Clocking Field Research Articles

Designing boron-cluster-centered zwitterionic Y-shaped clocked QCA molecules

Quantum-dot cellular automata (QCA) is a nanoscale, transistor-less device technology. A single molecule may provide an elementary QCA device known as a cell. Molecular redox centers function as quantum dots, and the configuration of mobile charge on the dots encodes device states useful for classical computing. Molecular QCA may support ultra-high device densities and THz-scale switching speeds at room temperature. An applied electric field may be used to clock molecular QCA, providing power gain to boost weakened signals, as well as quasi-adiabatic device operation for minimal power dissipation in QCA devices and circuits. A zwitterionic, Y-shaped, three-dot molecule may function as a field-clocked QCA cell. We focus on the design of a counterion built into the center of the cell. Ab initio computations demonstrate that choice of counterion determines the number of mobile charges for encoding the device state on the three quantum dots. We use or as the central counterionic linker for two different Y-shaped, three-dot QCA molecules. While both molecules support the desired device states, the number of trapped charges in the counterion determines the number of mobile holes on the molecular quantum dots. This, in turn, determines whether the device state is encoded by a hole or an electron. This choice of encoding determines how the molecular QCA cell responds to a clocking field. The two counterions studied here lead to two QCA molecules with opposite responses to the clock, similar to the complementary responses of PMOS and NMOS transistors to gated voltage control.

Clock Topologies for Molecular Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) is a low-power, non-von-Neumann, general-purpose paradigm for classical computing using transistor-free logic. Here, classical bits are encoded on the charge configuration of individual computing primitives known as “cells.” A cell is a system of quantum dots with a few mobile charges. Device switching occurs through quantum mechanical inter-dot charge tunneling, and devices are interconnected via the electrostatic field. QCA devices are implemented using arrays of QCA cells. A molecular implementation of QCA may support THz-scale clocking or better at room temperature. Molecular QCA may be clocked using an applied electric field, known as a clocking field. A time-varying clocking field may be established using an array of conductors. The clocking field determines the flow of data and calculations. Various arrangements of clocking conductors are laid out, and the resulting electric field is simulated. It is shown that that control of molecular QCA can enable feedback loops, memories, planar circuit crossings, and versatile circuit grids that support feedback and memory, as well as data flow in any of the ordinal grid directions. Logic, interconnect and memory now become indistinguishable, and the von Neumann bottleneck is avoided.

Robustness of majority gates based on nanomagnet logic

We studied the evolution of the magnetic state of a majority logic gate consisting of a cluster of five dipolarly-coupled nanomagnets, fabricated by e-beam lithography, under the application of a clocking field, using a combination of magneto-optical Kerr effect and magnetic force microscopy. The data were interpreted by advanced GPU-based micromagnetic simulations, where, in addition to the single ideal-shaped gate, a 3 × 3 array of “realistic gates”, whose shape is directly derived from scanning electron microscopy images, is considered. A fairly good agreement between measurements and simulations has been achieved, showing that asynchronous switching of nominally identical gates may occur, because of unavoidable structural and morphological imperfections. Moreover, a slight misalignment of 1°−2° of the clocking field with respect to the hard axis of the dots may be detrimental for the correct logic operation of the gates. It follows that reliable, error-free and reproducible operations in future magnetologic devices would require tight control and precision of both the lithographic process and the direction of the clocking field. Moreover, a significant improvement could be insured by a stronger dipolar coupling between the dots, for instance increasing their thickness and/or using materials with larger magnetization.

Balanced Magnetic Logic Gates in a Kagome Spin Ice

Nanomagnetic logic (NML) is a promising candidate to replace or complement traditional charged-based logic devices. Single NML gates such as the three-input majority gate are well studied, and their functionality has been verified experimentally. However, such gates suffer from a problem in that they sometimes produce erroneous output when integrated into circuits. A fundamental solution is offered by using balanced logic gates: gates for which the ground states corresponding to all possible input states have the same energy. We investigate how balanced gates can be created from kagome spin ice elements. We present a balanced NAND (and NOR) gate consisting of 19 dipole-coupled uniaxially anisotropic magnets. This gate can be either driven by an external clocking field or thermally driven. In the latter case, we numerically show that the gate has a reliability of at least 96%, a number which is shown to be robust against disorder. The presented gate provides a proof of concept for an artificial kagome spin ice NML gate.

