists with the long-range ferroelectric phase (tetragonal, T) under this MnO2 doping. Piezoelectric force microscopy (PFM) has revealed a special domain configuration, namely large striped and layered macro domains embedded with small nanodomains. This study provides a distinctive avenue to design BNT-based piezoelectric ceramics with high piezoelectric performance and temperature stability.

Ferroelectricity in the Al doped HfO2

Recently, hafnia (HfO2)-based ferroelectrics have attracted much attention due to their unique features such as superior ferroelectricity at an ultra-thin thickness, large coercive electric field, and compatibility with the standard silicon process platform. In this work, the growth of ferroelectric HfO2 thin films through Al doping have been demonstrated. Structural analysis showed that non-centrosymmetric polar orthorhombic phase HfO2 have been observed after doping Al. Interestingly, the ferroelectricity was shown to be dependent on the Al composition, where the remnant polarization as high as 2Pr = 22.9 μC/cm2 was obtained in HfO2 thin film with a 5.0 mol% Al doping. The mechanisms of structural transition to ferroelectric orthorhombic-phase owing to oxygen vacancies and generation of oxygen vacancies benefiting from Al doping were further studied by density functional theory calculations. The results shown in this work provide insights into the formation of ferroelectric HfO2 due to Al doping and further offer a simple way for controlled fabrication of ferroelectric HfO2 thin films.

Ferromagnetic Cu and ferrimagnetic Co layered hydroxides incorporating cinnamate derivative layers

A series of organic-inorganic hybrid materials TM(Xcin)(OH) were synthesized and their structural and magnetic properties were investigated to discuss their magneto-structural relations, where TM = Cu, Co and Xcin = cinnamic acid (1 for TM = Cu, 8 for TM = Co), 4-methylcinnamic acid (2 for TM = Cu, 9 for TM = Co), 4-chlorocinnamic acid (3 for TM = Cu, 10 for TM = Co), trans-p-coumaric acid (4 for TM = Cu), 4-(trifluoromethyl)cinnamic acid (5 for TM = Cu), 3-phenylpropionic acid (6 for TM = Cu), trans-styrylacetic acid (7 for TM = Cu) and 1,4-Phenylenediacrylic acid (11 for TM = Co). The compounds 1-11 possess organic-inorganic multi-layered structures, in which Xcin monocarboxylate bilayers (dicarboxylate monolayers for 11) alternate TM(OH)x inorganic monolayers. The Cu compounds1-7 exhibit soft ferromagnetic long-range-orderings below the Curie temperatures ranging from 10.8 to 14.7 K. In contrast, the Co compounds 8-11 exhibit ferrimagnetic long-range orderings with a large coercive field (Hc = 38 kOe for 8) below the Curie temperatures ranging from 44.0 to 57.0 K. We found that the TC values of 1-7 and 8-11 exist in a narrow temperature range although the inter-layer spacings were distributed in a wide range from 13.3 to 22.5 Å.

Ferroelectric Transistors for Memory and Neuromorphic Device Applications.

Ferroelectric materials have been intensively investigated for high-performance nonvolatile memory devices in the past decades, owing to their nonvolatile polarization characteristics. Ferroelectric memory devices are expected to exhibit lower power consumption and higher speed than conventional memory devices. However, non-complementary metal-oxide-semiconductor (CMOS) compatibility and degradation due to fatigue of traditional perovskite-based ferroelectric materials have hindered the development of high-density and high-performance ferroelectric memories in the past. The recently developed hafnia-based ferroelectric materials have attracted immense attention in the development of advanced semiconductor devices. Because hafnia is typically used in CMOS processes, it can be directly incorporated into current semiconductor technologies. Additionally, hafnia-based ferroelectrics show high scalability and large coercive fields that are advantageous for high-density memory devices. This review summarizes the recent developments in ferroelectric devices, especially ferroelectric transistors, for next-generation memory and neuromorphic applications. First, the types of ferroelectric memories and their operation mechanisms are reviewed. Then, issues limiting the realization of high-performance ferroelectric transistors and possible solutions are discussed. The experimental demonstration of ferroelectric transistor arrays, including 3D ferroelectric NAND and its operation characteristics, are also reviewed. Finally, challenges and strategies toward the development of next-generation memory and neuromorphic applications based on ferroelectric transistors are outlined.

