from Section IV - Spin-based devices
Published online by Cambridge University Press: 05 February 2015
Introduction
Nanomagnetic or spintronic circuits hold the promise of non-volatile and reconfigurable logic with low switching energy. One such circuit is the magnetic majority gate formed by concatenating several magnetic tunnel junctions together in such a manner that they interact with each other through a common ferromagnetic free layer to achieve the desired functionality. A key advantage of this configuration is that multiple majority gates can be concatenated together entirely in the magnetic domain without conversion to electric signals. The magnetic majority gates can in turn be concatenated together to form more complex circuits, such as a full magnetic adder circuit described here and simulated with a micromagnetic solver. The dynamics of magnetic polarization propagate through the adder circuit via the motion of magnetic domain walls and correspond exactly to the propagation of information through a ripple adder circuit. The switching speed and energy of the fundamental magnetic switching operation in the magnetic adder is comparable to the same fundamental switching operation in single magnetic gates or nanomagnetic memories. It provides a basis for estimating the operational speed and energy of the more complex magnetic circuits. A non-linear transfer characteristic ensures noise margin and signal restoration after every operation critical for Boolean logic.
The most common applications of spintronic devices in production today are non-volatile memories, namely magnetic random access memory (MRAM), which employ field induced switching of magnetic polarization. More recently, however, a much more efficient magnetic switching mechanism, based on current-induced switching, has been introduced and used to fabricate spin transfer torque RAM (STTRAM) memories [1]. It is natural to consider extending the physics of STTRAM to other magnetic logic functions [2], including the spin torque majority gate (STMG) described here. One obvious benefit of magnetic logic circuits is they are non-volatile, and hence do not suffer from standby power dissipation. A related benefit is that they can be turned on instantly since the circuit is non-volatile in the absence of input signals. In spite of these obvious advantages and the fact that numerous spintronic logic devices have been proposed, few of them have been fabricated and none have been demonstrated to function in an integrated circuit.
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