A layperson's introduction to spintronics
Introduction and motivation
In an effort to get more content on Tildes, I want to try and give an introduction on several 'hot topics' in semiconductor physics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.
Today's topic
I will start this series with an introduction to spintronics and spin transistors.
What is spintronics?
Spintronics is named in analogy to electronics. In electronics, the flow of current (consisting of electrons) is studied. Each electron has an electric charge, and by pulling at this charge we can move electrons through wires, transistors, creating modern electronics. Spintronics also studies the flow of electrons, but it uses another property of the electrons, spin, to create new kinds of transistors.
What are transistors?
Transistors are small electronic devices that act as an on-off switch for current. We can flip this on-off switch by sending a signal to the transistor, so that the current will flow. Transistors are the basis for all computers and as such are used very widely in modern life.
What is spin?
Spin arises from quantum mechanics. However, for the purpose of explaining spin transistors we can think of an electron's spin as a bar magnet. Each electron can be thought of as a bar magnet that will align itself to a nearby magnetic field. Think of it as a compass (the electron) aligning itself to a fridge magnet when it's held near the compass.
What are spin transistors and how do they work?
Spin transistors are a type of transistor whose on-off switch is created by magnets. We take two bar magnets, whose north poles are pointed in the same way, and put them next to each other, leaving a small gap between them. This gap is filled with a material through which the electrons can move. Now we connect wires to the big bar magnets and let current (electrons!) flow through both magnets, via the gap. When the electrons go through the first magnet, their internal magnets will align themselves to the big bar magnet. However, once they are in the gap the electrons' internal magnets will start rotating and no longer point in the same direction as the big bar magnets. So that when the electrons arrive at the second magnet, they will be repelled just like when you try to push the north poles of two magnets together. This means the current will not flow, and the device is off! So, how do we get it to turn on?
By exposing the gap to an electric field, we can control the amount of rotation the electrons experience (this is called the Rashba effect). If we change the strength of this electric field so that the electrons will make exactly one full rotation while crossing the gap, then by the time they reach the second big bar magnet they will once again be pointing in the right direction. Now the electrons are able to move through the second big bar magnet, and out its other end. So by turning this electric field on, the spin transistor will let current flow, and if we turn the electric field off, no current will flow. We have created an on-off switch using magnets and spin!
That's cool, but why go through the effort of doing this when we have perfectly fine electronics already?
The process of switching between the on and off states of these spin transistors is a lot more energy efficient than with regular transistors. These types of transistors leak a lot less too. Normal transistors will leak, meaning that a small amount of current will go through even when the transistor is off. With spin transistors, this leak is a lot smaller. This once again improves the energy efficiency of these devices. So in short, spin transistors will make your computer more energy efficient. This type of transistor can also be made smaller than normal transistors, which leads to more powerful computers.
Feedback and interest
As I mentioned, I wrote this post as a challenge to myself to explain modern physics to laypeople. Please let me know where I succeeded and where I failed. Also let me know if you like this type of content and if I should continue posting other similar topics in the same format.
Thank you for sharing and I think yours is definitely a success!
My reading may not strictly be that of a "layperson's" because my science background, but my specialization is elsewhere far far away. It's cool to read accessible introduction.
Now my questions: why are they more energy-efficient and less leaky compared to transistors?
I'll write this answer at a bit of a higher level than the post. A regular transistor puts a potential barrier between the input (source) and output (drain). By applying an electric field this potential barrier vanishes. It costs energy to raise and lower this barrier. Furthermore, quantum tunnelling will cause the electrons to tunnel through barriers (the chance that they will tunnel drops off for bigger barriers). So you need to make a huge barrier in the off-state to keep the transistor from leaking, but creating a huge barrier costs a lot of energy.
Spin transistors have a permanent barrier, the difference is that we orient electron spin so that they either see or don't see the barrier. So we don't need to invest energy into raising and lowering barriers. This also means they won't leak as much because we can just put a nice big barrier between the source and drain.
Well, I guess an analogy for the energy comparison could be "Operating a water lock uses much more energy than adjusting the little boats", right? :)
Again, thank you for your very nice write-up and I'll be very glad to read more :)
I like your idea, I'll add the tag. I hope others will use it too.
An enthusiastic yes from me. I consider myself a lifetime learner and welcome any opportunity to learn something new. In that spirit, a big stretch for me that was far outside of my discipline working was amateur astronomy and opened up a huge opportunity to learn about optics, physics, geometry, etc. but it was all in small bites as I could find time.
We live in a frantic world with many demands on our time so this condensed form with easy to understand language is an excellent remedy. I also appreciate how difficult it is to write. In my field, simplifying complex problems/issues for a lay person to understand presented a huge challenge.
I think you did a fantastic job.
Thank you!
Perhaps you could write similar posts on your discipline? :)
No, thank you as Its a fantastic idea and your execution is spot on. KUDOS.
I'm suffering from 'use it or lose it'. Although I have retained basic knowledge, my departure sometime ago means I am not up to date so I cannot consider myself qualified or an authority.
I do hope others will consider writing posts in their respective disciplines.
I know a little about a lot and I have opinions; lots and lots of opinions. :D
edited formatting
Is it possible for some amount of the electrons to spin just right to be realigned with the second magnet?
Also, small idea: Link to a good place for further reading. For folks outside this field, it can be hard to judge if a resource is good or not.
Short answer: I am not entirely sure but I think yes. Long/complicated answer: The process that causes this rotation is dependent on the electron momentum. Due to relativistic effects the electron will experience a magnetic field when moving, which is what causes the rotation. So I can imagine this rotation can be just right if the electron moves through the material with the right velocity. However, there are a lot of secondary effects to consider like scattering events with the material and the fact that the spin rotation is actually a quantum state.
In essence I like this idea, but my sources are review papers written for people in the field. So linking them wouldn't do much for the laypeople audience.
People seemed to have liked this post, so look forward to a post on artificial atoms in the near future!
Thanks for this explanation of the physics of spin transistors. I found it very interesting. I've got a bit of a self taught electronics background, so I'm going based on what little I've learned of electronics so far. I'm curious how these Spin transistors compare in electrical characteristics to traditional transistors, both NPN/PNP and MOSFETs.
Do these have an amplifying region like a traditional transistor, or are they only useful as an on/off switch?
Are their current limits comparable to MOSFETs or Solid State Relays for driving larger loads off microcontrollers logic pins, or are they useful only as logic transistors to be included in MCUs die designs as a power reducing option compared to traditional silicone transistors?
As far as I know such an application has been proposed, but I don't know if it has actually been implemented yet.
They have chiefly been studied as logic transistors. I will PM you a paper that contains the transistor I-V characteristics.