Computing with spins and magnets
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Introduction Recent experimental advances in spintronics and magnetics have merged the two fields and made possible the use of a single device, namely the magnetic tunnel junction (MTJ), to both read (R) information from magnets and to write (W) information onto magnets.1 Such devices are being widely researched and developed in industry and academia for spinbased memories. It seems natural to ask whether these advances in memory devices could also translate into a new class of logic devices based on nanoscale magnets or even single spins. What makes logic devices different from memory is the need for such devices to have gain and directionality. Memory bits act as isolated cells for information storage and read out. Logic devices must communicate with each other, requiring some bits as inputs and some as outputs. It is essential for inputs to write onto outputs and not the other way around, necessitating directionality. Moreover, since logic circuits require multiple cascaded logic bits for computing, information will be lost if not regenerated with gain. Such properties will subsequently enable building of complex circuits for Boolean logic, neural networks, etc. Complementary metal oxide semiconductor (CMOS) devices, which are the current workhorse of transistor-based logic technology, incorporate such characteristics. From this perspective, two categories for spin-based logic exist. In one category, logic is performed in conjunction with transistors and/or clocks2–8 to provide gain
and/or directionality. In the other case, such effects are built into spin devices.9–16 While covering both categories, our goal here is to present an overview of advancements in computing with spins and magnets from the macroscopic scale to the atomic scale.16–25
Magnetic tunnel junction Before we discuss spin-based logic, a brief point about MTJs is in order. MTJs currently act as the most important elemental building block for spin-based memories. A simplified schematic of an MTJ is shown in Figure 1a. It consists of two magnetic layers (one in dark blue and one in light blue) separated from each other by a thin tunnel barrier made of MgO. The two magnetic layers have different switching thresholds, where one of the layers is utilized as the fixed (reference) layer and the other one as the free (storage) layer. The role of the third magnetic layer (below Ru) is to cancel the stray field of the reference layer on the storage layer. Depending on the parallel (P) or anti-parallel (AP) orientation of the magnetization of the free and fixed layers, the resistance26,27 (Figure 1b) of the MTJ can change by more than 100%.27 In addition, the free layer in the MTJ can be electrically switched in a sub-200 ps time scale.28 The switching energy of the MTJ can be reduced, utilizing different mechanisms such as using a composite structure that preserves the thermal stability while reducing the switching energy.29 Since
Behtash Behin-Aein, GlobalFoundries, USA; [email protected] Jian-Ping Wang, University of Minnesota, USA; j
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