Joseph S. Friedman

All-Carbon Spin Logic

In all-carbon spin logic (ACSL), magnetoresistive graphene nanoribbons (GNRs) are connected by carbon nanotubes (CNTs) in logical circuits with extreme speed and energy-efficiency. The exceptional properties of low-dimensional carbon, in concert with electromagnetic wave-based signal propagation, provide the potential for terahertz operation and a 100x improvement in power-delay product.

All-Carbon Spin Logic Gate

The basis logic gate in ACSL is composed of a magnetoresistive GNR with a parallel CNT control wire adjacent to each GNR edge. Currents through the CNTs create magnetic fields along the GNR edges that control the global magnetic ordering, thereby modulating the GNR conductivity. A constant voltage is applied to the GNR, and the relative directions and magnitudes of the CNT input currents (IA and IB) can be used to switch the GNR output current (IO).

Binary logic states are defined in ACSL as large "1" and small "0" currents. Large CNT input currents are sufficient to switch the GNR edge magnetization between resistive antiferromagnetic (AFM) global ordering and conductive ferromagnetic (FM) global ordering. The two control wire inputs produce the logical OR function when the CNT currents are in opposite directions, and the XOR function when the CNT currents are in the same direction.
OR Gate

from Nature Communications 8, 15635 under CC BY 4.0
XOR Gate

Cascaded All-Carbon Spin Logic Circuits

ACSL circuits can be realized by using the GNR output currents IO as CNT control wire inputs to other ACSL logic gates. To enable cascading, GNRs are covalently bonded to CNT control wires that drive other GNR logic gates. Each GNR is connected by CNTs directly to V+ and V- nodes, with these CNTs routed adjacent to other GNRs to propagate information to cascaded GNR logic gates. Additionally, wired-OR gates can be used to perform the logical OR function without requiring a GNR device. As shown below, four GNR XOR gates and three wired-OR gates can be integrated to realize a one-bit full adder.

from Nature Communications 8, 15635 under CC BY 4.0

Performance & Energy Efficiency

The delay and power dissipation mechanisms in ACSL are quite different from conventional CMOS. As the state variable is current rather than voltage, there is no need for gates to accumulate charge with an RC delay. Instead, the combined ACSL interconnect and gate delays result from:
  • tmag, the time required for a changing CNT current to change the magnetic field at an adjacent GNR edge
  • tgnr, the time required for a changing magnetic field at a GNR edge to change the resistance of a GNR
  • tprop, the time required for a changing GNR resistance to change the CNT current adjacent to a cascaded GNR

from Nature Communications 8, 15635 under CC BY 4.0

The propagation time tprop is expected to dominate, with an electromagnetic wave speed of 800 km/s through the CNT. For a 400 nm CNT interconnect, the resulting total (interconnect + gate) delay is 500 fs, giving a clock frequency of 2 THz. Assuming a 1 V differential supply voltage, there is a potential for a 100x improvement in power-delay product as compared to 22 nm CMOS.

Related Publications

  1. J. S. Friedman, Cascaded All-Carbon Spin Logic based on Graphene Nanoribbon Magnetoresistance, Joint IEEE International Magnetics Conference & Conference on Magnetism and Magnetic Materials, Jan. 2019.
  2. J. S. Friedman, A. Girdhar, S. K. Heinrich-Barna, W. A. Chalifoux, J.-P. Leburton, A. V. Sahakian, 2D Carbon for Cascaded Spintronic Logic, International Conference on Superlattices, Nanostructures and Nanodevices, July 2018.
  3. S. K. Heinrich-Barna, J.-P. Leburton, J. S. Friedman, All-Carbon Spin Logic Sensor for RRAM Arrays, IEEE International Symposium on Circuits & Systems, May 2018 (invited).
  4. J. S. Friedman, A. Girdhar, R. M. Gelfand, G. Memik, H. Mohseni, A. Taflove, B. W. Wessels, J.-P. Leburton, A. V. Sahakian, Cascaded Spintronic Logic with Low-Dimensional Carbon, Nature Communications 8, 15635 (2017).
  5.   • Featured by the World Economic Forum (Article & Video), Design News, and Science Daily
  6. J. S. Friedman, B. W. Wessels, A. V. Sahakian, System and Method for Spin Logic, U.S. Patent #9,186,103 (2015).

  7. *from Nature Communications 8, 15635 under CC BY 4.0*