Defined in [the paper (Manipatruni-Nikonov)](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/3731/Beyond%2BCMOS%2Bcomputing%2Bwith%2Bspin%2Band%2Bpolarization.pdf), the figure of merit $ \lambda $ for any tunneling junction/CMOS transistor/switch/MtJ

$$ \lambda = \frac{E_{dissipation}}{E_{barrier}} $$

where $ E_{dissipation} \approx E_{switching} $ is the dissipation energy, which is also approximately equal switching energy.

- notice that $ E_{barrier} $ is always around $1-1.5 eV$, so $\lambda$ is really determined by **collective versus single-electron physics mechanism and the size of the cell** (anything else basic?)


In [another paper (Wang - Koester)](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5071/a16-Wang-2j6mokh.pdf) , the authors argue that $ \lambda \approx 1 $ is achievable. In particular, GSHE looks better than stt-MRAM. AFM and TI (at room temperature) can make significant breakthroughs, especially when made commercial (no sooner than 5 years?).

- for the electronic-charge switch, $\lambda $ is of order $ N_e \sim 400 $.

- for stt-MRAM, $ \lambda_{stt-mram} > 1000 $ is indicated, while in 2014 (from Wang-Koester) we can see $ \lambda_{stt-mram} \approx 4 * 10^5 $. This means that **basic STT-MRAM is not particularly energy-efficient.** From the table on [IMComputing page.](https://knowen.org/nodes/27132), the review in 2019 we can see $ \lambda_{stt-mram} \approx  10^4 $.


> the summary - **the smaller the dimensionless parameter $ \lambda $ is the better.** This parameter must be larger than 1 for any classical switching. Different switching devices need to optimize this parameter, **both CMOS and stt-mram are not at the possible, but very difficul physics-allowed, limit of this parameter.**

# key figures from [the paper (Wang - Koester)](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5071/a16-Wang-2j6mokh.pdf)

- excellent endurance of stt-MRAM (as well as commercial development)

![end](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5147/endurance.PNG 'end')

stt-mram has the best endurance among emerging NVMs, still behind by 1-2 orders of magnitude from SRAM and DRAM, see IMComputing

- Switching energy per write operation for stt-MRAM is a severe limitation due to $ \lambda_{stt-mram} > 1000 $

![sw](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5148/switching%2Benergy.PNG 'sw')

Write energy of stt-mram is about 2 orders of magnitude worse than for CMOS (1600 kT line). **stt-mram does not have here advantage against charge-based DRAM and SRAM in spite of being “collective spintronic switch”.** SRAM is about 2 orders better in write energy than stt-mram,  as you can see from the picture above. 

Topological Insulator and Sraintronics-based predicted to be very energy-efficient, but very far from being commercially ready (while in 2021y)

- the challenge of switching speed

distinguish between theoretical and realised versions

![speed](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5146/speed%2Bchallenges.PNG 'speed')

stt-mram is the best among experimentally shown to work.
stt- mram is slightly behind in write/read speed than SRAM (the fastest), but close (within 1 order of magnitude).

*AFM might become really good both in the switching energy category and the switching speed!*

- **speed and energy efficiency are two main goals for computing (AI hardware), so far as cost, non-volatility, and endurance are satisfactory (from the point of view of business applications)**

## the picture below shows "the ideal collective spintronic switch"



> The fundamental advantage of this approach over the semiconductor-based switch concept is its projected low operation energy.
> Fig. 1 **compares a generic spintronic switch and a generic electronic switch.**


![gg](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5128/pathway%2Bbasics.PNG 'gg')


The picture  might be misunderstood. **stt-mram is NOT the “ideal collective spintronic switch”.** 


## Which NVM-RAM is going to prove itself, even commercially, to be the “the ideal collective spintronic switch” in the (far?) future?