Applications In Memory Devices Research Articles

Strain and illumination triggered regulations of up-conversion luminescence in Er-doped Bi0.5Na0.5TiO3[sbnd]BaTiO3/Mica flexible multifunctional thin films

External stimuli induced effective regulations of luminescent material are of significant interest in the development of smart optical devices. Here, by simply doping with Er3+ in the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (BNTBT) ferroelectric host, using the bendable mica substrate, and exerting mechanical strain (bending) or light illumination (via photochromic reaction), the all-inorganic, highly-transparent and flexible Er-doped BNTBT/Mica luminescent-ferroelectric thin films were designed and fabricated, displaying strain-induced dramatically elevation of up-conversion photoluminescence (PL) intensity, suppression of PL concentration quenching, outstanding endurance and durability, convenient illumination-mediated PL quenching. And the strain-induced structural changes and local lattice distortions of the thin films were further explored through theoretical calculations and Raman measurement. Our results can supply the guidance of designing other luminescent-ferroelectric materials with controlled PL properties via easy mechanical/photo stimuli for expanding the application of multifunctional wearable memory devices.

Controllable Domain Walls in Two-Dimensional Ferromagnetic Material Fe3GeTe2 Based on the Spin-Transfer Torque Effect.

Recently, two-dimensional magnetic material has attracted attention worldwide due to its potential application in magnetic memory devices. The previous concept of domain walls driven by current pulses is a disordered motion. Further investigation of the mechanism is urgently lacking. Here, Fe3GeTe2, a typical high-Curie temperature (TC) two-dimensional magnetic material, is chosen to explore the magnetic domain dynamics by in situ Lorentz transmission electron microscopy experiments. It has been found that the stripe domain could be driven, compressed, and expanded by the pulses with a critical current density. Revealed by micromagnetic simulations, all the domain walls cannot move synchronously due to the competition between demagnetization energy and spin-transfer torque effect. In consideration of the reflection of high-frequency pulses, the disordered motion could be well explained together. The multiple stable states of the magnetic structure due to the weak exchange interaction in a two-dimensional magnet provides complex dynamic processes. Based on plenty of experiments, a cluster of domain walls could be more steady and move more synchronously under the drive of pulse current. The complication of domain wall motions presents a challenge in race track memory devices and two-dimensional magnetic material will be a better choice for application research.

Van der Waals Antiferroelectric Magnetic Tunnel Junction: A First-Principles Study of a CrSe2/CuInP2S6/CrSe2 Junction.

Magnetic tunnel junctions (MTJs), ferroelectric/antiferroelectric tunnel junctions (FTJs/AFTJs), and multiferroic tunnel junctions (MFTJs) have recently attracted significant interest for technological applications of nanoscale memory devices. Until now, most of them are based on perovskite oxide heterostructures with a relatively high resistance-area (RA) product and low resistance difference unfavorable for practical applications. The recent discovery of the two-dimensional (2D) van der Waals (vdW) ferroelectric (FE) and magnetic materials has opened a new route to realize tunnel junctions with high performance and atomic-scale dimensions. Here, using first-principles calculations, we propose a new type of 2D tunnel junction: an antiferroelectric magnetic tunnel junction (AFMTJ), which inherits the features of both MTJ and AFTJ. This AFMTJ is composed of monolayer CuInP2S6 (CIPS) sandwiched between 2D magnetic electrodes of CrSe2. The AFTJ with nonmagnetic electrodes of TiSe2 on both sides of CIPS and the asymmetric AFTJ with both CrSe2 and TiSe2 electrodes are also investigated. Based on quantum-mechanical modeling of the electronic transport, sizeable tunneling electroresistance effects and multiple nonvolatile resistance states are demonstrated. More importantly, a remarkably low RA product (less than 0.1 Ω·μm2) makes the proposed vdW AFMTJs superior to the conventional MFTJs in terms of their promising nonvolatile memory applications. Our calculations provide new guidance for the experiment and application of nanoscale memory devices.

