Electric field control of nonvolatile two-state magnetoelectric coefficient at room temperature in a hexaferrite

Cross coupling between magnetization and polarization in multiferroic materials provides a new controlling dimension for creating novel electric devices, particularly the electrical writing magnetic reading of low-power consumption nonvolatile random access memory. Despite its importance, rare single-phase materials that exhibit significant magnetoelectric phenomena at room temperature exist. We herein report that a spin-driven multiferroic hexaferrite Ba_1.1Sr_0.9Co₂Fe₁₁AlO₂₂ exhibits a prominent direct and converse magnetoelectric effect at room temperature. The electric field can manipulate magnetization as large as 0.1 µ_B/f.u. in the range of ±1 MV/m without any external magnetic field. Moreover, a feasible nonvolatile memory based on polycrystalline Ba_1.1Sr_0.9Co₂Fe₁₁AlO₂₂ is realized by recording the states of magnetoelectric coefficients, which is promising for further application in magnetoelectric electronics.     

Tailoring the multiferroic properties of BiFeO3 by co-doping Er at Bi site with aliovalent Nb,Mn and Mo at Fe site

Single phase multiferroic undoped BiFeO₃ notoriously suffers due to the poor spin–charge coupling resulting in limitations to device applications. The present work focuses on the tailoring of its multiferroic and magnetoelectric coupling properties by synthesizing multiferroic Bi_0.95Er_0.05Fe_0.98TM_0.02O₃ (TM = Nb, Mn and Mo) ceramics. The ferroelectric, magnetic, current leakage measurements and magnetoelectric effect were investigated. XRD along with the Reitveld refinement results confirms that all the samples possess perovskite based rhombohedral structure and reveals that doping of (Er, Nb), (Er, Mn) and (Er, Mo) induced the crystallographic distortion in the BFO lattice and hence induced a variation in the bond lengths and bond angle. Dual doping significantly enhanced the electrical, magnetic properties and magnetoelectric coupling as compared to BiFeO₃. Doping has lowered the leakage current by three to four orders compared to BFO. The lattice distortion, reduced leakage current and destruction of spin–cycloidal structure could be the origin for these improved features. The (Er, Nb) doped BiFeO₃ yields enhanced ferroelectric character with the maximum polarization value of 0.46 µC/cm², maximum ME coupling of 0.22 mV/cm at a magnetic field of 130 G, an improved magnetization with a remanance value of 0.0903 emu/g and the lowest leakage current density.     

Enhanced dielectric,ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba₄Sm₂Fe₂Nb₈O₃₀ (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO₄ is observed in the CS BSFN ceramics with columnar grains. Compared to Ba₄Sm₂Fe₂Nb₈O₃₀ ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm² and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba₄Sm₂Fe₂Nb₈O₃₀ is an intrinsic room temperature multiferroic materials.     

Lattice Defects Induce Multiferroic Responses in Ce,La‐Substituted BaFe0.01Ti0.99O3 Nanostructures

Single‐phase multiferroic Ba(Fe_0.67Ce_0.33)_0.01Ti_0.99O₃ (BFTO:Ce) and Ba(Fe_0.67La_0.33)_0.01Ti_0.99O₃ (BFTO:La) nanostructures were synthesized by a hydrothermal method (180°C/48 h). Rietveld refinement of X‐ray diffraction could confirm crystalline phase and lattice deformation by Ce, La into BFTO. The Ce and La doping induce nanoaggregation‐type BFTO nanostructural product due to their ionic size effect and chemical behavior with OH^? ions. Raman active modes show tetragonal phase and defects due to vacancies in the BFTO lattice. Photoluminescence spectrum involves multiple visible emissions due to defects/vacancies. The observed ferroelectric polarization is enhanced due to shape/size effect of nanoparticles, lattice distortion, and filling of d orbital in the perovskite BaTiO₃. The room‐temperature magnetic behavior is described due to antiferromagnetic interactions that strengthen by Ce and La doping. The zero‐field cooling and field cooling magnetic measurement at 500 Oe indicates antiferromagnetic to ferromagnetic transition. Dynamic magnetoelectric coupling was investigated, and maximum longitudinal magnetoelectric coefficient is 62.65 and 49.79 mV/cmOe, respectively, measured for BFTO:Ce and BFTO:La. The magnetocapacitance measurements induce negative values that described in terms of magnetoresistance and magnetic phase transition effects. The influence of oxygen vacancy on multiferroicity is evaluated by valance states of O ions.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Composition‐Dependent Microstructures and Properties of La‐, Zn‐, and Cr‐Modified 0.675BiFeO3–0.325BaTiO3 Ceramics

