[Maz]

A computer hard disk reader that uses a GMR sensor is equivalent to a jet flying at a speed of 30,000 kilometers (19,500 miles) per hour ... at a height of just one meter above the ground, and yet being able to see and catalogue every single blade of grass it passes over," [Ben Murdin, a physics professor at the University of Surrey in southeast England] said.

Impressive, eh?  This year's Nobel in physics was awarded for something that you actually use on a daily basis...but never think about. Albert Fert and Peter Grünberg get to split $1.5Million for their discovery of Giant Magnetoresistance (GMR).  I know; not only does it sound amazingly cool, but it's actually useful?  I mean, anti-particles, darkmatter, parallel universes and string theory are sexy and all, but come on.  This tech. actually is called Giant.  Not to mention that the discovery of GMR led to the field of spintronics, and the principles behind your iPod and 500 gigabyte hard drive.   

On to the science.

To start this discussion, I have to explain a little about spin and it's interaction with mag. fields. 
NOTE:  SKIP THIS SECTION IF YOU ALREADY KNOW THE BASIC BACKGROUND

Let's review.

So if you sent a beam of electrons through a magnetic field and had some quantum trash bags to collect the electrons upon exiting this mag. field, you'd see that 1 bag was full of spin up electrons, and 1 bag was full of spin down electrons.

One way you can attempt to rationalize this (and I know this is a little hand-wavy, but it'll do) is to imagine an infinitesimal charge, dq on the surface of the electron.  If we see that the electron is spherical and spinning, either clockwise or counterclockwise, then this dq rotates about as well.  So would all the other dq's on the surface of the electron.  You know that the flow of charge generates a magnetic field.  Through Maxwell's eq. you could show that if the electron is spinning clockwise it's z-axis, you'd see a magnetic field pointing in the "-z direction" (remember, no magnetic monopoles (we think) so the field lines could be thought of as initially going out the bottom of the electron. and then curling back up to arrive at the top), and if it was spinning counterclockwise, you'd see the field pointing in the +z direction.  These two dipole moments we call spin up and spin down, and they either align in parallel with the external magnetic field, or antiparallel with it.  As you're probably guessing, the parallel alignment is slightly lower in energy then the antiparallel.  This is known as the Zeeman effect.

END REVIEW

So GMR is, in my opinion, one really big and really cool trick.  I say so because it's one of those things that seems to me as obvious but not trivial.  There are a few different types of GMR applications that operate off of the same basic principle.  Here I'll talk about the most used one, the spin-valve.

If you take a ferromagnetic layer and polarize it, the unpaired electrons in the layer will align themselves to the external magnetic field.  These electrons are now what we call "spin polarized".  Do this for a second layer and place them next to each other, but separated by a non ferromagnetic layer.  Slap a potential difference across the two layers, and the electrons will maintain their polarization while moving through the circuit.  But when these electrons hit a material with a mag. field opposite their spin direction, they get flipped.  And here's the rub:  flipping the spins requires extra energy...in other words the electrical resistance is increased when the magnetic materials are polarized in the opposite directions (anti-parallel alignment). 

Who cares?  Well, you do, you just might not see it yet.  Being able to change the like this electrical resistance, which is easily detectable, is what computer guys call "non-volatile".  Meaning you don't require power to keep the changes made.  If you let the "low resistance" represent 1 and the "high resistance" represent 0, you can get digital logic.  Imagine doing what we just discussed, hundreds of billions of times on a 3.5" disc...and you've got yourself a hard drive.  This is a relatively simple type of spintronic device, which is why they say that the GMR discovery gave birth to the field of spintronics.
   
Spintronics is, in general, the development of technology which allows you to make use of the spins of electrons. If you can generate a current of like-spinned electrons, i.e. polarized electrons, and send them through a device that can detect and act based on the spin of these electrons, you have created a spintronic device.  It is sensitive not to voltage or charge or mass, but on the electron's spin.  That's a really big deal.  After-all, you've got an intrinsic 1 or 0 right there.

So the next time you strap that overpriced, little white noise machine onto your arm and go for a jog, say thanks to a couple of physicists who discovered something that turned out to be useful...in your lifetime.

Note:  This was by no means a rigorous discussion, and if anyone is interested in the topic, you could start a thread in the forums and see what grows.  For the blog though, this is maybe already too indepth.  I should have added some pretty pictures. 

Maz

[Mitch]

And yet, one non-grounded finger can completley screw it up.

[constant thinker]

Or a really hard kick from a frustrated person. 


(a computer from 1995 that just didn't want to work in 2001 proved this to me)