Characterizing Ferroelectric Properties of Hf0.5Zr0.5O2 From Deep-Cryogenic Temperature (4 K) to 400 K

Ferroelectric Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) thin film has obtained considerable attention for emerging non-volatile memory (eNVM) and synaptic device applications. To our best knowledge, the polarization switching of HZO has not been comprehensively investigated in wide-ranging temperatures from deep-cryogenic 4 K to elevated temperature 400 K within the same set of test structures. In this work, we experimentally characterize the reliability effects such as endurance (wake-up, fatigue, and breakdown), retention (including imprint), and small-signal response of the HZO capacitor from the lowest temperature reported (4 K) to the elevated temperature (400 K). We demonstrate one of the highest endurance cycles <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ &gt; 3.5\times 10^{10}$ </tex-math></inline-formula> among reported TiN/HZO/TiN capacitors with negligible wake-up/fatigue effects or retention degradation, all obtained at 4 K. Based on the experimental results, we further simulated ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistor (FeFET) to evaluate their potentials as cryogenic memories.

Nanoscale Multiferroic Properties at Room Temperature of Lead Zirconate Titanate Iron Tantalate for Memory Device Applications

The evolution of device miniaturization and the potential application for next-generation nonvolatile memory devices Pb(Zr0.53Ti0.47)0.60(Fe0.5Ta0.5)0.40O3 (PZTFT) can be used as a barrier in multiferroic tunnel junctions. We report single-phase magnetoelectric multiferroic ultra-thin PZTFT at room temperature. The magnetic and ferroelectric properties have been investigated down to 7 nm thickness which are requisite properties for multiferroic tunnel barriers for multi-states memory applications. In the present study, we show that it remains a switching multiferroic at room temperature down to at least thickness d = 7 nm, which is thin enough for tunnel junctions.

Switching-Modulated Phase Change Memory Realized by Si-Containing Block Copolymers.

The phase change memory (PCM) is one of the key enabling memory technologies for next-generation non-volatile memory device applications due to its high writing speed, excellent endurance, long retention time, and good scalability. However, the high power consumption of PCM devices caused by the high switching current from a high resistive state to a low resistive state is a critical obstacle to be resolved before widespread commercialization can be realized. Here, a useful approach to reduce the writing current of PCM, which depends strongly on the contact area between the heater electrode and active layer, by employing self-assembly process of Si-containing block copolymers (BCPs) is presented. Self-assembled insulative BCP pattern geometries can locally block the current path of the contact between a high resistive film (TiN) and a phase-change material (Ge2 Sb2 Te5 ), resulting in a significant reduction of the writing current. Compared to a conventional PCM cell, the BCP-modified PCM shows excellent switching power reduction up to 1/20 given its use of self-assembled hybrid SiFex Oy /SiOx dot-in-hole nanostructures. This BCP-based bottom-up process can be extended to various applications of other non-volatile memory devices, such as resistive switching memory and magnetic storage devices.

Room-temperature large magnetoelectricity in a transition metal doped ferroelectric perovskite

There is increasing interest in novel magnetoelectric (ME) materials that exhibit robust ME coupling at room temperature (RT) for advanced memory, energy, spintronics, and other multifunctional device applications, by making use of the ability to control polarization with a magnetic field and/or magnetization via an electric field. Obtaining ME materials with strong ME coupling, understanding the origin, and manipulating its processing along with composition to realize large ME coefficients at RT constitute an important step in multiferroic research. To address this, we have investigated the multiferroic and ME properties of Ni-doped $\mathrm{Pb}({\mathrm{Zr}}_{0.20}{\mathrm{Ti}}_{0.80}){\mathrm{O}}_{3}$ (PZT). We find that the ferroelectric $({T}_{\mathrm{C}}\ensuremath{\sim}700\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and weak ferromagnetic (\ensuremath{\sim}602 K) phase transitions of Ni-doped PZT are well above room temperature (RT), leading to a strong ME coupling coefficient (${\ensuremath{\alpha}}_{E,31}$) of $11.7\phantom{\rule{0.16em}{0ex}}\mathrm{mV}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}\phantom{\rule{0.16em}{0ex}}{\mathrm{Oe}}^{--1}$ (${H}_{\mathrm{ac}}=1\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ and $f=1\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$). While x-ray diffraction suggests a single-phase material, high-resolution transmission electron microscopy reveals regions with and without Ni present; thus magnetoelectric coupling between two phases is possible. First-principles calculations suggest the $(\mathrm{N}{\mathrm{i}}_{\mathrm{Pb}}{)}^{\ifmmode\times\else\texttimes\fi{}}$ defect is likely to be responsible for the experimental observed magnetism and ME coupling in Ni-doped PZT. We further demonstrate that Ni-doped PZT exhibits low loss tangent, low leakage current, large saturation polarization, and weak ferromagnetism. Ultimately, our work demonstrates that Ni-doped PZT is a cost-effective RT multiferroic with strong ME coupling.

