Joseph S. Friedman

CMAT: Complementary Magnetic Tunnel Junction Logic

Complementary MAgnetic Tunnel junction logic (CMAT) is a spintronic logic family enabling the cascading of MTJ gates. With a complementary structure analogous to CMOS, CMAT provides non-volatile logic that enables non-von Neumann architectures.

CMAT Logic Family Structure

Both CMOS and CMAT are composed of a voltage divider with complementary pull-up and pull-down networks. However, whereas CMOS (above left) outputs a voltage, CMAT (above right) emits a charge pulse at every switching event. This charge pulse is used as an input to the next gate, such as the inverter below. Starting from initial state (a) with a low resistance between V+ and the output (and a high resistance between V- and the output), a charge pulse through the control wire switches the states of the two MTJs. The charge pulse creates oppositely directed magnetic fields through the MTJs on both sides of the control, causing both to switch magnetization states. The resistances of pull-up and pull-down networks therefore switch, resulting in a switched voltage at the output node O.

These MTJ logic gates can be cascaded by connecting control wires from output nodes to a separate constant voltage. When the voltage switches at output node O1 in the ring oscillator below, charge flows through control wire I2, creating a magnetic field that causes the following inverter to switch. This cascading action can continue indefinitely. If the supply voltages V+ and V- are removed, the MTJ states are retained, enabling a power-saving sleep mode and non-volatile computation.

CMAT Logic Circuits

This direct cascading scheme provides complementary inputs and outputs, as the current can be routed across an MTJ in either direction. The logic gates are therefore more compact than CMOS, as exemplified by the CMAT basis gate (below left) which performs the AND, OR, NAND, and NOR functions. The cascaded current directions determine the function achieved by this four-MTJ multipurpose gate. As the wire structure is static, it would be misleading to refer to this gate as reconfigurable; however, it can be configured to perform any two-input AND or OR function by varying the combination of polarities. A 2:1 multiplexer can also be realized efficiently, requiring only six MTJs (below right).

Related Publications

  1. J. S. Friedman, A.V. Sahakian, Method for Computing with Complementary Networks of Magnetic Tunnel Junctions, U.S. Patent #9,711,200 (2017).
  2. J. S. Friedman, CMAT Non-Volatile Spintronic Computing: Complementary MTJ Logic, Proc. SPIE Spintronics IX, Aug. 2016 (invited).
  3. J. S. Friedman, A.V. Sahakian, Magnetic Tunnel Junctions with Control Wire, U.S. Patent #9,299,917 (2016).
  4. J. S. Friedman, D. Querlioz, A. V. Sahakian, Magnetoresistance Implications for Complementary Magnetic Tunnel Junction Logic (CMAT), Proc. IEEE/ACM International Symposium on Nanoscale Architectures, July 2015.
  5. J. S. Friedman, A. V. Sahakian, Complementary Magnetic Tunnel Junction Logic, IEEE Transactions on Electron Devices 61:4, 1207-1210 (2014).