Giant magnetoresistive effect - AbsoluteAstronomy.com

Giant magnetoresistance (GMR) is a quantum mechanical

Quantum mechanics

Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

magnetoresistance

Magnetoresistance

Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...

effect observed in thin-film structures composed of alternating ferromagnetic and non-magnetic layers. The 2007 Nobel Prize in physics

Nobel Prize in Physics

The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

was awarded to Albert Fert

Albert Fert

Albert Fert is a French physicist and one of the discoverers of giant magnetoresistance which brought about a breakthrough in gigabyte hard disks...

and Peter Grünberg

Peter Grünberg

Peter Andreas Grünberg is a German physicist, and Nobel Prize in Physics laureate for his discovery with Albert Fert of giant magnetoresistance which brought about a breakthrough in gigabyte hard disk drives.-Biography:...

for the discovery of GMR.

The effect is observed as a significant change in the electrical resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

depending on whether the magnetization

Magnetization

In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...

of adjacent ferromagnetic layers are in a parallel or an antiparallel

Antiparallel (electronics)

In electronics, two anti-parallel or inverse-parallel devices are connected in parallel but with their polarities reversed.One example is the TRIAC, which is comparable to two thyristors connected back-to-back , but on a single piece of silicon.Two LEDs can be paired this way, so that each protects...

alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment.

GMR is used commercially by hard disk drive manufacturers.

Discovery

GMR was first discovered in 1988, in Fe

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

/Cr

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

/Fe trilayers by a research team led by Peter Grünberg

Peter Grünberg

of Forschungszentrum Jülich (DE), who owns the patent. It was also simultaneously but independently discovered in Fe/Cr multilayers by the group of Albert Fert

Albert Fert

Albert Fert is a French physicist and one of the discoverers of giant magnetoresistance which brought about a breakthrough in gigabyte hard disks...

of the University of Paris-Sud (FR). The Fert group first saw the large effect in multilayers that led to its naming, and first correctly explained the underlying physics but, due to administrative reasons, they only filed the patent some days after the German team. The discovery of GMR is considered the birth of spintronics

Spintronics

Spintronics , also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.An additional effect occurs when a spin-polarized current is...

. Grünberg and Fert have received a number of prestigious prizes and awards for their discovery and contributions to the field of spintronics, including the 2007 Nobel Prize in Physics

Nobel Prize in Physics

Multilayer GMR

In multilayer GMR 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

RKKY

RKKY stands for Ruderman-Kittel-Kasuya-Yosida and refers to a coupling mechanism of nuclear magnetic moments or localized inner d or f shell electron spins in a metal by means of an interaction through the conduction electrons....

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

An antiparallel configuration is shown on the right and a parallel configuration on the left. FM stands for ferromagnetic metal, NM for normal metal, ↑ is a spin up electron and ↓ is a spin down electron. The vertical black arrows in the FM layers show the direction of the magnetisation. The arrows across the spin valves show the electron path. A bend in the path shows that an electron was scattered.

An electron passing through the spin-valve will be scattered more if the spin of the electron is opposite of the direction of the magnetisation in the FM layer. This principle is used to construct an equivalent electric circuit representation of the two configurations of the spin-valve. The size of the resistor represents the amount of resistance. Computation of the equivalent resistance for the antiparallel and parallel configurations shows that the parallel alignment has the lower resistance.

Research to improve spin valves is focused on increasing the magnetoresistance ratio by practical methods such as increasing the resistance between individual layers interfacial resistance, or by inserting half metallic layers into the spin valve stack. These work by increasing the distances over which an electron will retain its spin (the spin relaxation length), and by enhancing the polarization effect on electrons by the ferromagnetic layers and the interface. At the National University of Singapore, S. Bala Kumar and collaborators experimented with the interfacial resistance principle to show that magnetoresistance is suppressed to zero in NiFe/Cu/NiFe spin-valve at high amounts of interfacial resistance.

