Giant Magnetoresistance

CORE CONCEPTS Giant magnetoresistance Stephen Ornes laptops, cell phones, search engines, and Science Writer music players to scour densely packed hard drives and extract data in less time than it takes to push a button. Two European physicists, Albert Fert from Scientists have known for more than 150 sensitive sensors. The technology, called France and Peter Grünberg from Germany, years that a magnetic field can change giant magnetoresistance (GMR), manipu- independently demonstrated GMR in cus- electrical resistance in a material, ever since lates the magnetic properties of electrons tom-built devices in 1988, and in 2007 the Scottish physicist William Thomson (Lord and takes magnetoresistance to a new level. two shared the Nobel Prize in Physics— Kelvin) first measured the effect in iron. In GMR, minute fluctuations in a magnetic 10 years after the first GMR-based read-out That phenomenon, magnetoresistance, can field induce huge differences in electrical head showed up in a digital device. be tweaked according to the strength and resistance. Both scientists built devices by alternating orientation of the field, and has made it Electrical resistance measures how much layers of magnetic and nonmagnetic materi- possible for engineers to build sensors that opposition an electric current faces as it als, each layer only atoms thick, in a kind of could read the data stored as magnetic bits travels through a material. To compare it nanosandwich. Although their initial proto- in the hard drives of early computers. to driving: A current moving through high types worked only at supercold temperatures, However, as digital devices became smaller resistance is like trying to navigate a field other physicists soon invented the materials their hard drives became more crowded. The of boulders; low resistance is like zooming and methods to bring the technology to trendtowardminiaturizationmighthave unimpeded across salt flats. room temperature. stalled in the 1990s if it weren’tforthe Modern read-out heads—the speck-sized Read-out heads that use GMR have the advent of a new technology that powered gizmos responsible for pulling data from a same basic design as Fert’sandGrünberg’s the design and implementation of more disk—use GMR to make it possible for originals. In a read-out head that uses GMR, the separated magnetic layers are magnetized in opposite directions. An electric current moves through the magnetic-nonmagnetic- nanosandwich. However, because the intrin- sic magnetic properties of the electron (its spin) and one of the layer’smagneticfields point in different directions, the electrons in the current encounter high resistance. Enter the spinning hard drive: As the outermost magnetic layer of the read-out head moves over a magnetic bit in the spinning disk, this layer’s magnetic field aligns with that of the bit and of the other magnetic layer. As a result, the resistance drops off dramatically and the electrons in the current sail through. This dramatic change in resistance shows the binary state of the bit. Because GMR is so sensitive to even slight magnetic perturbations, bits can be supersmall and, therefore, packed more tightly on a disk. Although GMR now shows up in the memory-readers of digital devices, engineers are exploring ways to use it for all A read/write head sits above a computer hard disk, manipulating the stored information by kinds of memory, including random-access the use of giant magnetoresistance.©iStockphoto.com/BrandonSeidel memory, or RAM. 3710 | PNAS | March 5, 2013 | vol. 110 | no. 10 www.pnas.org/cgi/doi/10.1073/pnas.1302494110 Downloaded by guest on September 29, 2021.

Load more