Pure and 1.0 mol% La₂O₃, ZnO, and Cr₂O₃‐modified 0.675BiFeO₃–0.325BaTiO₃ (BF–BT) multiferroic ceramics were prepared and comparatively investigated. For pure and La‐, Zn‐, and Cr‐modified BF–BT, the average grain size is 415, 325, 580, and 395 nm, and the maximum dielectric constant temperature is 460°C, 430°C, 465°C, and 445°C, respectively. All additives weaken the ferroelectricity slightly. Zn‐ and Cr‐modifications dramatically enhance the room‐temperature magnetic properties, whereas La‐modification has almost no effect on magnetic property. Especially, the Cr‐modified BF–BT ceramics show switchable polarization and magnetization of 4.9 μC/cm² and 0.27 emu/g at room temperature, the magnetoelectric coupling is confirmed by the magnetization‐magnetic field curves measured on ceramics before and after electric poling. The mechanism responsible for the different effects of additive on microstructures and properties are discussed based on additive‐induced point defect and second phase as well as diffusion‐induced substitution. These results not only provide a promising room‐temperature multiferroic material candidate, but also are helpful to design new multiferroic materials with enhanced properties.     

A Comparative Study of the Performance of SrCo0.76Fe0.19Al0.1Ox and (SrCo0.8Fe0.2O3−δ)0.95(SrAl2O4)0.05 Mixed‐Conducting Membranes

Dense oxygen permeable ceramic membranes are promising materials for separating oxygen from air and for syngas production. The crystal structure, oxygen vacancy, microstructure, thermal expansion behavior, electrical conductivity, and oxygen permeability of two mixed‐conducting ceramics: 5 mol% SrAl₂O₄‐doped SrCo_0.8Fe_0.2O_{3 ? δ} (SCF‐SA) and SrCo_0.76Fe_0.19Al_0.1O_x (SCFA), with the same overall cation composition, were systematically investigated in this work. The main phases of SCFA and SCF‐SA are cubic perovskite and no orthorhombic brownmillerite appears from 700°C to 900°C in air, while SCF‐SA presents minor unidentified phases. SCF‐SA shows lower electrical conductivity than SCFA. SCFA membranes exhibit lower oxygen permeation flux than SCF‐SA. At 850°C, the oxygen permeation fluxes of SCFA and SCF‐SA are 1.23 and 1.46 mL/cm²/min, respectively, under the oxygen partial pressure gradient of 0.21 × 10⁵/1 × 10² Pa.     

Magnetic and electrical properties of single-phase multiferroic (1-x)Pb(Zr0.52Ti0.48)O3–xPb(Fe0.5Nb0.5)O3 thin films prepared by sol-gel route

Single-phase multiferroic (1-x)Pb(Zr_0.52Ti_0.48)O₃-xPb(Fe_0.5Nb_0.5)O₃ (0≤x≤0.5) thin films were synthesized by sol-gel route and characterized to understand their structural, electrical, and magnetic properties. The films were thermally treated by conventional furnace (CFA) and rapid thermal annealing (RTA). A pyrochlore-free perovskite phase is stabilized only by RTA in samples with high Fe³⁺/Nb⁵⁺ content. The films displayed excellent dielectric and ferroelectric properties in the whole concentration range, with saturated hysteresis loops and remanent polarization values of ～15μC/cm². Films with x>0.3 showed ferromagnetic behavior at room temperature. Consequently, the multiferroic behavior in the films occurs in a different concentration range than that observed in bulk ceramics. The origin of the weak ferromagnetism is discussed.     