Achieving Large Switchable Polarization and Enhanced Piezoelectric Response in BiFeO3‐PbTiO3 Solid Solution Ceramics

AbstractPerovskite materials based on BiFeO3‐PbTiO3 (BFPT) solid solutions are promising for various applications thanks to the extremely large spontaneous polarization (Ps) and existence of multiferroic morphotropic phase boundary. For applications in piezoelectric and memory devices, complete switching of Ps is needed, which is hard to achieve practically. In this work, a simple modified mixed‐oxide reaction method is developed allowing to prepare Dy‐ and Sm‐modified BFPT ceramics with significantly improved properties. The MPB compositions demonstrate well‐saturated ferroelectric hysteresis loops with large switchable remanent polarization of 60 µC cm−2 and enhanced piezoelectric properties with a large‐signal piezoelectric coefficient d33* = 214 pm V−1 and a direct piezoelectric coefficient d33 = 128 pC N−1, which is one and a half times larger than the best d33 value reported for the BFPT ceramics so far. The Curie temperature reaches 539 °C, suggesting a potential for high‐temperature applications. The domain structure is studied by piezoresponse force microscopy. It is found that the enhanced dielectric/piezoelectric properties are mainly due to polarization rotation/extension (intrinsic) mechanism, while the extrinsic contributions of the interphase and domain walls are virtually absent. The strategies for improvement of properties of BFPT and related materials are proposed.

Encapsulating viologen derivatives in anionic MOFs: Photochromism and photocontrolled luminescence

The tunable luminescent materials based on photochromic reactions have received considerable interest due to its potential applications in switches and optical memory devices. However, constructing such materials with ideal properties remains a challenge. In this context, two rare viologen-based host guest materials, 2[MV]·[Zn6(PMA)4(H2O)4] (1), 2[PBM]·[Zn2(PMA)2(H2O)6] (2), (MV·2NO3 ​= ​1,1'-dimethyl-(4,4'-bipyridine)-1,1'-diium dinitrate; PBM·2NO3 ​= ​4,4'-(1,4-phenylene)bis(1-methylpyridin-1-ium) dinitrate; H2PMA ​= ​pyromellitic acid), have been prepared by encapsulating viologen derivatives into the anionic metal-organic frameworks produced from pyromellitic acid ligands and zinc cations. Due to the presence of photoactive guest MV2+/PBM2+ and the short O⋯N+ distances between the oxygen atom of carboxylate groups and pyridine ring of guest, both complexes display fast-responsive reversible naked photochromism from colorless to blue or yellow upon UV–Vis irradiation At the same time, these host-guest materials exhibit good fluorescence emission behavior and their fluorescence can be reversible modulation under light irradiation. These are rare examples of fluorescence switching materials based on host-guest system that involve the electron-transfer photochromic process.

