magnetic moments

Last Christmas I bought my youngest niece, Cami, her very own iPod Nano, much to the relief of her older siblings, who were tired of Cami constantly "borrowing" theirs. Her reaction was priceless: she gasped, she squealed, tears came to her eyes, she hugged the little device to her chest with eyes cast heavenward in gratitude. (I predict a stellar career as an actress when she grows up.) She's not even 10 yet, so to her, it's just a wondrously magical device in a pretty color that holds her favorite tunes. She has no idea how it actually works, or that the iPod just won the 2007 Nobel Prize in Physics.

Okay, the iPod, per se, didn't win the Nobel Prize, just the fundamental physics research on which it is based. I just threw that comment in there to make the chronic curmudgeons grit their teeth at the media's insistence on trumpeting about the iPod instead of talking about the actual science being honored this week by the Nobel selection committee. But we here at Cocktail Party Physics join the rest of the physics blogosphere in congratulating Albert Fert (Universite Paris-Sud, Orsay, Franch) and Peter Gruenberg (Julich Research Center, Germany) on being awarded this year's Nobel Physics Prize for their  independent discovery of giant magnetoresistance (GMR), which has given us not just the iPod, but also high-density hard drives in general, high-density magnetic random access memory (MRAM) and a budding new field called spintronics. Anyone dying to get the fundamental physics scoop can check out this nifty background site (courtesy of IBM), or, for the truly technical minded, read the original papers on GMR as published in Physical Review Letters and Physical Review B.

Of course, if you really want to be all fundamental about it, this year's Nobel Prize in Physics has roots stretching all the way back to 900 BC, when a Greek shepherd named Magnes supposedly walked across a field of black stones, which pulled the iron nails out his sandals. He dubbed the region Magnesia. That's the legend, anyway, and who are we to argue about it? Certainly strange magnetic effects were known to the natural philosophers of antiquity, and by 1269 AD, some French dude named Petrus Peregrinus had figured out that so-called lode-stones -- basically naturally occurring spherical magnets of iron-rich ore -- could be used to make rudimentary compasses, although people didn't fully understand how or why they worked until 1600, when William Gilbert, court physicist to Queen Elizabeth I, discovered that the Earth itself is a giant magnet.

There were more mysteries of magnetism yet to come. In 1820, a humble physics professor at Copenhagen University named Hans Christian Oersted discovered that electricity and magnetism weren't two separate things, but were closely related -- more like flip sides of the same coin. He discovered this quite by accident while playing around in the lab with heated wires and compasses, among other scientific paraphernalia. He was merely trying to demonstrate the basics of magnetism and how electrical current can heat a wire, using a simple compass needle mounted on a wooden stand as a makeshift magnet. He noticed that every time the electric current switched on, the compass needle would jump in response. No doubt he thought to himself, "huh... that's funny!" -- which invariably heralds most major scientific breakthroughs. He found he could repeat the same effect, over and over again, in his lab, with more sophisticated apparatus. And the study of electromagnetism was born, bringing breakthrough after breakthrough as the giants of physics history (Faraday, Maxwell, Hertz, et al.) set themselves to the task of delving deeper into this phenomenon.

I won't even attempt to summarize the vast number of pioneering breakthroughs and revolutionary research that took place in the aftermath of Oersted's discovery -- although it is worth mentioning that back in 1857, famed British physicist Lord Kelvin demonstrated that electric resistance of materials such as iron could be influenced by a magnetic field (the basis for modern induction coils, the significance of which will become clear below). Let's just fast forward a couple of centuries to the 1980s, when two teams of researchers in France and Germany, led by Fert and Gruenberg, respectively, were building alternating ultra-thin layers of magnetic and non-magnetic atoms at the nanoscale. At those tiny scales quantum effects begin to hold sway, so fundamental properties of materials are altered.

For instance, an electron's spin becomes a much more important factor. "Spin" is a misleading term, in that in the minds of a layperson such as myself, it tends to conjure up an image of a spinning top. An electron spin is something quite different, and under the right circumstances, infinitely more powerful. To quote an article published last January by Lawrence Berkeley Laboratory:

"Spin is a quantum mechanical property that arises when the rotational momentum of a particle, in this case an electron, creates a tiny magnetic field. For the sake of simplicity, spin is given a direction either 'up' or 'down.' Just as the positive or negative values of an electrical charge can be used to encode data as the 0s and 1s of the binary system, so too can the up and down values of spin. Unlike charge-based data storage, however, spin-based storage does not disappear when the electrical current stops."

The shorthand version: Electron spins generate a magnetic field, and can be aligned either "up" or "down." The big breakthrough occurred when the French and German teams realized they could use spin orientation at the nanoscale to effect very large changes in the electrical resistance of layered materials, even if the changes in magnetic fields were very weak. That hunch proved correct. Fert and Gruenberg found that when all their nanoscale layers were aligned in the same direction, electrons with the same alignment would pass right through the material, but the electrons would be blocked if they had the opposite alignment. Independently of each other, they wondered what would happen if the layers were organized in an alternating up-down alignment. They found that in such instances, all electrons encountered some resistance, giving rise to a much higher level of resistance than had previously been observed.

