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Nobel prize recognizes GMR pioneers
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The 2007 Nobel Prize in Physics has been awarded jointly to Albert Fert of the Université Paris-Sud in France and Peter Grünberg of the Forschungszentrum Jülich in Germany "for the discovery of giant magnetoresistance". Their discovery, which both physicists made independently in 1988, led to a dramatic rise in the amount of data that can be stored on computer hard-disk drives. Fert and Grünberg share prize money totalling 10 million Swedish krone (about $1.5m). Giant magnetoresistance, or GMR, is the sudden change in electrical resistance that occurs when a material consisting of alternating ferromagnetic and non-magnetic metal layers is exposed to a sufficiently high magnetic field. In particular, the resistance becomes much lower if the magnetization in neighbouring layers is parallel and much higher if it is antiparallel. This change in resistance is due to "spin up" and "spin down" electrons scattering differently in the individual layers.
GMR has since been used to develop extremely small and sensitive read heads for magnetic hard-disk drives. These have allowed an individual data bit to be stored in a much smaller area on a disk, boosting the storage capacity greatly. The first commercial read heads based on GMR were launched by IBM in 1997 and GMR is now a standard technology found in nearly all computers worldwide and is also used in some digital cameras and MP3 players.
In Grünberg's original work, he and his team studied an iron/chromium/iron trilayer system that showed an decrease in resistance of 1.5%. Fert and colleagues, in contrast, studied an iron/chromium multilayer system in which the electrical resistance decreased by nearly 50%. "These films started out as being very esoteric, but it turned out that they would have great practical importance," says Tony Bland, a physicist from the University of Cambridge. "They paved the way for substantial information densities of commercial disk drives. It also paved the way for new physics, such as tunneling magnetoresistance (TMR), spintronics and new sensor technology, for example biosensors. The caveat is that GMR has already become old technology and people are now interested in TMR for future technology." TMR gives rise to a more pronounced resistance change in small applied fields than is found in GMR devices. Albert Fert was born in 1938 in Carcassone, France, and received a PhD in physics in from Université Paris-Sud, Orsay in France. He is now also scientific director of CNRS/Thales Unité Mixte de Physique in Orsay. Peter Grünberg was born in 1939 in Pilsen (now in Czech Republic) and is a German citizen. He gained his PhD in physics from the Technische Universität Darmstadt, Germany. Grünberg, who holds a patent on GMR, originally submitted his paper slightly before Fert, although Fert's was published first. "But whereas Fert was able to describe all the underlying physics, Grünberg immediately saw the technological importance," adds Bland. Source: Physicsworld 'Ubiquitous' technology Professor Ben Murdin of the University of Surrey, UK, said giant magnetoresistance, or GMR, was the science behind a ubiquitous technological device. "Without it you would not be able to store more than one song on your iPod!" he explained. "A computer hard-disk reader that uses a GMR sensor is equivalent to a jet flying at a speed of 30,000 kmph, at a height of just one metre above the ground, and yet being able to see and catalogue every single blade of grass it passes over."
The breakthrough underpins how data is read from hard disks GMR involves structures consisting of very thin layers of different magnetic materials. For this reason it can also be considered "one of the first real applications of the promising field of nanotechnology", the Royal Swedish Academy of Sciences said in a statement. "Applications of this phenomenon have revolutionised techniques for retrieving data from hard disks," the prize citation said. "The discovery also plays a major role in various magnetic sensors as well as for the development of a new generation of electronics." Source: BBC Giant magnetoresistance Giant magnetoresistance (GMR) is a quantum mechanical effect, a type of magnetoresistance effect, observed in thin film structures composed of alternating ferromagnetic and nonmagnetic metal layers. The effect manifests itself as a significant decrease in electrical resistance in the presence of a magnetic field. In the absence of an applied magnetic field the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers, and it decreases to a lower level of resistance when the magnetization of the adjacent layers align due to an applied external field. The spins of the electrons of the nonmagnetic metal align parallel or antiparallel with an applied magnetic field in equal numbers, and therefore suffer less magnetic scattering when the magnetizations of the ferromagnetic layers are parallel. The effect is exploited commercially by manufacturers of hard disk drives. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.
Founding results of Fert et al. Multilayer GMRTwo or more ferromagnetic layers are separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe). At certain thicknesses the RKKY coupling between adjacent ferromagnetic layers becomes antiferromagnetic, making it energetically preferable for the magnetizations of adjacent layers to align in anti-parallel. The electrical resistance of the device is normally higher in the anti-parallel case and the difference can reach more than 10% at room temperature. The interlayer spacing in these devices typically corresponds to the second antiferromagnetic peak in the AFM-FM oscillation in the RKKY coupling. The GMR effect was first observed in the multilayer configuration, with much early research into GMR focusing on multilayer stacks of 10 or more layers. Spin valve GMR Two ferromagnetic layers are separated by a thin (about 3 nm) non-ferromagnetic spacer, but without RKKY coupling. If the coercive fields of the two ferromagnetic electrodes are different it is possible to switch them independently. Therefore, parallel and anti-parallel alignment can be achieved, and normally the resistance is again higher in the anti-parallel case. This device is sometimes also called a spin valve.
Spin-valve GMR Spin valve GMR is the configuration that is industrially most useful, and is used in hard drives. Granular GMRGranular GMR is an effect that occurs in solid precipitates of a magnetic material in a non-magnetic matrix. In practice, granular GMR is only observed in matrices of copper containing cobalt granules. The reason for this is that copper and cobalt are immiscible, and so it is possible to create the solid precipitate by rapidly cooling a molten mixture of copper and cobalt. Granule sizes vary depending on the cooling rate and amount of subsequent annealing. Granular GMR materials have not been able to produce the high GMR ratios found in the multilayer counterparts. Source: Wikipedia
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