Ferroelectric control of the Néel vector in L10-type antiferromagnetic films

How to efficiently manipulate the N\'eel vector of antiferromagnets (AFM) by electric methods is one of the major focuses in current antiferromagnetic spintronics. In this work, we investigated the ferroelectric control of magnetism in AFM L10-MnPt/BaTiO3 bilayers structures by using first-principles calculation. We studied the effect of ferroelectric polarization reversal on magnetic crystalline anisotropy (MCA) of L10-MnPt films with different interface structures. Our results predict a large perpendicular MCA in L10-MnPt films with Pt-O interface, while an in-plane MCA with Mn-O interface when they are interfaced with ferroelectric BaTiO3. In addition, the magnitude of MCA for both interfaces can be modulated efficiently by the polarization reversal of BaTiO3. The ferroelectric control of MCA has been analyzed based on second order perturbation theory, and it can be mainly attributed to the ferroelectric polarization driven redistribution of Pt-5d orbital occupation around Fermi energy. Especially, for Mn-O interface, the N\'eel vector can be switched between in-plane [100] and [110] directions, or even from in-plane to out-of-plane at certain film thickness by reversing ferroelectric polarization. Our results may provide a non-volatile concept for ferroelectric control of N\'eel vector in L10-antiferromagnets, which could stimulate experimental investigations on magnetoelectric effect of antiferromagnets and promote its applications in low-power consumption spintronic memory devices.

Forming-free Pt/Al2O3/HfO2/HfAlOx/TiN memristor with controllable multilevel resistive switching and neuromorphic characteristics for artificial synapse

Controllable multilevel resistive switching (RS) and neuromorphic characteristics emerges as a promising paradigm to build power-efficient computing hardware for high density data storage memory and artificial intelligence. Nevertheless, the current nonvolatile memory still endures from reliability and variability of the memristors. In this work, Pt/Al2O3/HfO2/HfAlOx/TiN multilayer memristor was prepared by using atomic layer deposition (ALD) to examine the well-regulated multilevel RS and neuromorphic properties. The memristor was found to demonstrate admirable RS properties, including forming-free, low operating voltage (Set/Reset), high switching ratio (>100), multi-level retention time (104 s), and good durability (1000 switching cycles). Furthermore, seven and four resistance states can be accomplished by modulating CC through set-operation and stop-voltage during the reset-operation. By modulating the multi-level resistance state, the electronic synapse can simulate synaptic plasticity, such as potentiation/depression, paired pulse facilitation (PPF) and spike-timing-dependent plasticity (STDP). Results show that a multilayer memristor has potential in the application of multilevel data storage memory and bionic portable electronic devices.

Large spin–orbit torque efficiency in PtBi2 film

Bulk PtBi2 has attracted much attention for its topological semi-metallic electronic properties and highly promising applications in spintronics. Here, we report large spin–orbit torque (SOT) efficiency in the sputtered PtBi2 alloy with the trigonal-phase. From spin–torque-induced ferromagnetic resonance measurements, the SOT efficiency of 5 nm PtBi2 is estimated to be ∼0.2. Moreover, the spin Hall conductivity of PtBi2 [∼1 × 105 ℏ/2e (Ω m)−1] is comparable to that of topological materials, such PtTe2 and Bi2Te3. The PtBi2 film has much lower resistance than that of Bi-based topological materials, which makes it a useful candidate for application. The results suggest that the PtBi2 alloy is promising for applications in magnetic memory and logic devices driven by SOT.

Charge Storage of Isolated Monolayer Molybdenum Disulfide in Epitaxially Grown MoS2/Graphene Heterostructures for Memory Device Applications.

We epitaxially grew bilayer molybdenum disulfide (MoS2) on monolayer graphene by sulfurizing a molybdenum-trioxide film (MoO3) which was deposited with thermal evaporation. The Hall mobilities of graphene before and after the growth of MoS2 are similar, indicating that the underlying 2D layer was little affected during the deposition and sulfurization. Through the atomic-layer etching, the topmost layer of MoS2 is isolated from the source and drain electrodes. The top-gate transistor with the isolated monolayer MoS2 on top of the graphene channel exhibits hysteresis of drain current as the gate voltage varies. This may be due to the weak tunneling through 2D layers bonded by the van der Waals force in the absence of an external electric field. The long retention time of the device features robust charge storage around the isolated MoS2 layer. The one-transistor-zero-capacitor memory module based on this thin heterostructure of 2D materials can be advantageous for applications in dynamic random access memories with reduced thickness.

