The Giant Magnetoresistive Head: A giant leap for IBM Research
It's called the Giant Magnetoresistive effect.
Ten years ago, it hadn't even been discovered. But now, after intense and dedicated research and development, "giant magnetoresistance" -- or GMR for short -- makes its mass-market debut in IBM's record-breaking 16.8-gigabyte hard disk drive for desktop computers using a special GMR structure developed at IBM called a spin valve.
Most people don't give their hard drive a second thought -- until they run out of disk space. If this describes you, read on. Once you understand the beauty of the GMR/spin valve head, you will never feel the same way about your hard disk drive again.
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What is it?The "giant magnetoresistive" (GMR) effect was discovered in the late 1980s by two European scientists working independently: Peter Gruenberg of the KFA research institute in Julich, Germany, and Albert Fert of the University of Paris-Sud . They saw very large resistance changes -- 6 percent and 50 percent, respectively -- in materials comprised of alternating very thin layers of various metallic elements. This discovery took the scientific community by surprise; physicists did not widely believe that such an effect was physically possible. These experiments were performed at low temperatures and in the presence of very high magnetic fields and used laboriously grown materials that cannot be mass-produced, but the magnitude of this discovery sent scientists around the world on a mission to see how they might be able to harness the power of the Giant Magnetoresistive effect.
IBM Research Arrives on the SceneStuart Parkin and two groups of colleagues at IBM's Almaden Research Center, San Jose, Calif, quickly recognized its potential, both as an important new scientific discovery in magnetic materials and one that might be used in sensors even more sensitive than MR heads.
Parkin first wanted to reproduce the Europeans' results. But he did not want to wait to use the expensive machine that could make multilayers in the same slow-and-perfect way that Gruenberg and Fert had. So Parkin and his colleague, Kevin P. Roche, tried a faster and less-precise process common in disk-drive manufacturing: sputtering. To their astonishment and delight, it worked! Parkinís team saw GMR in the first multilayers they made. This demonstration meant that they could make enough variations of the multilayers to help discover how GMR worked, and it gave Almaden's Bruce Gurney and co-workers hope that a room-temperature, low-field version could work as a super-sensitive sensor for disk drives.
The Nitty GrittyThe key structure in GMR materials is a spacer layer of a non-magnetic metal between two magnetic metals. Magnetic materials tend to align themselves in the same direction. So if the spacer layer is thin enough, changing the orientation of one of the magnetic layers can cause the next one to align itself in the same direction. Increase the spacer layer thickness and you'd expect the strength of such "coupling" of the magnetic layers to decrease. But as Parkin's team made and tested some 30,000 different multilayer combinations of different elements and layer dimensions, they demonstrated the generality of GMR for all transition metal elements and invented the structures that still hold the world records for GMR at low temperature, room temperature and useful fields. In addition, they discovered oscillations in the coupling strength: the magnetic alignment of the magnetic layers periodically swung back and forth from being aligned in the same magnetic direction (parallel alignment) to being aligned in opposite magnetic directions (anti-parallel alignment). The overall resistance is relatively low when the layers were in parallel alignment and relatively high when in anti-parallel alignment. For his pioneering work in GMR, Parkin won the European Physical Society's prestigious 1997 Hewlett-Packard Europhysics Prize along with Gruenberg and Fert.
Searching for a useful disk-drive sensor design that would operate at low magnetic fields, Bruce Gurney and colleagues began focusing on the simplest possible arrangement: two magnetic layers separated by a spacer layer chosen to ensure that the coupling between magnetic layers was weak, unlike previously made structures. They also "pinned" in one direction the magnetic orientation of one layer by adding a fourth layer: a strong antiferromagnet. When a weak magnetic field, such as that from a bit on a hard disk, passes beneath such a structure, the magnetic orientation of the unpinned magnetic layer rotates relative to that of the pinned layer, generating a significant change in electrical resistance due to the GMR effect. This structure was named the spin valve.
To see an animation of how MR and GMR recording heads work, click here. Gurney and colleagues worked for several years to perfect the sensor design that is used in the new disk drives. The materials and their tiny dimensions had to be fine-tuned so they 1) could be manufactured reliably and economically, 2) yielded the uniform resistance changes required to detect bits on a disk accurately, and 3) were stable -- neither corroding nor degrading -- for the lifetime of the drive. "That's why it's so important to understand the science," Parkin says. "IBM's intensive studies of GMR enabled us to enhance considerably the performance of some low-field sensors."
The chief source of GMR is "spin-dependent" scattering of electrons. Electrical resistance is due to scattering of electrons within a material. By analogy, consider how fast it takes you to drive from one town to another. Without obstacles on a freeway, you can proceed quickly. But if you encounter heavy traffice, accidents, road construction and other obstacles, you'll travel much slower.
Depending on its magnetic direction, a single-domain magnetic material will scatter electrons with "up" or "down" spin differently. When the magnetic layers in GMR structures are aligned anti-parallel, the resistance is high because "up" electrons that are not scattered in one layer can be scattered in the other. When the layers are aligned in parallel, all of the "up" electrons will not scatter much, regardless of which layer they pass through, yielding a lower resistance.
For an animation showing how electrons of different spins scatter
within a GMR structure, click here.