A Universal Nonvolatile Processing Environment

After many decades of stunning progress in the shrinking of complementary metal-oxide-semiconductor (CMOS) devices, the steadily increasing difficulty in handling physical limitations as well as the rapidly increasing production and investment costs for each new technology generation will stop CMOS scaling in the not-too-distant future. Among the most challenging problems for further performance gains today are the static power dissipation as well as the interconnection delay and the associated energy for information transport.1, 2 A very efficient solution to the static leakage power problem is to simply turn off unused parts of a circuit. However, this causes the previously stored information to vanish and requires energyand time-wasting recovery cycles, when the dormant circuit parts are powered up. Thus, in order to avoid information loss during shutdown, nonvolatile elements must be incorporated. Due to its CMOS compatibility, nonvolatility, high endurance, and fast operation, spintronics is a promising avenue for adding nonvolatility to circuits.3 The term spintronics is very general and covers a vast number of devices with an extreme variety in operating principles and practical feasibility for commercial applications.3, 4 In this chapter, we concentrate on what, in our opinion, appears to be the most feasible technology for large-scale integration in the next few years: the combination of CMOS with nonvolatile magnetoresistive random-access memory (MRAM). Indeed, the integration of CMOS and magnetic tunnel junctions (MTJs) is not only likely but already available in the form of nonvolatile stand-alone MRAM arrays and embedded DRAM,5 and the introduction of further commercial products will surely follow.6–9 Importantly, the all-electrical magnetization manipulation in modern MRAM by spin transfer torque (STT) renders the wires for separate magnetic field generation superfluous and also significantly reduces the MTJ switching energy. Technological advances, such as the exploitation of free-layer perpendicular magnetic anisotropy and use of MgO tunnel barriers, have led to a further reduction in switching energy, as well as improved scalability.10 Promising