Photoassisted Electron-Ion Synergic Doping Induced Phase Transition of n-VO2/p-GaN Thin-Film Heterojunction.

As a typical correlated metal oxide, vanadium dioxide (VO2) shows specific metal-insulator transition (MIT) properties and demonstrates great potential applications in ultrafast optoelectronic switch, resistive memory, and neuromorphic devices. Effective control of the MIT process is essential for improving the device performance. In the current study, we have first proposed a photoassisted ion-doping method to modulate the phase transition of the VO2 layer based on the photovoltaic effect and electron-ion synergic doping in acid solution. Experimental results show that, for the prepared n-VO2/p-GaN nanojunction, this photoassisted strategy can effectively dope the n-VO2 layer by H+, Al3+, or Mg2+ ions under light radiation and trigger consecutive insulator-metal-insulator transitions. If combined with standard lithography or electron beam etching processes, selective doping with nanoscale size area can also be achieved. This photoassisted doping method not only shows a facile route for MIT modulation via a doping route under ambient conditions but also supplies some clues for photosensitive detection in the future.

First principles investigation of physically conductive bridge filament formation of aluminum doped perovskite materials for neuromorphic memristive applications

Perovskites had gathered considerable attention as they presented interesting properties and were studied for their potential applications. In this theoretical work, device-to-device comparison using two different materials for state of art of memristors are explored. Effect of substitutional doping of Al atom (Alsub) with the different concentration (25%, 50%, 75% and 100%) being source of noise (external perturbation) on magnetoelectric properties of two perovskites GdFeO3 and NdFeO3 are investigated for studying resistive switching phenomena. We examined formation energy and magnetoelectric properties of the perovskites using Vienna ab initio simulation package (VASP) based on density functional theory. Calculated formation energies showed that 75% Al-NdFeO3 is the most stable element of this study. Density of states and spin polarized density of states showed that conductivity of both composites increased with increasing concentration of Al and became maximum for 75% doping concentration. This confirmed the constructive role of noise (dopant being external perturbation). Strong conductive channel illustrated in isosurface charge density plots with stronger charge transfer from integrated charge density plots of 75% doping concentration confirmed this result. Electronic properties endorsed that 75% Alsub doped NdFeO3 with enhanced electronic character is highly preferable to exercise in resistive switching (RS) mechanism for future memory device applications.

Magnetic properties of NH4Al(SO4)2•12H2O: Evidence of glass behavior based on proton orbitals

The polarity of the magnetic susceptibility χ(T) of ammonium alum NH4Al(SO4)2•12H2O single crystals below its ferroelectric Curie temperature TC is identified to depend only on the cooling rate. To explain this, we consider reorienting distorted NH4+ to carry weak magnetic and electric dipole moments that are linearly coupled. Slow cooling allows the dipole moments to align which enhances χ(T) to become positive. However, due to energy degeneracy of rotational motion and geometric frustration of the NH4+, faster cooling rates render the system to become an orientational glass state causing χ(T) to remain negative. Specific heat measurements also show the presence of excess entropy below a negative anomaly near 53 K. Our findings identify a possible new class of non-equilibrium system with potential applications in non-volatile memory devices.

Mechanical Motion in Crystals Triggered by Solid State Photochemical [2+2] Cycloaddition Reaction.

Some special crystals respond to light by jumping, scattering or bursting just like popping of popcorn kernels on a hot surface. This rare phenomenon is called the photosalient (PS) effect. Molecular level control over the arrangement of light-responsive molecules in microscopic crystals for macroscale deformation or mechanical motion offers the possibility of using light to control smart material structures across the length scales. Photochemical [2+2] cycloaddition has recently emerged as a promising route to obtain photoswitchable structures and a wide variety of frameworks, but such reaction in crystals leading to macroscopic mechanical motion is relatively less explored. Study of chemistry of such novel soft crystals for the generation of smart materials is an imperative task. This minireview highlights recent advances in solid-state [2+2] cycloaddition in crystals to induce macroscale mechanical motion and thereby transduction of light into kinetic energy.