Contiguous clock lines for pipelined nanomagnet logic

In this paper we study pipelined nanomagnet logic by simulating and comparing varying adjacent clock line structures. Unlike previous simulations, a realistic clock line shape is used in simulations to obtain a more accurate idea of whether or not these clock lines function properly. First, we simulate individual clock lines using Ansys Maxwell 2D according to the parameters of the fabricated clock lines. Then, these clock lines are placed adjacent to one another to simulate how data can propagate from one clock zone to another. Adjusting the clock line layer structure minimizes a dip in the magnetic field at the clock zone boundaries from 35% minimum below the clocking field to a 16% dip. These magnetic field profiles are then used in the object-oriented micromagnetic framework (OOMMF) to simulate lines of nanomagnets. By reducing the gap between contiguous clock lines, we show error free data propagation in the form of a ferromagnetically coupled line of nanomagnets.

Innovative orderly programmable in‐plane majority gates using trapezoid shape nanomagnet logic devices

Programmable circuit components are indispensable for nanomagnet logic (NML, also called magnetic quantum-dot cellular automata) devices. Two alternative programmable majority gates (PMGs) (PMG1 and PMG2) for in-plane NML are proposed. The presented designs both utilise trapezoid shape nanomagnets (nanomagnet with a slanted edge) to form the structures. With only a uniform layout and a homogeneous clocking field, the proposed structures can orderly reconfigure eight input combinations of the majority gates, and they also demonstrate superior performance over the regular shape nanomagnet-based PMG. The innovative design could find technological application in the field of majority gate-based programmable magnetic circuits (e.g. programmable nanomagnetic Full Adder).

Misalignment-free signal propagation in nanomagnet arrays and logic gates with 45°-clocking field

A key obstacle for the application of Magnetic Quantum-dot Cellular Automata (MQCA) is the misalignment of clocking field, which results in low stability for both signal propagations within nanomagnet array and logic operation in majority gates. Here, we demonstrate that a reversal clocking field applied at 45° off the hard axis, with progressively reduced amplitude, applied to a shape-tuned nanomagnet array fabricated by e-beam lithography, helps intrinsically eliminate the misalignment sensitivity of the elements and results in correct signal propagation. Further, least reversal steps and reduced field amplitude was required owing to the 45°-clocking field. This clocking field was also tested for majority gates (OR function) and characterized by Magnetic Force Microscopy demonstrating correct output. This novel design provides high stability for signal propagation and logic operation of MQCA and potentially paves way for its application.

1-Bit Full Adder in Perpendicular Nanomagnetic Logic using a Novel 5-Input Majority Gate

In this paper, we show that perpendicular Nanomagnetic Logic (pNML) is particularly suitable to realize threshold logic gate (TLG)-based circuits. Exemplarily, a 1-bit full adder circuit using a novel 5-input majority gate based on TLGs is experimentally demonstrated. The theory of pNML and its extension by TLGs is introduced, illustrating the great benefit of pNML. Majority gates based on coupling field superposition enable weighting each input by its geometry and distance to the output. Only 5 magnets, combined in two logic gates with a footprint of 1.95 μm 2 and powered by a perpendicular clocking field, are required for operation. MFM and magneto-optical measurements demonstrate the functionality of the fabricated structure. Experimental results substantiate the feasibility and the benefits of the combination of threshold logic with pNML.

Information transport in field-coupled nanomagnetic logic devices

The information transport in field-coupled nanomagnetic logic (NML) systems is demonstrated by investigating signal propagation in a circular chain of magnets. Design criteria for the magnet layout, signal injection timing, and the required clocking field are presented. The strong interaction between the magnets is estimated by simulations and verified by hysteresis curve measurements. Signal transmission in the magnetic wire is confirmed by magnetic force microscopy measurements, especially the propagation of a metastable pair of magnets with parallel magnetization. For the first time, a field-coupled magnetic logic device is successfully operated for hundreds of clocking cycles. Extensive studies verify the reliability and robustness of information transport in field-coupled NML systems from perpendicular magnetic media.

