
Introduction to spintronics - Phithagoras
http://www.physics.umd.edu/rgroups/spin/intro.html
======
maxander
Layman questions:

\- What are the potential benefits of spintronics to technology? Would it make
electronics cheaper, smaller, more efficient, ...? Would it allow something
that we currently couldn't do?

\- "For example, whether placing a semiconductor in contact with another
material would impede spin transport across the interface is far from well-
understood." ...This seems like an eminently testable thing- just take a
semiconductor, put it in contact with something else, put a, uh, spin current
(layman!) through them and check for impedance. What hilariously ignorant
mistake am I making? : )

~~~
tagrun
> Would it make electronics cheaper, smaller, more efficient

As the name implies, it uses transport of spin rather than electrons
themselves. The electrons may not move at all (you can have a spin current
with no electric current at all). So it'd no longer be electronics.

The current premise is more dense memory devices with lower power consumption.

> This seems like an eminently testable thing- just take a semiconductor, put
> it in contact with something else, put a, uh, spin current (layman!) through
> them and check for impedance. What hilariously ignorant mistake am I making?

Well, where do I start?

Think about these: how do you generate a spin current? How much is it? How do
you measure it? (hint: no, there is no such thing as spinmeter) How do you
store/switch/manipulate bits? How does it effect magnetization, temperature,
electric current, chemical potential? (these are all intricately coupled
things) What kind of spin waves is it going to generate? Can you treat them as
quasi particles? (magnons) Will there be a flow of magnons? What are the
transport properties and how does an interface affect it (it's not easy to
isolate and measure only a single transport coefficient in practice)? What is
your order parameter? Are we talking about a ferromagnet, antiferromagnet or
ferrimagnet? Are they interfaced? What are the transport properties of your
interface? What is your Landau free energy (from which you can calculate
effective field or ground state)? Which phase of matter are you in? Helical
phase? Skyrmion crystal phase? Canted/polarized phase? What is your geometry?
What kind of stray field does it generate? What are your anisotropies? Does
your system have a strong spin-orbit copuling? What is your overall spin
transfer torque? How do all these depend of temperature / thermal
fluctuations? etc etc etc.

And keep in mind that almost all the physical observables we're talking about
are really really _really_ tiny. We're talking about quantum mechanical
effects.

I hope by now it is clear that you have to come up with an "impedance" on your
own tailored to your system, taking all the relevant physical aspects of your
system into account (which requires a good background in condensed matter
physics). There is no generic "impedance" of "spinmeter" that will work in any
situation.

And generally, the issue is not measuring spin current or transport properties
in an existing system. And you certain don't want to make devices randomly and
measure its properties until something nice comes out miraculously: you have
to "engineer" the desired properties (which typically requires very specific
conditions), from ground up.

So it's a little different from connecting a lamp and a resistor to a battery
and see whether it lights up or not. Suffice it to say that it's more
complicated than your average high school science experiments.

~~~
maxander
Your paragraph after "where do I start" is so incomprehensible that I'm not
entirely sure you're not simply spouting random technobabble at me. But I get
the idea. :D

------
stephengillie
[deleted]

~~~
tagrun
> "Spin" in quantum physics is more of an artistic interpretation of the term.
> It's similar to how Autism and other disorders have a "spectrum", but aren't
> divided into colors.

> With wave-particle duality, spin signifies how in-phase one particle's wave
> part is with the wave part of other particles.

What? Is this a joke? (I'm serious, because it's not even wrong)

Edit: Since I wrote my reply, the parent was fist deleted, and then edited
into a quote from a blog post which tries to explain spin to layman. Well,
whatever.

~~~
ttctciyf
Arrived too late to see the post you're replying to, but isn't it fair to say
that an electron's spin, though it corresponds to a notion of angular momentum
mathematically, is a little different to everyday ideas of spin? Not least in
that spin-half particles like the electron must "rotate" 720 rather than 360
degrees before they get back where they started.

~~~
tagrun
There's an accidental similarity between the groups SO(3) and SU(2), which
works for spin. There are other quantum numbers such as flavor SU(3) for there
is no mathematical coincidence and such analogies won't work.

The differences between SU(2) and SO(3) go beyond just "rotate 720 rather than
360 and you're back where you started".

While I understand the urge of expressing new things in terms of what you
know, sometimes, analogies aren't helpful and distort reality, and you just
need to accept that there can be things in the nature that doesn't correspond
to anything in your daily life in the real sense of the word.

That being said, the original parent post (which I quoted, so it's actually
there) is total nonsense.

~~~
stephengillie
Could you recommend a good entry point for understanding group theory? It's
always been particularly impenetrable, and the wiki page doubly so. Book or
video or blog post or even private college course - anything would be
appreciated.

Is spin the _exact_ same physical phenomena as 'macroscopic' spin, but on a
quantum scale? If not, just curious, why use the same word?

~~~
tagrun
I can recommend Howards Georgi's Lie Algebras in Particle Physics which is a
good introduction in the context of particle physics (yes, it covers both Lie
algebras and groups).

No, it is nothing like the angular momentum of a spinning object. The name is
rooted in a historical misconception. It is a form on angular momentum, which
caused the misunderstanding. As of today, we still do not have a deeper
understanding of spin in quantum mechanics: it's just some intrinsic angular
momentum with SU(2) symmetry, and the value of angular momentum is in general
different between particles (corresponding to the irreducible representations
of SU(2), and the angular momentum is mostly 1/2 or 1 in units of reduced
Planck constant except for some exotic/composite particles).

