Nobel prize recognizes GMR pioneers
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
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.
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
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
Two 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
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 is the configuration that is industrially most
useful, and is used in hard drives.
Granular 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
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