Effect of annealing temperature on the multiferroic properties of 0.7BiFeO3–0.3Bi0.5Na0.5TiO3 solid solution prepared by sol–gel method

Room-temperature multiferroic 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics have been prepared by the sol–gel method. We have discussed the annealing temperature dependence of the multiferroic properties. The samples are annealed at 1023, 1123, 1223 and 1323 K for 3 h, respectively. X-ray diffraction patterns identify that all samples are pure. Scanning electron micrographs present the increasing grain size with higher annealing temperature. Magnetic, ferroelectric and dielectric properties are enhanced obviously with the increase in annealing temperature. The coexistence of ferroelectric and ferromagnetic properties is also proved at room temperature. In addition, it reveals that the optimal annealing temperature accompanied with favorable multiferroic properties of 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics is near 1223 K.     

Structure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of BiFeO3‐BaTiO3‐Bi0.5Na0.5TiO3 Lead‐Free Multiferroic Ceramics

Lead‐free multiferroic ceramics of BiFeO₃‐BaTiO₃‐Bi_0.5Na_0.5TiO₃ have been prepared by a conventional ceramic technique. The microstructure, multiferroic, and piezoelectric properties of the ceramics have been studied. The ceramics sintered at 1000°C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x = 0.02. After the addition of Bi_0.5Na_0.5TiO₃, two dielectric anomalies are observed at high temperatures (T_m ~ 510°C–570°C and T₂ ~ 720°C). The phase transition around T_m becomes wider gradually with increasing x. The ferroelectricity, piezoelectricity, and ferromagnetism of the ceramics are significantly improved after the addition of Bi_0.5Na_0.5TiO₃. High resistivity (~1.3 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 27.4 μC/cm²), good piezoelectricity (d₃₃ =140 pC/N, k_p = 31.4%), and weak magnetic properties (M_r =0.19 emu/g) are observed.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Enhanced room-temperature magnetoelectric coupling effects in c-axis oriented polycrystalline BaSrCo2-xMgxFe11AlO22

The influence of applied magnetic field during annealing process as well as of Mg doping on the room-temperature magnetoelectric coupling effects in BaSrCo_2-xMg_xFe₁₁AlO₂₂ are experimentally studied through the magnetization, magnetodielectric, and magnetoelectric current measurements. Hexaferrite samples of Co₂Y were found to be highly oriented by an applied magnetic field (H_o) during the annealing process, leading to an enhancement of the room-temperature magnetoelectric coupling effects. Although the substitution of nonmagnetic Mg ions in Co sites tends to reduce the ferromagnetism at macroscopic scale, a proper amount of Mg doping content facilitates the superexchange interaction between the adjacent magnetic blocks; meanwhile modulates the magnetic anisotropy in the samples. An appropriate adjustment of the competition between the anisotropy and the superexchange could enhance the magnetoelectric coupling at room temperature, which can be confirmed by the magnetic-field-induced dielectric constant and current density study.     

Dielectric and magnetic properties of (Pb,Ti) co-doped BiFeO3 multiferroic ceramics

In this study, Bi_0.9Pb_0.1Fe_1-xTi_xO₃ (0.05 ≤ x ≤ 0.20) multiferroic ceramics were prepared through solid-state reaction. The influence of Pb, Ti partial substitutions on the dielectric and magnetic properties of BiFeO₃ multiferroic ceramics was investigated and discussed in detail. X-ray diffractions confirm rhombohedral perovskite phase formation (R3c space group). Scanning electron microscopy (SEM) was employed to investigate the morphology, revealing a cuboidal microstructure with bimodal distribution of grain sizes. Magnetic studies were carried out and the results reveal a slight enhancement of saturation magnetization with Ti concentration increasing. The present data indicates that Bi_0.9Pb_0.1Fe_1-xTi_xO₃ can be used as multifunctional material in different magnetoelectric applications.     

Thermal annealing effects on the energy storage properties of BST ceramics

Thermal annealing treatments with different atmospheres (air, oxygen, and reducing atmospheres, respectively) were employed for the conventionally sintered (Ba_0.4Sr_0.6)TiO₃ (BST) ceramics. The effect of thermal annealing on the energy storage properties of BST ceramics was investigated, where oxygen vacancies played an important role. The dielectric loss, bulk resistivity and dielectric breakdown strength (BDS) were found to be sensitive to the annealing process, leading to the different energy densities in the range of 0.30‐0.80 J/cm³. Temperature‐dependent dielectric measurement and thermally stimulated depolarization current analysis were conducted to understand the impacts of oxygen vacancies on the macroscopic properties, which were found to be closely associated with the annealing conditions.     

