G. Csaba

Magnetic QCA systems

The field-coupled QCA architecture has emerged as a candidate for providing local interconnectivity for nanodevices, and offers the possibility to perform very dense, high speed, and low power computing in an altogether new paradigm. Magnetic interactions between nanomagnets are sufficiently strong to allow room-temperature operation. We are investigating the fabrication and testing of arrays of nanomagnets for this purpose, and have found that by tailoring their shapes, strong coupling can be observed. This paper will present recent work of the Notre Dame group on magnetically coupled QCA.

Applications of High-Capacity Crossbar Memories in Cryptography

This paper proposes a new approach for the construction of highly secure physical unclonable functions (PUFs). Instead of using systems with medium information content and high readout rates, we suggest to maximize the information content of the PUF while strongly reducing its readout frequency. We show that special, passive crossbar arrays with a very large random information content and inherently limited readout speed are suited to implement our approach. They can conceal sensitive information over long time periods and can be made secure against invasive physical attacks. To support our feasibility study, circuit-level simulations and experimental data are presented. Our design allows the first PUFs that are secure against computationally unrestricted adversaries, and which remain so in the face of weeks or even years of uninterrupted adversarial access. We term the new design principle a “SHIC PUF,” where the acronym SHIC stands for super high information content.

Read-Out Design Rules for Molecular Crossbar Architectures

This paper investigates the behavior of large-scale crossbar memory arrays, built from molecular switches. We construct SPICE models based on experimental I(V) curves and investigate how critical circuit parameters (read-out margin, power dissipation, and speed) scale with circuit size. We concentrate on the read-out process. We explore the effect of nonlinear/rectifying elements placed at the junctions and conclude that scalable crossbar memories could be built using molecules with nonlinear, nonrectifying behavior in the molecular I(V) curve. The ultimate achievable storage capacity of these arrays is estimated and prescriptions for optimized molecular switches are provided.

Nanomagnetic Logic: Error-Free, Directed Signal Transmission by an Inverter Chain

In this paper we present a new clocking method for non-reciprocal information transmission by an inverter chain of focused ion-beam engineered Co/Ni nanomagnets. The functionality of the clocking method is proven experimentally. Local focused ion-beam irradiation is used in order to define the direction of the information flow. The nanodots of the chain are clocked synchronously by a global external field. Information is transmitted synchronously between neighboring dots due to field-coupling. Magnetic force microscopy is used to reveal the magnetization state of the nanomagnets after every field pulse to prove the correct transmission of the signal. The switching characteristics of the fabricated nanodots and coupling forces between neighboring dots are analyzed optically using the magneto-optical Kerr effect. A detailed explanation is given for the occurrence of possible errors in a field coupled inverter chain.

Power dissipation in nanomagnetic logic devices

We investigate power dissipation phenomena in individual and coupled nanoscale magnets using micromagnetic simulations. We demonstrate that nanomagnet dots pumped with slowly varying external fields can dissipate as small as a few-ten millielectronvolts of energy per switching. This makes coupled nanomagnets promising candidates for realizing low-power computing devices.

Field-coupled nanomagnets for interconnect-free nonvolatile computing

Exploiting the magnetic interaction of field-coupled single-domain magnets is promising for rad-hard, nonvolatile, dense and highly parallel digital information processing. The shortcomings of metal wiring are overcome as no current flow is needed for the logic operation. In principle only few kBT energy per switching event are dissipated [1]. Magnetic computing structures have been demonstrated before [2–4]. Here electrical in- and output, magnetic signal branching and amplifier-like structures are fabricated, which will allow for complete computing systems operating at room temperature.

Low Temperature Rectifying Junctions for Crossbar Non-Volatile Memory Devices

ZnO-based Schottky junctions fabricated at low temperature are proposed as selectors for crossbar non-volatile memory devices. Rectifying ratio over 10n 7n and forward current density as high as 10n 4n A/cmn 2n are reported. Results of the integration with NiO based switching memory elements are also shown.

Security applications of diodes with unique current-voltage characteristics

Diodes are among the most simple and inexpensive electric components. In this paper, we investigate how random diodes with irregular I(U) curves can be employed for crypto and security purposes. We show that such diodes can be used to build Strong Physical Unclonable Functions (PUFs), Certificates of Authenticity (COAs), and Physically Obfuscated Keys (POKs), making them a broadly usable security tool. We detail how such diodes can be produced by an efficient and inexpensive method known as ALILE process. Furthermore, we present measurement data from real systems and discuss prototypical implementations. This includes the generation of helper data as well as efficient signature generation by elliptic curves and 2D barcode generation for the application of the diodes as COAs.

Magnetic Logic Devices Based on Field-Coupled Nanomagnets

Nanomagnets that exhibit two distinct stable states of magnetization can be used to store digital bits. This phenomenon is already applied in today’s magnetic random access memories (MRAM). In addition, interacting networks of such nanomagnets, with physical spacing on the order of 10 nm between them, have been proposed to propagate and process binary information by means of magnetic coupling. The application of field-coupled nanomagnets for digital logic circuits was described in a concept called magnetic quantumdot cellular automata (MQCA) [1][2]. MQCA offer very low power dissipation and high integration density of functional devices. In addition, it can operate over a wide temperature range from near absolute zero to the Curie temperature of the employed ferromagnetic material. We introduce a logic gate similar to that proposed by Parish and Forshaw [3], which performs majority-logic operation. The nanomagnets are arranged in a cross-geometry as shown in Fig. 1, where the dipole coupling between the nanomagnets produces ferromagnetic and antiferromagnetic ordering of the magnetic states. Consider that the magnetic state of nanomagnets A, B, and C can be set by some inputs, and that a horizontal external magnetic field, called the clock-field, can allow the system to relax to its ground state. Then, majority logic operation can be performed by the central nanomagnet M, and the result can be transferred to another nanomagnet labeled as “out”. The cross-geometry can be extended, and inputs can be provided by adding nanomagnets that are oriented along the clock-field. Varying the position of these horizontally elongated nanomagnets, all eight input combinations in the majority-logic truth table can be tested. Figure 1. (a) An elongated polycrystalline NiFe alloy nanomagnet exhibits two stable magnetic states in the direction of its longest axis. (b) Majority gate geometry built up from nanomagnets. We demonstrate room temperature operation of majority gates made of NiFe alloy and fabricated by electron-beam lithography on silicon. Dipolar ordering in the nanomagnetnetworks is imaged by magnetic force microscopy (MFM), and the operation is explained by means of micromagnetic simulations. Figure 2 introduces a particular majority gate, in which the magnetic state of A is set to be the opposite to that of B and C. In this gate, A, B, and C all exert torque on the central dot’s magnetic moment in the same direction. Simulations show that, as the clock-field is reduced, the switching of the nanomagnets inside the gate begins at the input dots. The central dot switches after A, B, and C, and the switching propagates along the antiferromagnetically-coupled chain to the right. The figure demonstrates the final states after two independent experiments, in which the clock-field was applied in opposite directions. The MFM data shows the correct alignment of the magnetic moments in both cases.

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.