Vortex core reversal by elastic waves in ferromagnetic materials

Magnetic vortex has attracted increasing attention due to its potential application in magnetic logic and memory devices. With the development of straintronics, acoustic waves could remotely control the topologically nontrivial magnetic bits such as magnetic vortices. Understanding the dynamic response of magnetic vortex subjected to elastic waves is crucial for the future logic and memory devices. In the present work, a dynamic phase field model is developed to investigate the dynamic magnetoelastic coupling behavior between elastic waves and magnetic vortices. Phase field simulations show that the magnetic vortex core can flip over under the elastic waves with gigahertz frequency. Compared with the results of conventional phase field model without the elastodynamic term, it is found that the strain gradient caused by elastic wave plays an important role in the reversal of vortex core. When the strain gradient in the ferromagnetic material reaches a certain value, the vortex core is likely to flip over. The elastic waves with larger magnitudes and frequencies have the larger strain gradient, which are more easily to cause the reversal of vortex core. In addition to the strain gradient, magnetic damping constant also has great influence on the vortex core reversal by changing the precession process of magnetization. Larger magnetic damping constant makes the smaller precession of magnetization, which retards the reversal of vortex core. The results of this study suggest a mechanical way for the remote control of magnetic vortices by elastic waves.

Manipulating density of magnetic skyrmions via multilayer repetition and thermal annealing

The magnetic skyrmion, a tiny magnetic texture that holds promise as the next-generation information carrier, has been widely studied in recent years. A fine tunability of skyrmion density is required for its real applications in novel memory and logic devices. Here, we report on the manipulation of skyrmion density at room temperature in a Pt/Co/Ta/MgO system composed of multiple repetitions with perpendicular magnetic anisotropy. N\'eel-type skyrmions are observed by performing Lorentz transmission electron microscopy and magnetic force microscopy. The influence of structure repetition on skyrmions is experimentally investigated and further understood by micromagnetic simulations. The variation of skyrmion density with repetition number is mainly attributed to magnetic anisotropy. Meanwhile, thermal annealing is utilized to regulate the skyrmion density, where skyrmion-related properties exert a competitive effect. Our findings provide alternative means to manipulate skyrmion density, further allowing for optimized engineering of skyrmion-based devices.

Perpendicularly magnetized ferromagnetism in Mn/Al bilayer thin films on Si substrates induced by temperature dependent ion beam mixing

The perpendicularly magnetized τ-phase was achieved in Mn/Al bilayer thin films by ion beam mixing using 400 keV Xe ions. The effect of annealing and irradiation on the structural, microstructural, and magnetic properties of Mn/Al bilayer thin films was investigated in the present study. Mn/Al bilayer thin films (thickness ∼97 nm) were deposited on silicon substrates by the evaporation technique. These as-deposited films were irradiated to the fluences of 5 × 1016 and 1 × 1017 ions cm−2 at three substrate temperatures (Room temperature ∼300 K, 450 K, and 623 K). As-deposited films were also annealed at 450 K and 623 K for 5 h. The ferromagnetic τ-phase was enhanced in annealed and irradiated thin films confirmed from synchrotron x-ray diffraction patterns, MFM images, and magnetic hysteresis curves. The average grain size estimated from the AFM study was found in the range of 78–192 nm. The domain patterns obtained in MFM images confirmed the ferromagnetic nature of the annealed and irradiated films. The hysteresis curves measured at 5 K show the maximum coercivity (7111 Oe) for the sample annealed at 450 K and the highest value of saturation magnetization (60 emu cc−1) for the film irradiated to the fluence of 1 × 1017 ions cm−2 at the substrate temperature of 450 K. Perpendicularly magnetized τ-phase was achieved in the films irradiated to the fluence of 1 × 1017 ions cm−2 at different temperatures. The formation of the ferromagnetic phase by ion beam mixing can be attributed to the combined effect of ballistic mixing and thermal spike mechanisms. This study suggests that the ferromagnetic properties of MnAl thin films can be modified by selecting the proper annealing and irradiation conditions to make them useful for applications in spintronics and magnetic memory devices.