Digital data, generated by corporate and individual users, is growing day by day due to a vast range of digital applications. Magnetic hard disk drives (HDDs) currently fulfill the demand for storage ...space, required by this data growth. Although flash memory devices are replacing HDDs in applications like mobile phones, laptops, and desktops, HDDs cover the majority of digital data stored in the cloud and servers. Since the capacity growth of HDDs is slowing down, it is essential to look for a potential alternative. One such alternative is domain wall (DW) memory, where magnetic domains in the form of two-dimensional or three-dimensional wires are used to store the information. DW memory (DWM) devices should satisfy the four basic operations, such as writing (nucleating domains or inserting DWs in memory element), storing (stabilizing DWs), shifting (moving DWs), and reading (reading magnetization direction). An external magnetic field or spin-transfer torque can be used to write the information. Spin–orbit torque or electric field may be used for shifting the DWs. The information can be read using tunneling magnetoresistance. The domains may be stored along the tracks using artificial pinning potentials. The absence of moving parts makes the DWM consume less power as compared to HDDs, and be more robust. The potential to stack many layers to store information in three dimensions makes them potentially a large storage capacity device. In addition to memory, DW devices also offer a route for making synaptic devices for neuromorphic computing.
Despite these potential advantages of DWM, significant advances in research are needed before DWM could become commercially viable. One of the major challenges associated with DWM is DW dynamics. Many problems, such as controlled DW motion, the stability of domains, reducing the dimensions of the DW devices are still to be addressed. Artificial pinning sites fabricated through either geometrical or non-geometrical methods have been proposed for controlling DW motion. This review paper presents a survey of the investigations carried out so far and the future perspective of such devices.
The discovery and precise manipulation of atomic‐size conductive ferroelectric domain walls offers new opportunities for a wide range of prospective electronic devices, and the emerging field of ...walltronics. Herein, a highly stable and fatigue‐resistant nonvolatile memory device is demonstrated, which is based on deterministic creation and erasure of conductive domain walls that are geometrically confined in a topological domain structure. By introducing a pair of delicately designed coaxial electrodes onto the epitaxial BiFeO3 film, a center‐type quadrant topological domain with conductive charged domain walls can be easily created. More importantly, reversible switching of the quadrant domain between the convergent state with highly conductive confined walls and the divergent state with insulating confined walls can be realized, resulting in an apparent resistance change with a large on/off ratio of >104 and a technically preferred readout current (up to 40 nA). Owing to restrictions from the clamped quadrant ferroelastic domain, the device exhibits excellent restoration repeatability over 108 cycles and a long retention of over 12 days (>106 s). These results provide a new pathway toward high‐performance ferroelectric‐domain‐wall memory, which may spur extensive interest in exploring the immense potential in the emerging field of walltronics.
A prototype memory based on conductive ferroelectric domain‐walls confined in a topological quadrant domain is demonstrated, which exhibits a resistive‐switching characteristic with a large on/off ratio >104 and high readout current, as well as an excellent endurance over 108 cycles and a long retention over 12 days.
Racetrack memory (RTM) is a novel spintronic memory-storage technology that has the potential to overcome fundamental constraints of existing memory and storage devices. It is unique in that its core ...differentiating feature is the movement of data, which is composed of magnetic domain walls (DWs), by short current pulses. This enables more data to be stored per unit area compared to any other current technologies. On the one hand, RTM has the potential for mass data storage with unlimited endurance using considerably less energy than today's technologies. On the other hand, RTM promises an ultrafast nonvolatile memory competitive with static random access memory (SRAM) but with a much smaller footprint. During the last decade, the discovery of novel physical mechanisms to operate RTM has led to a major enhancement in the efficiency with which nanoscopic, chiral DWs can be manipulated. New materials and artificially atomically engineered thin-film structures have been found to increase the speed and lower the threshold current with which the data bits can be manipulated. With these recent developments, RTM has attracted the attention of the computer architecture community that has evaluated the use of RTM at various levels in the memory stack. Recent studies advocate RTM as a promising compromise between, on the one hand, power-hungry, volatile memories and, on the other hand, slow, nonvolatile storage. By optimizing the memory subsystem, significant performance improvements can be achieved, enabling a new era of cache, graphical processing units, and high capacity memory devices. In this article, we provide an overview of the major developments of RTM technology from both the physics and computer architecture perspectives over the past decade. We identify the remaining challenges and give an outlook on its future.
The dynamic contribution of domain walls (DWs) to application‐relevant dielectric and piezoelectric properties of ferroelectrics and relaxor‐based oxide ceramics has been in the focus since the ...earliest studies on these complex multifunctional materials. Despite the vital importance of DWs in the material design for targeted applications, the understanding of these interfaces has been significantly advanced only recently owing to the concurrent development of analytical methodologies probing the structure of samples in situ and at different length scales. In this contribution, we present two recent cases that have raised particular attention and controversy. The first is on the electrically conductive DWs in BiFeO3 (BFO), whereas the second is on low‐angle DWs characteristic for relaxor ferroelectric Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN–PT). Using nonlinear piezoelectric measurements with the support from multiscale structural analysis, we illustrate with the two case studies how, on one hand, the local electrical conductivity in BFO and, on the other hand, the structural disorder inherent to PMN–PT affect DW dynamics, leading to emerging macroscopic effects. By clarifying some of the aspects of these particular interfaces, we hope that the presented results will provide guidance to analytical approaches for identifying the key microscopic mechanisms contributing to macroscopic functional properties of ferroelectric and related materials.
