Proposal for an all-spin logic device with built-in memory Behtash Behin-Aein 1 * , Deepanjan Datta 1 , Sayeef Salahuddin 2 and Supriyo Datta 1 * The possible use of spin rather than charge as a state variable in devices for processing and storing information has been widely discussed 1,2 , because it could allow low-power operation and might also have applications in quantum computing. However, spin-based experiments and proposals for logic appli- cations typically use spin only as an internal variable, the term- inal quantities for each individual logic gate still being charge- based 3–8 . This requires repeated spin-to-charge conversion, using extra hardware that offsets any possible advantage. Here we propose a spintronic device that uses spin at every stage of its operation. Input and output information are rep- resented by the magnetization of nanomagnets that communi- cate through spin-coherent channels. Based on simulations with an experimentally benchmarked model, we argue that the device is both feasible and shows the five essential characteristics 9,10 for logic applications: concatenability, non- linearity, feedback elimination, gain and a complete set of Boolean operations. Our proposal has certain features in common with previous work. For example, as in a previous proposal for spin-based logic 4 , it uses non-local spin signals 11,12 . However, whereas ref. 4 requires sophisticated circuitry to amplify these signals and create the Amperian magnetic fields to switch nanomagnets, our proposal uses the non-local spin signal to directly switch a nanomagnet, which constitutes the input to the next stage. Other related examples include the magnetic quantum cellular automata (MQCA) architec- ture 13–15 and domain wall logic 16 , which use magnetic represen- tation of information and do not require spin-to-charge conversion. In all these cases, however, information is communi- cated through Amperian magnetic fields generated by current-car- rying wires. In contrast, our use of spin currents should allow communication signals to be selectively routed between specific input and output magnets (logic bits) that need not be nearest neighbours. In short, we are presenting a vision for an all-spin logic using nanomagnets as digital spin capacitors to store infor- mation and spin currents to communicate (Fig. 1a), with versatility comparable to that of standard charge-based architectures that use charge capacitors to store information and charge currents to com- municate (Fig. 1b). It also has the potential for low-power operation 17,18 that can enable continued downscaling. The basic concept underlying the all-spin logic device (ASLD) is illustrated in Fig. 1a. The bistable nanomagnets can be switched between their stable states representing binary data (right- or left- magnetized in Fig. 1a) if enough torque is exerted on them. Information stored in the magnetization direction of an input magnet is used to generate a spin current that can be routed along a spin-coherent channel to a desired location, where it deter- mines the final state of the output magnet based on the spin–torque phenomenon 19–22 . Overall, what is achieved is the switching of an output magnet in accordance with the information provided by the input magnet using energy provided by V supply . Both the infor- mation and the energy are conveyed through the channel spin current. Later, we will describe an alternative scheme (Fig. 2) in which the energy is delivered directly from V supply to the output magnet, and only the information is conveyed by the channel spin current. This could be an advantage, but at the expense of a more complicated clocking scheme. Interestingly, the COPY operation described above can be changed to a NOT operation simply by changing the polarity of the voltage applied to the input. A negative voltage injects majority spins (that is, spins parallel to the magnetization of the input magnet), the pres- ence of which turns the output magnet parallel to the input magnet, but a positive voltage extracts majority spins, the absence of which turns the output anti-parallel to the input. Input magnet Output magnet V supply All-spin switch GND a b Output capacitor Charge-based switch V supply (V dd ) Input capacitor GND 2 2 3 3 3 4 1 3 1 5 5 Magnetic free layer 1 Isolation layer 2 Tunnelling layer 3 Channel/interconnect 4 Contact 5 5 5 Figure 1 | An all-spin logic device (ASLD) with built-in memory. a, The input logic bit controls the state of the corresponding output logic bit with the energy coming from an independent source, V supply . In the ASLD, information is stored in the bistable states of magnets. Corresponding inputs and outputs communicate with each other via spin currents through a spin-coherent channel, and the state of the magnets is determined by the spin–torque phenomenon. The channel material can comprise metals or semiconductors, with the latter having higher spin coherence length. The tunnelling layers could be made of oxides or Schottky barriers. The isolation layers could be composed of electrostatic barriers or insulation layers. The magnets and contacts are metallic. b, In a charge-based switch, information (charge/no charge) on an input capacitor controls the state of the switch in the box, which then determines the state (charge/discharge) of the output capacitor. 1 School of Electrical and Computer Engineering and NSF Network for Computational Nanotechnology (NCN) Purdue University, West Lafayette, Indiana 47907, USA, 2 School of Electrical Engineering and Computer Science, UC Berkeley, Berkeley, California 94720, USA. *e-mail: behinb@purdue.edu; datta@purdue.edu LETTERS PUBLISHED ONLINE: 28 FEBRUARY 2010 | DOI: 10.1038/NNANO.2010.31 NATURE NANOTECHNOLOGY | VOL 5 | APRIL 2010 | www.nature.com/naturenanotechnology 266 © 2010 Macmillan Publishers Limited. All rights reserved.