9. Sense Amplifier for Spin-Based Cryogenic Memory Cell

One of the primary drawbacks limiting the use of superconductive electronics is the lack of fast and dense memory capable of operating within a cryogenic environment. Recent research suggests the use of cryogenic spin-based memory—magnetic tunnel junctions and spin valves. In this chapter, a sense amplifier topology for a spin-based cryogenic memory cell is proposed and described. The nanocryotron (nTron) device is used as a driver for a spin-based memory element—the cryogenic orthogonal spin transfer device. A clocked DC-to-SFQ converter is used as a sense amplifier to resolve small differences in the readout current. The sense amplifier produces a variable number of SFQ pulses to represent different analog states by passing or blocking input clock pulses. This clock is derived from the system clock by synchronizing the read pulse to the same clock signal. These output pulses are counted and converted into a binary form. The sense amplifier exploits the specific shape of the nTron output waveform characterized by an L/R time constant to achieve the resolution of low magnetoresistance (MR) memory cells and is adaptable to different nTron sizes, bias currents, and spin-based devices. The dynamic power dissipation and resolution of the sense amplifier can be adjusted by the frequency of the applied clock signal, allowing the resolution to be reduced for high MR devices. The sense amplifier consists of two Josephson junctions, requiring little area, particularly in comparison to a standard nTron device, and can therefore be connected to each column of the memory array.