Multistate Memory Devices Research Articles

Bulk axial Landau levels in the helically magnetized Weyl Hamiltonian and application to multistate memory devices

Bulk axial Landau levels in the helically magnetized Weyl Hamiltonian and application to multistate memory devices

Magnetoelectric behaviour of Pb-free Na0.5Bi0.5TiO3-Ni0.5Co0.5Fe2O4 particulate novel ceramic composites

Magnetoelectric (ME) multiferroic composites are more promising candidates for low-power consumption devices, next-generation spintronic devices, and multistate memory devices since they exhibit significantly higher Magnetoelectric interaction over single-phase multiferroics. In this regard magnetic behaviour, dielectric, and magnetoelectric characteristics of a novel Pb-free (1-x) Na0.5Bi0.5TiO3-(x) Ni0.5Co0.5Fe2O4, (x = 0, 0.2, 0.4, 0.6, 0.8 & 1) particulate composites were explored. Solid-state synthesis was adopted to prepare the single-phase Na0.5Bi0.5TiO3 (NBT), Ni0.5Co0.5Fe2O4 (NCFO), and their composites. The presence of rhombohedral NBT and cubic inverse spinel NCFO in the composites were confirmed with X-ray diffraction (XRD) patterns. Microstrain (ε) induced in the prepared samples was estimated by using the Williamson-Hall (W-H) method. The piezoelectric NBT phase shows the tensile strain and the ferromagnetic NCFO shows the comprehensive strain. Among all the composites 0.8NBT-0.2NCFO shows a higher strain value of 0.012%, and the decrement in the strain values was observed upon increases in ferrite concentration. Backscattered scanning electron microscope (BS-SEM) images revealed the existence of highly dense uniformly distributed elements and Energy dispersive spectroscopy (EDS) confirmed the existing elements in the composites. UV–vis absorbance spectra disclosed a rise in energy gap values from 1.14 eV to 3.0 eV with increasing NBT percentage. Variations of dielectric permittivity and dielectric loss with respect to AC frequency were studied at room temperature (RT), and the 0.2NBT-0.8NCFO sample shows a high dielectric permittivity of 8500 at ∼100Hz. M − H (Magnetization-Magnetic field) hysteresis loops demonstrate the pure ferrite phase has a coercive field (Hc) = 578 Oe, remnant magnetization (Mr) = 21.43 emu/g and saturation magnetization (Ms) = 71.29 emu/g. The Ms and Mr values in the composites are enhanced with increasing ferrite percentage. Studies on ME behaviour of (1-x) NBT-(x) NCFO, (x = 0.2,0.4,0.6 & 0.8) show the coupling effect between both the phases at room temperature and the highest coupling coefficient value 3.16mV/Cm. Oe was observed for the 0.8NBT-0.2NCFO further the reasons and mechanisms are discussed.

Emergence of Multiple Hysteresis Cycles and Polarization Plateaus: A Monte Carlo Investigation of Fullerene-Like System

This study uses Monte Carlo simulations to study the emergence of polarization plateaus and hysteresis cycles within fullerene-like systems. To explore the dielectric properties of the system, we also applied the Blume-Capel Ising model, which considered critical, coercive and saturation fields. Temperature, ferrielectric parameter, and crystal field all influenced the system’s behavior. Multi-state memory devices, spintronics, and materials characterization can benefit from the discovery of multiple loops and polarization plateaus.

Greatly enhanced tunneling electroresistance in ferroelectric tunnel junctions with a double barrier design

We propose that the double barrier effect is expected to enhance the tunneling electroresistance (TER) in the ferroelectric tunnel junctions (FTJs). To demonstrate the feasibility of this mechanism, we design a model structure of Pt/BaTiO3/LaAlO3/Pt/BaTiO3/LaAlO3/Pt double barrier ferroelectric tunnel junction (DB-FTJ), which can be considered as two identical Pt/BaTiO3/LaAlO3/Pt single barrier ferroelectric tunnel junctions (SB-FTJs) connected in series. Based on density functional calculation, we obtain the giant TER ratio of 2.210 × 108% in the DB-FTJ, which is at least three orders of magnitude larger than that of the SB-FTJs of Pt/BaTiO3/LaAlO3/Pt, together with an ultra-low resistance area product (0.093 KΩμm2) in the high conductance state of the DB-FTJ. Moreover, it is possible to control the direction of polarization of the two single ferroelectric barriers separately and thus four resistance states can be achieved, making DB-FTJs promising as multi-state memory devices.

