REVIEW Copyright © 2008 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoelectronics and Optoelectronics Vol. 3, 1–14, 2008 MagneticQuantum-DotCellularAutomata:Recent DevelopmentsandProspects A. Orlov 1 ∗ , A. Imre 1 2 , G. Csaba 3 , L. Ji 1 , W. Porod 1 , and G. H. Bernstein 1 1 University of Notre Dame, Center for Nano Science and Technology, Notre Dame, IN 46556, USA 2 Currently at Argonne National Laboratory, Materials Science Division and Center for Nanoscale Materials, Argonne, IL 60439, USA 3 Technical University of Munich, Institute for Nanoelectronics, Munich, D-80333, Germany Quantum-dot Cellular Automata (QCA) is a computational paradigm that uses local physical cou- pling between nominally identical bistable building blocks (cells) assembled into arrays to per- form binary logic functions. QCA offers low power dissipation and high integration density of functional elements. Depending upon the choice of local fields causing interactions between the cells, different types of QCA are possible, such as magnetic, electronic, or optical. Here we dis- cuss recent developments in the field of magnetic QCA (MQCA) all-magnetic logic where planar, magnetically-coupled, nanometer-scale magnets are assembled into the networks that perform binary computation. The nanomagnets are defined by electron beam lithography. We demonstrate theoperationofbasicelementsofMQCAarchitecturesuchasbinarywire,threeinputmajoritylogic gate, and their combination, and discuss interfacing such systems with conventional CMOS-based logic. Keywords: CONTENTS 1. Introduction ........................................ 1 2. Fabrication, Measurements, and Simulations Techniques ......................................... 7 3. MQCA Devices: Binary Wires ......................... 7 4. MQCA Devices: Majority Logic Gate .................... 8 5. MLG-Binary Wire Combination Device .................. 11 6. Development of the Input and Readout for MQCA .......... 11 7. Alternative Techniques for MQCA Fabrication ............. 12 8. Summary .......................................... 13 Acknowledgments ................................... 13 References and Notes ................................ 13 1. INTRODUCTION The use of the phenomenon of magnetism for informa- tion processing goes back to the end of XIX century. In 1888, Oberlin Smith suggested the use of permanent magnetic impressions for the recording of sound. The recording of the human voice on a steel piano wire was first carried out in 1898 by a Danish inventor Valdemar Poulsen, whose invention gave rise some 30 years later to a magnetic tape recording industry. With the creation of the first computers, the use of magnetic storage elements ∗ Author to whom correspondence should be addressed. such as tapes, cores, and later magnetic disks, have become widespread. Early on in the computer era, several attempts were made to develop all-magnetic logic, most notably using such devices as “laddics” and “transfluxors.” These devices were magnetic ferrite elements of complex shape, inter- connected by windings of copper wire. For example, the laddic 1 was an element that had the appearance of a small ladder cut out of a ferrite with wire windings serving as inputs and outputs. By controlling the switching path through the structure, any Boolean function could be pro- duced. The switching speeds of a few tenths of a microsec- ond and repetition rates of a few hundred kHz were reported. 2 During the infancy of semiconductor process- ing, these numbers looked rather attractive. Moreover, even almost half a century later, all-magnetic logic devices are still unsurpassed in terms of their reliability, nonvolatile data retention and radiation hardness. In the early sixties, several functional all-magnetic com- puters, which were able to withstand the electromagnetic pulses from nuclear detonations and lightning surges, were built for niche applications such as aeronautics and rail- road depots. One such computer was built for the United States Air Force in 1962. 3 With a clock rate of 600 kHz, it was capable of performing more than 12,000 additions or subtractions of 24-bit words per second. The processor’s J. Nanoelectron. Optoelectron. 2008, Vol. 3, No. 1 1555-130X/2008/3/001/014 doi:10.1166/jno.2008.004 1