Structure, dielectric and energy storage properties of Bi(Mg2/3Nb1/3)O3 modified Na0.5Bi0.5TiO3-NaNbO3 ceramics

Na0.5Bi0.5TiO3-based ceramics are considered to be a prospective material for energy storage applications due to their unique phase transition and crystal structure. However, the large remanent polarization (Pr) and coercive field (Ec) limit their application in energy storage devices. In this work, the composition-dependent structure, dielectric properties and energy storage properties of Bi(Mg2/3Nb1/3)O3 (BMN) modified Na0.5Bi0.5TiO3-NaNbO3 (NBTNN) ceramics were systematically investigated. The introduction of BMN promotes the formation of local random field, which effectively suppresses the hysteresis loss along with Pr, and form “slender” P-E hysteresis loops. The maximum recoverable energy density of 5.26 J/cm3 is obtained in the optimum composition 0.8NBTNN-0.2BMN, which is about three times more than that of the undoped one, while yielding a high energy efficiency of 78%. In addition, the introduction of BMN into NBTNN ceramic results in excellent temperature stability and significant enhancement in band gap of the material. The improved energy storage performance of BMN modified NBTNN ceramic indicates their potential applicability in dielectric capacitors.

Radical‐Bridged Heterometallic Single‐Molecule Magnets Incorporating Four Lanthanoceniums

AbstractThe syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6]{CoI[(μ3‐HAN)RE2Cp*4]2} (1‐RE) and [K(Crypt)]2{CoI[(μ3‐HAN)RE2Cp*4]2} (2‐RE) containing hexaazatrinaphthylene radicals (HAN⋅3−) and four rare earth (RE) ions are reported. 1‐RE shows isolable species with ligand‐based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2‐RE via one‐electron chemical reduction. Strong electronic communication in mixed‐valency supports stronger overall ferromagnetic behaviors in 2‐RE than 1‐RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1‐Dy and 2‐Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2‐Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1‐Dy. Ligand‐based mixed‐valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln‐SMMs.

Radical-Bridged Heterometallic Single-Molecule Magnets Incorporating Four Lanthanoceniums.

The syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6 ]{CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (1-RE) and [K(Crypt)]2 {CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (2-RE) containing hexaazatrinaphthylene radicals (HAN⋅3- ) and four rare earth (RE) ions are reported. 1-RE shows isolable species with ligand-based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2-RE via one-electron chemical reduction. Strong electronic communication in mixed-valency supports stronger overall ferromagnetic behaviors in 2-RE than 1-RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1-Dy and 2-Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2-Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1-Dy. Ligand-based mixed-valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln-SMMs.

Structural, magnetic and thermoelectric properties of double perovskite Nd2Co1-xCrxMnO6

Double perovskite Nd2Co1-xCrxMnO6 (x = 0, 0.1, 0.2, 0.3) samples were synthesized by a sol-gel method, and their structural, magnetic and thermoelectric properties were investigated. X-ray diffraction analysis showed that all crystallized in a monoclinic P21/n structure. The lattice parameters and unit cell volume increase with increasing Cr content. Two ferromagnetic transitions were observed in the Cr-doped samples, which can be attributed to the exchange interactions of the ordered Co2+–O–Mn4+ and the disordered Cr3+/Co3+–O–Mn3+ clusters, respectively. A large coercive field of 10.6 kOe was achieved at 10 K for the sample with x = 0.2. The magnetic transition temperatures and the magnetic moments decrease with increasing Cr content, which combined with the Raman analysis indicated a decrease in the degree of the B-site ordering. Both the electrical conductivity (σ) and Seebeck coefficient (S) increase over the temperature range of 190–1100 K with Cr doping. The undoped Nd2CoMnO6 exhibits a colossal S of 2273 μV/K at 220 K and has a negative S between 300 and 700 K with a maximum of − 24 μV/K at 325 K. All the Cr-doped samples are p-type semiconductors. The conduction mechanism follows the small polaron hopping model.