Optimization of magnetoelectric coupling in BiFeO3-BaTiO3-MnFe2O4 tri-phase composites for ultra-sensitive devices

Multiferroic materials with coupled electric and magnetic orderings have astonished the scientific community by exhibiting magnetoelectric (ME) coupling, viable for multistate memory devices. Facile synthesis of multiferroic composites have further triggered these delightful attributes. Here we present a series of tri-phase composites of the form; (1–x)(0.7BiFeO3 + 0.3BaTiO3) + xMnFe2O4, synthesized by a facile two-step method. The phase-pure synthesis was confirmed using X-ray diffraction. For energy storage capability, a composition with x = 0.02, reveals the recoverable energy density of ∼0.22 mJ/cm3 followed by the highest efficiency of 64.02%, making such composition favourable for energy storage perspectives at minute inclusion of the spinel phase. The characteristic I-V curves were used to explain the leakage current of BiFeO3 (BFO). The positive up and negative down (PUND) sequence demonstrates the significant variation in switching charge density. The gradual increase in grain size resulted in a lowering of the coercivity values up to 301 Oe at x = 0.10 having 6.31 emu/g as saturation magnetization (MS) value along with MR/MS ratio of 0.33. The linear response shown by ME coupling at x = 0.06 along with the afore mentioned attributes make this composition a potential candidate for ultra-sensitive magnetoelectric devices.

Enhanced magnetoelectric effect in Lead-Free piezoelectric BaZr0.2Ti0.8O3 − 0.5 Ba0.7Ca0.3TiO3 and Fe-rich magnetostrictive Co0.8Fe2.2-xDyxO4 nanocomposites for energy harvesting applications

The design, discovery and development of magnetoelectric (ME) composites with enhanced properties and performance opens up further opportunities to explore new scientific aspects and to develop next-generation electromagnetic devices. Nevertheless, the engineered complex ME composites with multi-phase interfacial chemistry have rarely been reported. In this context, in the present work, enhanced magnetoelectric phenomenon reported for the ME-composites made using lead-free ferroelectric phase BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 (BZT-BCT) and magnetostrictive phase Co0.8Fe(2.2-x)DyxO4 (CFDO). The ME composites were synthesized by varying the magnetostrictive CFDO phase, where Dy(x) varied as x = 0.000(ME1), 0.025 (ME2), 0.050 (ME3) and 0.075 (ME4). The effect of varying CFDO phase on the structure, morphology, microstructure, density, piezoelectric, dielectric, ferroelectric and magnetoelectric properties of the ME-composites is evaluated. X-ray diffraction analyses of all ME-composites confirm the tetragonal structure of the BZT-BCT ferroelectric phase and spinel cubic structure of the CFDO ferrimagnetic phase. Raman spectroscopic analyses also confirm and validate the respective phases with a clear signature presence of the characteristics modes. Granular, dense microstructure with a uniform particle size distribution evidenced in scanning electron microscopy imaging analyses. The physical density measurements indicate that the ME composites attain a density in the range of 6.0–6.4 g/cm3; density increases with increasing CFDO phase. Frequency dispersion profiles of the dielectric constant exhibit a maximum dielectric constant for ME1 composite while a sharp decrease in dielectric constant at higher frequencies is noted with increasing CFDO phase in the ME-composites. Ferroelectric properties evaluated by measuring the polarization (P) – electric field (E) hysteresis loops at 300 K indicate the presence of typical P-E loops in all ME composites confirming that the ferroelectric properties are retained even after combining with the magnetic phase. The ME4 composite demonstrate a 0.573 squareness ratio, which is useful for multistate memory device applications. The ME4 composites further characterized by a moderately higher remnant polarization (Pr = 2.24 µC/cm2). All the synthesized ME-composites are promising for permanent memory storage device (FeRAM) applications. Piezoelectric coefficient measurements indicate a maximum piezoelectric coefficient of 116 pC/N for the ME4 composite while the ME-coefficient measurements demonstrate higher ME-coefficients of 250 mV/(cm-Oe) for ME2 composite. Based on the comprehensive and comparative studies of the ME-composites with variable magnetostrictive phase, the structure-chemistry-property correlation in these ME-composites established for possible applications in electromagnetic devices and energy harvesting applications.