The promise of topologically vortex‐like magnetic spin textures hinges on the intriguing physical properties and theories in fundamental research and their distinguished roles as high‐efficiency ...information units in future spintronics. The exploration of such magnetic states with unique spin configurations has never ceased. In this study, the emergence of unconventional (anti)meron chains from a domain wall pair is directly observed at zero field in 2D ferromagnetic Fe5−xGeTe2, closely correlated with significant enhancement of the in‐plane magnetization and weak van der Waals interactions. The simultaneous appearance of a large topological Hall effect is observed at the same temperature range as that of the abnormal magnetic transition. Moreover, the distinctive features of the (anti)meron chains and their collective dynamic behavior under external fields may provide concrete experimental evidence for the recent theoretical prediction of the magnetic‐domain‐wall topology and endorse a broader range of possibilities for electronics, spintronics, condensed matter physics, etc.
The generation of (anti)meron chains from conventional domain walls is directly observed at zero field in 2D ferromagnetic Fe5−xGeTe2, which closely correlates with the weak van der Waals interaction and temperature‐dependent spin anisotropy transformation. The simultaneous topological Hall effect and the collectively dynamic behavior of the (anti)meron chains under external fields experimentally highlight the theoretical prediction of magnetic‐domain‐wall topology.
Successful preparation of perpendicularly magnetized Co thin layers on Pt seed-layers using atomic layer deposition (ALD) technique with Co(PF3)4H as a precursor is reported. The residual phosphorus ...content in the samples, due to low deposition temperatures to realize ALD, was reduced effectively by tuning ALD parameters. Spin-orbit-torque driven current-induced domain wall motion was observed in the strip containing ALD Co layer prepared with the low phosphorus condition.
Observation of a new type of nanoscale ferroelectric domains, termed as “bubble domains”—laterally confined spheroids of sub‐10 nm size with local dipoles self‐aligned in a direction opposite to the ...macroscopic polarization of a surrounding ferroelectric matrix—is reported. The bubble domains appear in ultrathin epitaxial PbZr0.2Ti0.8O3/SrTiO3/PbZr0.2Ti0.8O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high‐resolution piezoresponse force microscopy (PFM), and is corroborated by aberration‐corrected atomic‐resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel–Bloch‐like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab‐initio‐based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.
Nanoscale spheroid domains—“bubble domains”—sub‐10 nm in lateral size with local dipoles self‐aligned in a direction opposite to the polarization of the surrounding ferroelectric matrix are reported in ultrathin epitaxial ferroelectric heterostructures. Incommensurate dipolar order and symmetry breaking is found within these domains, which leads to local polarization rotation and consequently mixed Néel–Bloch‐like character to the bubble domain walls.
Display omitted
Fluorite-structure ferroelectrics — in particular the orthorhombic phase of HfO2 — are of paramount interest to academia and industry because they show unprecedented scalability down ...to 1-nm-thick size and are compatible with Si electronics. However, their polarization switching is believed to be limited by the intrinsically high energy barrier of ferroelectric domain wall (DW) motions. Here, by unveiling a new topological class of DWs, we establish an atomic-scale mechanism of polarization switching in orthorhombic HfO2 that exhibits unexpectedly low energy barriers of DW motion (up to 35-fold lower than given by previous conjectures). These findings demonstrate that the nucleation-and-growth-based mechanism is feasible, challenging the commonly held view that the rapid growth of the oppositely polarized domain is impossible. Building on this insight, we describe a strategy to substantially reduce the coercive fields in HfO2-based ferroelectric devices. Our work is a crucial step towards understanding the polarization switching of HfO2, which could provide a means to solve the key problems associated with operation speed and endurance.
Domain wall nanomagnet (DWNM)-based devices have been extensively studied as a promising alternative to the conventional CMOS technology in both the memory and logic implementations due to their ...non-volatility, near-zero standby power, and high integration density characteristics. In this paper, we leverage a physics-based model of a DWNM device to design a highly scalable current-mode majority gate to achieve a novel one bit full-adder (FA) circuit. The modeled DWNM specifications are calibrated with the experimentally measured data. The functionality of the proposed DWNM-based FA (DWNM-FA) is verified using a SPICE circuit simulator. The detailed analysis and the calculations have been performed to realize the proposed DWNM-FA delay and power consumption corresponding to the various induced input currents at different operating temperatures. The power-delay product of DWNM-FA is examined to tune the operation within the optimum induced input current region to obtain desired power-delay requirements over a range of 200 μA to 1 mA at temperatures from 298 to 378 K. Finally, the comparison results exhibit 52% and 49% area improvement as well as 41% and 31% improvement in device count complexity over CMOS-based and magnetic tunnel junction-based FA designs, respectively.
PDF