Highly stable signal propagation in a consecutively tuned nanomagnet array

A key function of magnetic quantum-dot cellular automata (MQCA) is signal propagation in the nanomagnet array, for which a clocking field is required. However, the misalignment of the clocking field and the resultant low stability for signal propagation is one of the main challenges for its application. Here, we modeled and fabricated a progressively shape-tuned nanomagnet array combined with a reversal clocking field with progressively reduced amplitude. Based on micromagnetic simulations, Fe nanomagnet arrays were fabricated by electron beam lithography and their magnetization states characterized by magnetic force microscopy demonstrated correct signal propagation against clocking field misalignment up to ±5°. Furthermore, cascade-like signal propagation was observed. This novel design provides high stability and directional control in signal propagation within the nanomagnet array and potentially paves the way for addressing the misalignment issue in MQCA structures.

Magnetic Quantum Cellular Automata-Based Logic Computation Structure: A Full-Adder Study

Magnetic quantum cellular automata (MQCA) device has drawn much attention recently due to its substantial advantages of nonvolatile data retention, potentially low power and room temperature operation. This paper theoretically demonstrates pipelined and robust logic computation architecture for MQCA (MQCA full adder). The architecture is based on different three-direction-input magnetic majority gate and utilizes a data blocking region. Data flow and computation synchronization are realized by proposed six-zone time division quasiadiabatic clocking. Micromagnetic simulation demonstrates the function validity of proposed MQCA full adder, and comparation with previous conceptual designs as to their area and complexity shows that new architecture presents superior performance. Furthermore, we investigate non-ideal characteristics of MQCA full adder. How clocking field magnitude affects signal propagation of the MQCA full adder is examined. The results show that there exists an optimal 'null' clocking field magnitude to drive the circuit.

Theoretical study and experimentation of magnetic quantum-dot cellular automata corner structure

Magnetic quantum-dot cellular automata (MQCA) corner structure consists of both ferromagnetic coupling and anti-ferromagnetic coupling. Signal propagation of corner structure for magnetic material Fe is theoretically investigated by applying an external clocking field method. Magnetization evolution image on signal propagation of the corner is presented by micromagnetic simulation; it is shown that Fe nanomagnets can ensure stable switching of corner including two kinds of lengthy coupling. The proposed corner structure is then fabricated by electron beam lithography, thermal evaporation and lift-off technologies. Scanning electron microscopy and magnetic force microscopy measurement results demonstrate perfect pattern and correct signal propagation of two types of coupling.

Performance of Magnetic Quantum Cellular Automata and Limitations Due to Thermal Noise

Operation parameters of magnetic quantum cellular automata are evaluated for the purposes of reliable logic operation. The dynamics of the nanomagnets is simulated via the Landau–Lifshitz–Gilbert equations with a stochastic magnetic field corresponding to thermal fluctuations. It is found that in the macrospin approximation, the switching speed does not change under scaling of both size and distances between nanomagnets. Thermal fluctuations put a limitation on the size of nanomagnets: when we consider a majority gate that features a biaxial anisotropy as a stabilizing mechanism and a uniform clocking field, the gate error rate becomes excessive for nanomagnets smaller than about <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{200 nm}$</tex></formula> at room temperature.

Landauer Clocking for Magnetic Cellular Automata (MCA) Arrays

Magnetic cellular automata (MCA) is a variant of quantum-dot-cellular automata (QCA) where neighboring single-domain nanomagnets (also termed as magnetic cell) process and propagate information (logic 1 or logic 0) through mutual interaction. The attractive nature of this framework is that not only room temperature operations are feasible but also interaction between neighbors is central to information processing as opposed to creating interference. In this work, we explore spatially moving Landauer clocking scheme for MCA arrays (length of 8, 16, and 32 cells) and show the role and effectiveness of the clock in propagating logic signal from input to output without magnetic frustration. Simulation performed in object oriented micromagnetic framework suggests that the clocking field is sensitive to scaling, shape, and aspect ratio.