A skyrmion helicity-based multistate memory in synthetic antiferromagnets

Skyrmions in synthetic antiferromagnets (SAFs) are famous for being immune to the skyrmion Hall effect and hold the advantages of significantly higher speed for motion and smaller size than ferromagnetic systems. Therefore, skyrmions in SAFs are promising in spintronic devices. Here, we investigated the formation and in-plane-current-driven motion of bilayer skyrmions in the absence of Dzyaloshinskii–Moriya interaction by using micromagnetic simulations. Then, we studied the spacing variation between consecutive skyrmionic bits on the antiferromagnetic coupled nanotrack; it is found that there is an equilibrium distance between two Bloch skyrmions with opposite helicities. We also propose a reasonable method to distinguish skyrmions with opposite helicities. Finally, we displayed that the SAF skyrmion could pass through impurities due to topological protection. Based on these results, we designed the skyrmion helicity-based multistate memory devices in the SAF system, which have the advantages of high density and energy efficiency.

Metal-Organic Precursor Synthesis, Structural Characterization, and Multiferroic Properties of GdFeO3 Nanoparticles.

GdFeO3 nanoparticles were fabricated by a facile metal–organic precursor method using citric acid as a complexing agent. The phase purity and structural analysis by powder X-ray diffraction and FTIR studies indicates that the material is highly crystalline with an orthorhombic structure. Electron microscopic (TEM and SEM) studies of rare earth ferrites reveal worm-shaped nanoparticles with an average grain size of 95 nm. The high-resolution TEM study provides an insightful image, which shows an interplanar spacing of approximately 0.12 nm that corresponds to the (112) crystalline plane. A high surface area of 231.5 m2 g–1 has been achieved with a mesoporous texture, which in turn gives a high dielectric constant. Well-defined hysteresis is obtained with a saturation magnetization of 17.5 emu g–1, remanent magnetization of 3.9 emu g–1, and coercive field of −446 Oe. Room-temperature ferroelectricity in GdFeO3 nanoparticles has been found for the first time with no leaky current and hence may be used in multistate memory devices.

Enhanced magnetoresistance and electroresistance at high temperature in a nano-matrix manganite

Magnetoresistance (MR) is highly exploitable to future spintronics devices, such as random-access memory, magnetic sensors, and spin-based neuromorphic electronics. Conventionally the enhanced MR in manganite always companies a sacrifice of Curie temperature, which limits its widespread application potential. Here we report a high-temperature survived MR in a La0.67Sr0.33MnO3 (LSMO) film grown on a piezoelectric substrate. The lattice mismatch between the film and substrate creates a self-assembled nano-matrix with two types of nanoscale matrices, namely [001]- and [101]-orientated LSMO magnetic domains. In this structure, the MR can reach -67% at even 350 K. Furthermore, the resistance of LSMO is electric-field-tunable to multiple resistance states by electrically modulating the biaxial strain imposed by the underlying piezoelectric substrate, an additional ∼14.2% electroresistance (ER) can be obtained. The achievements of high-temperature substantial MR and its electrical tunability in the nano-matrix manganite are of use to future multi-state memory devices in both spintronics and straintronics.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Utilizing spin currents from the dual surfaces of a heavy metal Pt layer for simultaneous spin-torque switching in FeTb/Pt/FeTb trilayers

The spin–orbit torques (SOTs) induced by spin currents in the heavy metal/ferromagnet (HM/FM) bilayer are of great importance for enabling nonvolatile, low power consumption spintronics. In the standard HM/FM bilayers, only the spin current at the interface is employed for implementing SOTs. Another part of the spin current with the opposite spin polarization, which is generated at the opposite side of the HM layer, is supposably not put into action. Here, we will show the experimental feasibility of utilizing spin currents from the dual surfaces of the HM Pt layer. In particular, through synthesizing perpendicularly magnetized Fe1−x1Tbx1/Pt/Fe1−x2Tbx2 trilayers, we demonstrate that spin currents at the dual surfaces of the Pt layer can be simultaneously used to deterministically switch the perpendicular magnetization vectors of the neighboring Fe1-xTbx layers, through the current-induced SOTs. Based on the second harmonic measurement, we further quantify the enhanced efficiency of the current-induced SOTs. A three-dimensional spintronic device that can host an exponentially increased magnetization states is also demonstrated. Our results could be important for enabling more energy efficient spin-orbitronics that could be used in multistate memory and spin logic devices.