How much higher? According to the Nobel background materials, Fert's group observed "a magnetization-dependent change of resistance of up to 50%," while the German group, led by Gruenberg, saw a 10% difference. Fert used many more layers, giving rise to a stronger GMR effect, but even Gruenberg's 10% difference was a vast improvement: with traditional magnetoresistance, barely a single percent of change in resistance had ever been observed. You didn't need to be a rocket scientist to know this was a major breakthrough; Gruenberg had the foresight to file a patent as he was writing up the results for publication, realizing (as did Fert) that such an effect would have an enormous impact on technology.

On a practical level, this was very good news for magnetic hard drive technology. Hard drives store data in patterns of magnetic fields, and metal induction coils (thank you, Lord Kelvin!) are then used to read out the data. The problem was that there were some fundamental physical constraints that limited how small those induction coils  (and data bits) could get; the magnetic signals quickly became too faint to be accurately detected at those smaller scales. The GMR effect meant that even a tiny magnetic field -- like the one on the surface of a computer hard drive -- can trigger a very large change in electrical resistance, and this effect can be exploited to "read" any data encoded in that surface's magnetic orientation. So much more information can be packed into a very small space, and still be "read."

Even better, later research found that one didn't need to create the ultra-thin layers using the labor-intensive and costly process of epitaxy; a sputtering process worked just fine, and could be easily scaled up to commercial volumes, because the GMR effect wasn't dependent on creating highly precise (at the atomic level) layers. The first devices based on GMR appeared in 1997, and the rest is history. MP3 players have been shrinking ever since: I have a full-sized iPod with a whopping 60 GB storage capacity, and a teensy iPod Shuffle for gym workouts that is about the size of a postage stamp, yet can still hold a good 250 songs.

GMR has also given rise to a magnetic alternative to dynamic random access memory (DRAM) called, appropriately, Magnetic Random Access Memory (MRAM).  Everyone's familiar in theory with RAM: it's the temporary storage of data on your computer before you hit "save" and your hard work gets copied over to the hard drive for more permanent storage. At least, that's what happens if all goes well and there isn't a power outage, or a faulty sector on the hard drive, or, or, or...  My senior year of college, I was feverishly writing the final draft of my senior thesis, and got so involved in the actual writing that I failed to keep saving the file every half hour or so. And wouldn't you know it, I got the Blue Screen of Death and lost much of the last third of my revised paper. Foul language filled the computer lab for a good 10 minutes before I resigned myself to my fate and stayed up all night to rewrite what I'd lost.

That sort of thing is still happening occasionally to computer users everywhere, even all these years later. GMR could put an end to such tragedies. Thanks to GMR (and its offspring, Tunneling Magnetoresistance), it is possible, in principle, to use these effects for both reading and writing information, so MRAM could be used as a permanent working memory without a hard drive's dependence on electric power.  Computers using MRAM memory chips don't need to be booted up to move hard-drive data into memory, and the chips can store more data in a much smaller space, and access it much more quickly, than with conventional DRAM. Ultimately this means even more miniaturization, something that any product using embedded computers would welcome (cars, for instance). And that's not even counting the budding field of spintronics.

So, sure, GMR is fundamental physics, but a big part of its importance stems from its usefulness for a broad range of applications -- some of them potentially revolutionary. I understand why some physicists are a bit miffed at all the play the iPod has been getting in the press as a result of this, but honestly, the Nobel website itself specifically plays up that aspect, and made the selection in part because it's such a terrific example of how fundamental research can lead to cutting-edge technology, which in turn feeds back into fundamental research -- in this case, possibly enabling future breakthroughs in nanotechnology and quantum computing.

Personally, I'd find it gratifying  if I were a physicist and my basic research revolutionized the music industry the way the iPod has done. Fert, for one, doesn't seem to be too upset about all the attention being paid to the iPod and all the other spinoff technologies made possible by GMR. "These days, when I go to my grocer and see him type on a computer, I say, 'Wow, he's using something I put together in my mind.' It's wonderful," he told the Associated Press.

Nor is he kicking himself over not filing a patent claim, like Gruenberg did when the effect was first discovered. He's got half of a $1.5 million Nobel Prize to play with, after all, and told France's Inter Radio about his plans to share some of that with the colleagues who aided his ground-breaking research. Oh, and he also plans to buy some new sails for his windsurfers. Fert is Jen-Luc Piquant's new personal hero, and I can understand why. When I get to be 69, I hope I'm as relaxed, confident and personally fulfilled, and still physically capable of controlling a windsurfing contraption -- perhaps listening to my implanted (and waterproof!) MP3 player, made even tinier thanks to advances like GMR.

Comments

[Chas.]

The way I heeard it, Olmstead was conducting a classroom demonstration when he laid his wire carrying a current near a compass. This is supoposidly the only time a scientific discovery was made in a classroom. A prize is/was given to science teachers in his name because of this.

[M. Simon]

One of the important properties of GMR is speed of response. A large slow effect wouldn't be nearly so useful.

[Cros Vincent]

Hi Jennifer

I am a disciple and also a collaborator of Albert Fert. It is not worth to say that I am a very lucky scientist and more generally very lucky guy to have the privilege to work since about 15 years with Albert. I would like to warmly thank you for your article that I have found perchance. It is a nice feeling to see that a person like you that does not know him personally, is able to discribe in a very friendly (and true) manner his way of thinking and living. As a confidence, let me tell you that if everybody could be as friendly and good like Albert is (and I suppose I do not have to mention his inteligence), everything would go more more better on earth!!

I will transmit to him the link to your blog and I can assure you that he will appreciate it.
Best regards
Vincent