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Major Breakthrough In Spintronics Research
Invisible Pink Unicorn writes "Spintronics is the field of research into developing devices that rely on electron spin rather than electron charge to carry information. A major advance has been made by the Naval Research Laboratory (NRL), where they have for the first time generated, modulated, and electrically detected a pure spin current in silicon. Progress in this field is expected to lead to devices which provide higher performance with lower power consumption and heat dissipation. Basic research efforts at NRL and elsewhere have shown that spin angular momentum, another fundamental property of the electron, can be used to store and process information in metal and semiconductor based devices. The article abstract is available from Applied Physics Letters."
Spins are transferred from electron to electron as the spins flip. Imagine a series of bar magnets. You can flip one magnet and it will affect the energy of the next one, and then the next one etc. The exact solution is difficult to calculate quite often, but in general, if you have a high population of spin up electrons localized in one area, the spins will tend to diffuse away from that via a few mechanisms:
1. The spin ups will turn to spin downs and cause nearby spin downs to turn spin up.
2. The spin up electrons will move to the right (just picking a random direction), and this will be compensated for by having spin down move left. The result is a net spin current with no net charge current.
To generate this, a spin polarized charge current is generally used. In this paper they used a ferromagnet contact as a source. The setup is basically a 3-way intersection.
Lead 1 is just a floating lead not connected to any ground.
Lead 2 is the ferromagnetic lead
Lead 3 is a ground connection
A voltage is applied between Lead 2 and Lead 3 causing an electrical current to flow. The electrons come out of the ferromagnet partially polarized. This current then goes into the ground Lead 3. All charge current flows from Lead 2 to Lead 3. However, the excess spin up electrons in the junction cause spins to diffuse down the floating Lead 1. No charge current flows down Lead 1 because it has nowhere to go. The result is a net spin current with no net charge current.
All particles have an associated spin, just as all particles have an associated net charge.
Spintronic devices make use of the spin of electrons. Whence, Spintronics.
Sig free's the way to be.
well, it's 1am and i'm writing up my phd thesis draft in... spintronics... so i'll jump on this as best i can, having skimmed the article (but not the press release because really, what science comes from press releases)
the idea for spintronic devices is to use different device physics utilizing the spin of charge carriers vs just their charge. a common device is a GMR read head on hard drives - developed in '88, widespread now. the next step is to make transistors that use spin - this requires a new class of materials (GMR is a metal/macro structure effect), essentially making non-magnetic materials ferromagnetic is the goal. (personally i use ZnO, not Si, but the idea is similar). if you use a ferromagnetic semiconductor of some kind, then there is better charge transfer to other semiconductors vs a ferro metal to semiconductor... and then what you're looking for is a material that has a long spin polarization lifetime (time before the knocking around flips the spin and all of a sudden you have no polarization). so i *think* that they mean a spin current to be something that is 100% spin polarized (ie all spin up) - which means that if aligned with an applied magnetic field there will be minimal scattering therefor lower resistivity and lower heat/phonon interaction. vs. the case of a partially polarized or random spin up/down distribution where the available states in a material subjected to a field are only open to half of the free carriers (ie only spin up states are available because of the field, so only spin up electrons are efficient carriers). all this is very much so like GMR heads, obviously (well i suppose to me).
i've met the authors at conferences, and i'm sure they're less than thrilled with this being labeled a "major breakthrough" though i'm sure they like a bit of the attention. this is pretty cool and interesting stuff that they've done, but it isn't a breakthrough - just another piece of an extremely large and complex puzzle.
to email me: take my
Well, I'm not a particle physicist either, but I did my master's thesis on this type of system(*) So, how close are they to applications?
First of all, they've measured from 5K up to 80K which is about quarter of room temperature, practically there by solid state physics standards (see the Nature paper). Considering that the effect didn't dwindle by more than half in that range, that's a very good sign that it could be brought up to room temperature. The problem is in getting the electrons "lined up" enough. In the Nature paper they estimate that they see about 30% more spin up than down electrons, but for real applications you'd like to get a lot closer to "100% polarization". I guess that problem might be solvable, but it includes a lot of putting very thin layers of material on single crystal with quite extreme tolerances. Then again, chip fab tolerances are quite extreme already.
In cases like this it's hard to figure out how well stuff will work without actually trying, and that's what this recent paper is about: They've built a transistor-like device with technology similar to that which would be used for mass production and measured a (tiny) effect. Now it's a matter of optimisation, and they might just get there, but it'll be years at least before it's time to start drafting the chip layout.
(*) Hint: If it involves building huge accelerators to crash particles together at pseudo-Big-Bang conditions, it's particle physics. If it involves sticking little pieces of semiconductor into a magnet at 5 Kelvin it's solid state physics. Not that solid state physicists don't use particles, we're all over electrons, phonons, excitons, magnons and many other 'ons that ignorants dismiss as "quasi"particles. Hrmph!
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