Effect of Substrate on Structure and Multiferrocity of (La,Mn) CoSubstituted BiFeO3 Thin Films

In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO₃‐based thin films at room temperature. (La, Mn) cosubstituted BiFeO₃ (BFOLM) thin films were deposited on differently lattice mismatched single‐crystal substrates to manipulate the strain states in the as‐deposited films. All the films with 30‐nm thick CaRuO₃ bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X‐ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm² for the films grown on SrTiO₃, (La, Sr)(Al, Ta)O₃, and LaAlO₃, respectively. It is found that the room‐temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.     

Significantly enhanced dc electrical resistivity and piezoelectric properties of Tb-modified CaBi2Nb2O9 ceramics for high-temperature piezoelectric applications

Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) is considered to be one of the most potential high-temperature piezoelectric materials due to its high Curie temperature T_c of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare-earth element Tb ions compositional adjustment. The nominal compositions of Ca_1−xTb_xBi₂Nb₂O₉ (abbreviated as CBN-100xTb) have been fabricated by conventional solid-state reaction method. The composition of CBN-3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 10⁶ Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb³⁺ ions for Ca²⁺ ions reduce the oxygen vacancy concentrations and increase the band-gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature-dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low-temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN-3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d₃₃ of ∼13.2 pC/N and a high T_c of ∼966°C. Moreover, the CBN-3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb-modified CBN for high-temperature piezoelectric applications.     

Highly Conductive p‐Type Zinc blende SiC Thin Films Fabricated on Silicon Substrates by Magnetron Sputtering

Polycrystalline silicon carbide (SiC) thin films were fabricated on Si(100) substrates using radio‐frequency magnetron sputtering followed by annealing at 1300°C in an Ar atmosphere. The SiC films exhibited a zinc blende structure with planar and point defects as detected by X‐ray diffraction and Raman spectroscopy. The SiC films were p‐type conductive with electrical resistivity as low as 2.8 × 10^?3 Ω·cm at room temperature. The p‐type character of the SiC films can be explained in terms of the Si vacancies in the C‐rich environment as evidenced by Raman spectroscopy.     

Improved Energy Storage Properties Accompanied by Enhanced Interface Polarization in Annealed Microwave‐Sintered BST

Microwave‐sintering (MWS) technique was employed to fabricate dense (Ba_0.4Sr_0.6)TiO₃ (BST) ceramics. With respect to the high dielectric loss at room temperature, induced by the formation of oxygen defects during the MWS under vacuum atmosphere (?60 kPa), the as‐sintered samples were thermally annealed in air to reduce tanδ and recover the insulating performance. Accompanied by the decreased tanδ, the energy storage properties for annealed MWS BST were optimized, with increasing energy density (γ) from 0.77 to 1.15 J/cm³ and energy efficiency (η) from 60% to 82%. The lower oxygen vacancy concentrations were believed to account for the enhanced insulating characteristics of grain boundaries and contribute to the improved properties after annealing. Electrical characterization of grain and grain boundary by impedance spectroscopy demonstrated that the annealing preferentially modified the grain boundary. In addition, resistances extracted from the high temperature impedance analysis were found to be inadequate for evaluating the electrical characteristics of materials affected by extrinsic mechanisms, such as the interfacial polarization. For comparison, annealing effect on energy storage properties were also discussed for conventionally sintered BST.     

Thermal treatment effect on the magnetoelectric properties of (Ni0.5Zn0.5)Fe2O4/Pt/Pb(Zr0.3Ti0.7)O3 heterostructured thin films

Magnetoelectric (ME) property modulation in heterostructured (Ni_0.5Zn_0.5)Fe₂O₄/Pt/Pb(Zr_0.3Ti_0.7)O₃ (NZFO/Pt/PZT) thin films on platinized Si substrate by thermal annealing condition variation was studied. In an attempt to prevent interfacial reaction between NZFO and PZT layers during high temperature annealing, thin Pt layer was deposited which can serve as inter-diffusion barrier as well as electrode. The ferroelectric, magnetic, and ME properties of the heterostructured film were noticeably modulated due to microstructural evolution and clamping relaxation developed during thermal annealing process. Room temperature ME voltage coefficient of the heterostructured thin films was enhanced with increasing annealing temperature and reached to 29 mV/cm·Oe when annealed at 650 °C.