Three-dimensional chiral magnetization structures in FeGe nanospheres

Skyrmions, spin spirals, and other chiral magnetization structures developing in materials with an intrinsic Dzyaloshinskii-Moriya interaction display unique properties that have been the subject of intense research in thin-film geometries. Here, we study the formation of three-dimensional chiral magnetization structures in FeGe nanospheres by means of micromagnetic finite-element simulations. In spite of the deep submicron particle size, we find a surprisingly large number of distinct equilibrium states, namely, helical, meron, skyrmion, chiral-bobber, and quasisaturation states. The distribution of these states is summarized in a phase diagram displaying the ground state as a function of the external field and particle radius. This unusual multiplicity of possible magnetization states in individual nanoparticles could be a useful feature for multistate memory devices. We also show that the magnetodipolar interaction is almost negligible in these systems, which suggests that the particles could be arranged at high density without experiencing unwanted coupling.

Magnetoelectricity in vertically aligned nanocomposites: Past, present, and future

Vertically aligned magnetoelectric nanocomposites (ME VANs) self-assembled from piezoelectric perovskite and magnetostrictive spinel phases are appealing due to their unique pillar-array-like morphologies and enhanced ME properties. The strain-mediated ME effect leads to various field-dependent characteristics that are tunable, which provides application pathways in microelectronics. The microstructure formation mechanisms of ME VANs have been explored by phase field simulations. Several effective approaches to modify and improve the ME properties of VANs are discussed, including orientations, deposition conditions, and substrate types. Potential applications are also discussed, such as field tunable “adaptive” microelectronics and multistate memory devices.

Multistate Memory Enabled by Interface Engineering Based on Multilayer Tungsten Diselenide.

The diversification of data types and the explosive increase of data size in the information era continuously required to miniaturize the memory devices with high data storage capability. Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for flexible and transparent electronic and optoelectronic devices with high integration density. Multistate memory devices based on TMDs could possess high data storage capability with a large integration density and thus exhibit great potential applications in the field of data storage. Here, we report the multistate data storage based on multilayer tungsten diselenide (WSe2) transistors by interface engineering. The multiple resistance states of the WSe2 transistors are achieved by applying different gate voltage pulses, and the switching ratio of the memory can be as large as 105 with high cycling endurance. The water and oxygen molecules (H2O/O2) trapped at the interface between the SiO2 substrate and WSe2 introduce the trap states and thus the large hysteresis of the transfer curves, which leads to the multistate data storage. In addition, the laminated Au thin film electrodes make the contact interface between the electrodes and WSe2 free of dangling bond and Fermi level pinning, thus giving rise to the excellent performance of memory devices. Importantly, the interface trap states can be easily controlled by a simple oxygen plasma treatment of the SiO2 substrate, and subsequently, the performance of the multistate memory devices can be manipulated. Our findings provide a simple and efficient strategy to engineer the interface states for the multistate data storage applications and would motivate more investigations on the trap state-associated applications.

Magnetic State Control via Field-Angle-Selective Switching in Asymmetric Rings

Switching the chirality of the vortex state in asymmetric ferromagnetic rings is interesting for multistate memory devices, logic elements, and stray-field-based rotation sensors. This study shows that different magnetic states can be configured by carefully tuning the magnetic field angle. Using time-resolved scanning electron microscopy with polarization analysis to image the magnetization dynamics of these rings, the authors detect competing switching pathways for certain field angles. These different pathways do not change the resulting magnetic states, though, which is advantageous for engineering reliable devices for a range of potential spintronic applications.

Analysis of recoverable and energy loss density mediated by Ni/Cr co-doping in BiFeO3

BiFeO3 is the only multiferroic material in which coexistence of ferroelectricity and anti-ferromagnetism can be seen at room temperature. However, small strength of coupling is a main hurdle for its use in multistate memory devices. The single or co-doping of any other material can improve its multiferroicity. In this regard, we synthesized a series with varying concentration of Ni at the place of Cr in BiFe0.9Cr0.1−xNixO3 (x = 0, 0.03, 0.07, 0.1) using sol‒gel auto-combustion route. The rhombohedrally distorted cubic perovskite structure has been observed throughout the series, while the micrographs obtained from scanning electron microscopy exposed that Ni-free composition had diffused grains with no sharp boundaries. Addition of Ni not only reduced the average grain size but also increased the ferroelectric response. Energy density calculations were performed using charge–discharge curves which revealed that increasing Ni/Cr ratio at the Fe site enhanced the energy storage capability. Dielectric study revealed that real part of dielectric constant decreased and concentration-dependent variation remained consistent with morphological change. Impedance spectroscopy exposed that Ni substitution caused an increase in its resistive response, while the relaxation phenomenon and nature of transportation were examined through the shifting of peaks in the complex modulus spectrum.