A high performance from the spin valve is achieved using a large GMR. The GMR ratio is maximised by finding the optimal resistance and polarization of the interface between layers.

Spacer materials include Cu (copper), and ferromagnetic layers use NiFe (permalloy

Permalloy

Permalloy is a nickel-iron magnetic alloy, with about 20% iron and 80% nickel content. It is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields...

), which are both widely studied and meet industrial requirements.

Granular GMR

Granular GMR is an effect that occurs in solid precipitates of a magnetic material in a non-magnetic matrix. To date, granular GMR has only been 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

Annealing (metallurgy)

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...

. Granular GMR materials have not been able to produce the high GMR ratios found in the multilayer counterparts.

GMR and Tunnel magnetoresistance (TMR)

Tunnel magnetoresistance

The Tunnel magnetoresistance is a magnetoresistive effect that occurs in magnetic tunnel junctions . This is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough , electrons can tunnel from one ferromagnet into the other...

(TMR) is an extension of spin valve GMR in which the electrons travel with their spins oriented perpendicularly to the layers across a thin insulating tunnel barrier (replacing the non ferromagnetic spacer). This allows a larger impedance, a larger magnetoresistance

Magnetoresistance

value (~10x at room temperature) and a ~0 temperature coefficient to be achieved simultaneously. TMR has now replaced GMR in disk drives, in particular for high area densities and perpendicular recording. TMR has led to the emergence of MRAM

MRAM

Magnetoresistive Random-Access Memory is a non-volatile computer memory technology that has been under development since the 1990s. Continued increases in density of existing memory technologies – notably flash RAM and DRAM – kept it in a niche role in the market, but its proponents...

memories and reprogrammable magnetic logic devices.

Applications

GMR has triggered the rise of a new field of electronics called spintronics

Spintronics

which has been used extensively in the read heads

Disk read-and-write head

Disk read/write heads are the small parts of a disk drive, that move above the disk platter and transform platter's magnetic field into electrical current or vice versa – transform electrical current into magnetic field...

of modern hard drives and magnetic sensor

Sensor

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

s. A hard disk storing binary information can use the difference in resistance between parallel and antiparallel layer alignments as a method of storing 1s and 0s.

A high GMR is preferred for optimal data storage density. Current perpendicular-to-plane (CPP) Spin valve

Spin valve

A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on...

GMR currently yields the highest GMR. Research continues with older current-in-plane configuration and in the tunnelling magnetoresistance (TMR) spin valves which enable disk drive densities exceeding 1 Terabyte per square inch.

Hard disk drive manufacturers have investigated magnetic sensors based on the colossal magnetoresistance

Colossal magnetoresistance

Colossal magnetoresistance is a property of some materials, mostly manganese-based perovskite oxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field...

effect (CMR) and the giant planar Hall

Planar Hall sensor

The planar Hall sensor is based on the planar Hall effect of ferromagnetic materials.It measures the change in anisotropic magnetoresistance caused by an external magnetic field in the Hall geometry. The sensor responds to magnetic field components in the sensor plane as opposed to the ordinary...

effect. In the lab, such sensors have demonstrated sensitivity which is orders of magnitude stronger than GMR. In principle, this could lead to orders of magnitude improvement in hard drive data density. , only GMR has been exploited in commercial disk read-and-write head

Disk read-and-write head

s because researchers have not demonstrated the CMR or giant planar hall effects at temperatures above 150K.

Magnetocoupler is a device that uses giant magnetoresistance (GMR) to couple two electrical circuits galvanicly isolated and works from AC down to DC.

Vibration measurement in MEMS

Microelectromechanical systems

Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...

systems.

Detecting DNA or protein binding to capture molecules in a surface layer by measuring the stray field from superparamagnetic label particles.

External links

Giant Magnetoresistance: The Really Big Idea Behind a Very Tiny Tool National High Magnetic Field Laboratory
Presentation of GMR-technique (IBM Research)
Nobel prize in physics 2007 - Nobel Foundation (also
Prize Walf

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