ABSTRACT
There is increasing interest in implementing digital logic with single-domain nanomagnets instead of traditional transistors as the former have the potential to be extremely energy-efficient binary switches. Transistors switch by moving electrical charge into or out of their active regions. If this process is carried out 66nonadiabatically, then it dissipates an amount of energy equal to at least NkTln(1/p), where N is the number of electrons (information carriers) moved into or out of the device, T the temperature, and p the bit error probability associated with random switching of the device (Zhirnov et al. 2003; Salahuddin and Datta 2007). On the other hand, if logic bits are encoded in two stable magnetization orientations along the easy axis of a shape-anisotropic single-domain nanomagnet (or the single-domain magnetostrictive layer of a multiferroic nanomagnet), then switching between these orientations can take place by dissipating only approximately kTln(1/p) of energy, regardless of the number of spins (information carriers) in the nanomagnet (Salahuddin and Datta 2007). This results from the fact that exchange interaction between spins makes all the approximately 104 spins in a single-domain nanomagnet of volume approximately 105 nm3 behave collectively like a giant single spin (Salahuddin and Datta 2007; Cowburn, et al. 1999a) (a single-information carrier) (Salahuddin and Datta 2007). Ideally, all of these spins will rotate in unison when the nanomagnet switches from one stable magnetization state to the other. This is schematically explained in Figure 4.1. As a result, for the same bit error probability p, the ratio of the minimum energy that must be dissipated to switch a nanomagnet to that dissipated to switch a nanotransistor will be approximately 1/N << 1. The mutual interaction between spins leading to collective dynamics, which is absent in the case of charges, makes the nanomagnet switch intrinsically more energy-efficient than the transistor switch.*
