2007 Nobel Prize, Physics: Giant Magnetoresistance

To the typical consumer, it may seem that the process of miniaturization is an inexorable march towards ever smaller devices. Every year, manufacturers parade a line of slimmer mp3 players, smaller phones, and thinner laptops – and somehow, each diminutive device has more features than its predecessor did the year before. But miniaturization is not simply the natural flow of consumer electronics; manufacturers do not decide again and again that perhaps this is the year to pack more memory into a smaller device. Rather, these changes rely on continued scientific breakthroughs. One such breakthrough, the discovery of giant magnetoresistance, was recently recognized with the 2007 Nobel Prize in Physics.

In 1988, two separate research teams, headed by Albert Fert and Peter Grünberg, discovered a new phenomenon known today as giant magnetoresistance. This phenomenon has been utilized to produce very sensitive hard drive read-out heads, making possible increasingly small hard drives (1).

Until recently, hard drives functioned on a similar concept – magnetoresistance. Information on a hard drive was, and still is, stored in a binary format – groups of ones and zeros. To achieve this physically, a small area representing a “1” would be magnetized in one direction, while an area representing a “0” would be magnetized in a different direction. Magnetoresistance made it possible to read these areas of different magnetization (2).

To read the hard disk, a read-out head with a small metal conductor on the end would pass over the magnetized areas, and its changing resistance would be monitored. An area magnetized parallel to the direction of the conductor decreases its resistance – this would be read as a “0”. Next up, the read-out head might encounter an area magnetized perpendicularly to it – this would increase the conductor’s resistance, a reading of “1” (2).

This method worked well, but there was a problem. As hard drives became smaller and smaller, the magnetized areas also shrunk – and therefore became weaker and weaker. It became increasingly difficult for the read-out heads of the time to decipher the weak signals. This is where giant magnetoresistance came into play.

In 1988, Albert Fert and Peter Grünberg were researching materials that would exhibit greater changes in resistance when exposed to magnetization. Both were using ultra-thin (only a few atoms thick) layers of magnetic and non-magnetic metals – Grünberg’s team used a trilayer of Fe/Cr/Fe while Fert’s team utilized stacks of Fe/Cr (2).

What they found when they exposed the layered metals to magnetized areas was a giant leap in resistance – far greater than anyone imagined was possible. Fert’s material showed a greater leap, largely due to the dozens of layers he used, as well as the extremely low temperature at which he performed his experiment. But both experiments showed changes in resistance far greater than any previously achieved using conventional materials (1).

At a major magnetic conference that year, the International Colloquium on Magnetic Films and Surfaces in Le Creusot, France, both Fert and Grünberg were scheduled to give talks. Both men presented their findings on the magnetoresistances of the thinly layered materials they had constructed. Afterwards, Grünberg remarked, “Yes, we obviously found the same kind of an effect” (1).

Nearly a decade passed before giant magnetoresistance could be put to practical application – largely because the ultra-thin layered metals were so expensive to produce. Stuart Parkin, a researcher from IBM, was the first to demonstrate a cheap yet still effective way to achieve giant magnetoresistance. By 1997, the first hard drive read-out heads utilizing the phenomenon were being produced (1).

The new read-out heads paved the way for the continued miniaturization of hard drives. Because the materials developed by Fert and Grünberg showed much greater changes in resistance, they were more sensitive to smaller, weaker areas of magnetization. With the new, super-sensitive read-out heads, more information could be stored on increasingly small hard drives (1).

The ultra thin metal layers also represented one of the first applications of the nascent field of nanotechnology. The usefulness of giant magnetoresistance has served to encourage further research into nanotechnology, which has had applications in a number of other disciplines. Said Fert, “the nanotechnologists are a wonderful tool for the physicists, and for the biologists, and for the chemists” (3).

Furthering this interdisciplinary theme, giant magnetoresistance may even prove applicable to biology. According to Grünberg, some labs are researching a technique to detect genetic material using magnetoresistance. In this method, specific gene sequences could have “magnetic beads” attached to them; the antigenes would bind complementary sections of DNA, and “via magnetoresistive sensors you can then detect genetic material” (4).

This ability to detect DNA may prove to be one of the long term applications of giant magnetoresistance. But even in the short term, the utility of this phenomenon can be seen all around us. Especially as people across America head to the malls in search of this year’s cutest little gadget, Fert and Grünberg’s discovery will be having a great impact, in a very tiny way.

References:
1. “The Nobel Prize in Physics 2007: Information for the Public” The Royal Swedish Academy of Sciences, (2007). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2007/info.pdf (November 24, 2007).
2. “The Nobel Prize in Physics 2007: Scientific Background” The Royal Swedish Academy of Sciences, (2007). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2007/phyadv07.pdf (Novermber 24, 2007).
3. A. Fert, interview (2007). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2007/fert-telephone.html (November 24, 2007).
4. P. Grünberg, interview (2007). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2007/grunberg-telephone.html (November 24, 2007).