Albert Fert of the Université Paris-Sud inOrsay, France, and Peter Grünberg of theForschungszentrum in Jülich, Germany, have been awarded the 2007 Nobel Prize in physics for theirdiscovery of the phenomenon known as giant magnetoresistance (GMR). The two scientists independently discovered the phenomenon and published their results in 1988 and 1989, respectively. This work has previously been recognized by the American Physics Society and awarded the 1994 James C. McGroddy Prize for New Materials.
Magnetoresistance is nothing new in science—it is the change in electrical resistance of a material when it is in the presence of an external magnetic field. It was measured150 years ago by W. Thomson (Lord Kelvin), who found that the resistance of iron and nickel would change depending upon the orientation of the magnetic field relative to the material. What he discovered is now referred to as anisotropic magnetoresistance, a property of materials that arises from electron spin-orbit coupling. Normally, this is a weak phenomenon, as general magnetoresistance changes the conduction in a material by only a few percent at most.
Even though this phenomenon is weak, it led to the development of many important technologies, including the parts that allow us to read and write to disks. Prior to the discovery of GMR materials, the best known alloy for exploiting magnetoresistance was permalloy (Ni20Fe80), and it represented little improvement over the materials used in Lord Kelvin's time. The major breakthroughs came when the two groups started experimenting with magnetic multilayers, stacks of alternating ferromagnetic and non-magnetic metals where each layer is only a few nanometers thick. The first materials used by Furt and Grünberg's groups had stacks of iron and chromium. Testing carried out on these early magnetic mutilayers showed a decrease in resistance of up to 50 percent—far greater than any seen previously. This radical increase appeared to be an entirely new phenomenon, which was named GMR.
GMR relies on a combination of magnetism and electron spin to cause changes in conductance in the stacks of magnetic layers used by the researchers. When adjacent layers have the same magnetic orientation, electrons of a single spin type can move easily between them. When the magnetic fields in neighboring layers are opposed, electrons of both spin types are scattered, causing high electrical resistance. A more detailed discussion of how a GMR magnetic multilayer works can be read in the background paper compiled by the Class for Physics of the Royal Swedish Academy of Sciences (PDF).
While the GMR phenomenon was only discovered 20 years ago, it has already found many practical applications,mainly in the read heads used in high density computer storage. Other uses are still in the development phase; nonvolatile, low-power, high-density magnetic random access memory (MRAM) that is based on GMR materials may be the successor to DRAM that is found in most PCs today. The applications of this technology are still in their infancy, but some suggest that materials that exploit this phenomenon could eventually lead us to practical optical or quantum computers.