Non-volatile resistance switching in LaNiO3 films on PMN-PT substrates

Using perovskite oxide films of LaNiO3 as a research object, we demonstrate that the lattice strain has a huge effect on the electron transport properties of the film. The LaNiO3 film was deposited on the PMN-PT substrate, a large change in resistance is generated under the electric field. The substrate will be polarized under an electric field and induced large lattice strain due to the converse piezoelectric effect, the resistivity of the LaNiO3 film is modulated up to three orders of magnitude. Moreover, multiple stable resistance states with endurance properties and good retention can be obtained at room temperature. The modulation on resistive states of LaNiO3 films under an electric field can be used for other oxide films and provides a simple and low energy consumption way to construct multistate resistive memory devices.

MnO2-doping induced enhanced multiferroicity in Bi0.83Sm0.17Fe0.95Sc0.05O3 ceramics

Multiferroic material BiFeO3 (BFO) that possesses spin and dipole orders simultaneously at ambient temperature shows great potential in multi-state memory and spintronic devices. Nevertheless, the presence of low polarization and weak magnetism in ceramics heavily restricts their applications. Here, the charge compensation method was utilized to reduce leakage current and enhance dielectric breakdown strength (Eb) by introducing MnO2 in Bi0.83Sm0.17Fe0.95Sc0.05O3 ceramics. It was revealed that all the ceramics have the coexistence of polar R3c and nonpolar Pnma phases. With increasing MnO2, Eb is significantly improved, leading to the enhanced ferroelectric polarization. A high remanent polarization of ≈51.54 μC/cm2 was achieved in the composition with 0.9 wt. % MnO2 doping, which is the maximum in the reported BFO-based ceramics. Meanwhile, a rather high remanent magnetization of ≈0.31 emu/g was obtained in the same compound. Therefore, this work reveals an interesting route to tailor BFO-based ceramics toward enhanced multiferroicity.

Magneto-electric coupling and multifunctionality in BiFeO3–CoFe2O4 core-shell nano-composites

The presence of magneto-electric composites in the form of core-shell nanostructures improves their multifunctional characteristics, which makes them suitable for use in diverse fields ranging from medicine to data storage industry. Among these composites, the perovskite/spinel combination has a huge potential. To explore the multifunctional response of this arrangement, we synthesised a series of (1-x)BiFe0.95Mn0.05O3/xCoFe2O4 (x = 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) nano core-shells by a sonication-assisted sol-gel auto combustion method. Through structural analysis, the peaks corresponding to both perovskite and spinel phases were identified and the number of peaks belonging to each phase were found to vary according to their composition. Surface images captured using scanning tunnelling microscopy confirmed the formation of a CoFe2O4 core covered with Mn-doped BiFeO3 shells. The effect of increased concentration of CoFe2O4 on dielectric properties was analysed on the basis of the local charge transport mechanism in a broad frequency range. With the help of PE loops, energy density was calculated to investigate the potential of these compositions in energy storage devices. Magnetic response analysis revealed that saturation magnetisation increased as CoFe2O4 contents were increased while coercivity remained almost constant. The significant signal of the magneto-electric effect indicated that these composites have a potential for application in multistate memory devices.

Design of a multifunctional polar metal via first-principles high-throughput structure screening

Intrinsic polar metals are rare, especially in oxides, because free electrons screen electric fields in a metal and eliminate the internal dipoles that are needed to break inversion symmetry. Here we use first-principles high-throughput structure screening to predict a new polar metal in bulk and thin film forms. After screening more than 1000 different crystal structures, we find that ordered BiPbTi2O6 can crystallize in three polar and metallic structures, which can be transformed between via pressure or strain. In a heterostructure of layered BiPbTi2O6 and PbTiO3, multiple states with different relative orientations of BiPbTi2O6 polar displacements, and PbTiO3 polarization, can be stabilized. At room temperature, the interfacial coupling enables electric fields to first switch PbTiO3 polarization and subsequently drive 180° change of BiPbTi2O6 polar displacements. At low temperatures, the heterostructure provides a tunable tunnelling barrier and might be used in multi-state memory devices.