Giant magnetoresistance: Wikis

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Founding results of Fert et al.

Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers.

The effect manifests itself as a significant decrease (typically 10–80%) in electrical resistance in the presence of a magnetic field. In the absence of an external magnetic field, the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers. The result is high-resistance magnetic scattering as a result of electron spin.

When an external magnetic field is applied, the magnetization of the adjacent ferromagnetic layers is parallel. The result is lower magnetic scattering, and lower resistance.[1]

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.

Hard disk drive manufacturers are looking at such as colossal magnetoresistance effect (CMR) and giant planar Hall effect magnetic sensors. In the lab, such sensors have demonstrated sensitivity orders of magnitude stronger than GMR, and in principle could lead to orders of magnitude improvement in hard drive data density. As of 2003, GMR is the only one used in commercial disk read-and-write heads, because researchers have not yet demonstrated CMR or giant planar hall effect at temperatures above 150 K. [2]

Contents

Discovery

GMR was discovered in 1988 in Fe/Cr/Fe trilayers by a research team led by Peter Grünberg of the Jülich Research Centre (DE), who owns the patent. It was also simultaneously but independently discovered in Fe/Cr multilayers by the group of Albert Fert 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. The discovery of GMR is considered the birth of spintronics. 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.

Types of GMR

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 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

Spin-valve GMR

In spin valve GMR two ferromagnetic layers are separated by a thin non-ferromagnetic spacer (~3 nm), 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.

Research to improve spin valves is intensely focused on increasing the MR 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. The magnetic properties of nanostructures (and all properties) are dominated by surface and interface effects due to the high local ratio of atoms as compared to the bulk.

At the National University of Singapore, Z.Y. Leong and collaborators experimented with the interfacial resistance principle to show the magnetoresistance is suppressed to zero in NiFe/Cu/NiFe spin-valve at high amounts of interfacial resistance.

The crucial trick for maximizing GMR ratio is to find the optimal resistance and polarization of the interface between layers to yield high performance from the spin valve.

Spacer materials include Cu (copper), and ferromagnetic layers use NiFe (permalloy), which are both widely studied and meet industrial requirements.

Current perpendicular to plane (CPP) Spin valve GMR is the configuration that currently yields the highest GMR and thus is the configuration used in hard drives. Research is ongoing in the older current-in-plane configuration, and in the tunneling magnetoresistance (TMR) spin valves which enable disk drive densities exceeding 1 Terabyte per square inch.

Pseudo-spin valve

Pseudo-spin valve devices are very similar to the spin valve structures. The significant difference is the coercivities of the ferromagnetic layers. In a pseudo-spin valve structure a soft magnet will be used for one layer; where as a hard ferromagnet will be used for the other. This allows the applied field to flip the magnetization of one layers before the other, thus providing the same anti-ferromagnetic affect that is required for GMR devices. For pseudo-spin valve devices to work they generally require the thickness of the non-magnetic layer to be thick enough so that exchange coupling is kept to a minimum. It is imperative to prevent the interaction between the two ferromagnetic layers in order to exercise complete control over the device.

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

GMR's relation to TMR

Tunnel magnetoresistance (TMR) is an extension of spin valve GMR in which the (electrons) spins travel perpendicularly to the layers across a thin insulating tunnel barrier (replacing the non ferromagnetic spacer). By doing so, one can simultaneously achieve a larger impedance (therein matching that of circuit electronics), a larger magnetoresistance value (~10x at room temperature), and a ~0 temperature coefficient. TMR has now replaced GMR in disk drives, in particular for high areal densities and perpendicular recording, and has fueled the emergence of competitive MRAM memories. It is also the building block for numerous spin electronics applications such as reprogrammable magnetic logic devices.

Applications

GMR has been used extensively in the read heads in modern hard drives and magnetic sensors. An application of the TMR effect, which is closely related to the GMR effect, is in magnetoresistive random access memory (MRAM), a type of non-volatile semiconductor memory. GMR has triggered the rise of a new field of electronics called spintronics.

References

  1. ^ Stoner-Leeds
  2. ^ Stephen Cass (July 2003). "Flash of Brilliance or Flash in the Pan? Giant planar Hall effect could be the next big thing in data storage". IEEE Spectrum: 18. 

See also

External links


Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and non magnetic layers. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.

The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel 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.

Contents

Discovery

GMR was first discovered in 1988, in Fe/Cr/Fe trilayers by a research team led by Peter Grünberg of the Jülich Research Centre (DE), who owns the patent. It was also simultaneously but independently discovered in Fe/Cr multilayers by the group of Albert Fert 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. The discovery of GMR is considered the birth of spintronics. 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.

Types of GMR

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 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

In spin valve GMR two ferromagnetic layers are separated by a thin non-ferromagnetic spacer (~3 nm), 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.

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

Pseudo-spin valve

Pseudo-spin valve devices are very similar to the spin valve structures. The significant difference is the coercivities of the ferromagnetic layers. In a pseudo-spin valve structure a soft magnet will be used for one layer; where as a hard ferromagnet will be used for the other. This allows an applied field to flip the magnetization of the hard ferromagnet layer. For pseudo-spin valves, the non-magnetic layer thickness must be great enough so that exchange coupling minimized. This reduces the chance that the alignment of the magnetisation of adjacent layers will spontaneously change at a later time......

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. 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 (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 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 memories and reprogrammable magnetic logic devices.

Applications

GMR has triggered the rise of a new field of electronics called spintronics which has been used extensively in the read heads of modern hard drives and magnetic sensors. 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 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 effect (CMR) and the giant planar Hall 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. As of 2003, only GMR has been exploited in commercial disk read-and-write heads 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.[1]

Vibration measurement in MEMS systems.[1]

Detecting DNA or protein binding to capture molecules in a surface layer by measuring the stray field from superparamagnetic label particles.[1]

See also

References

  1. ^ a b c "Novel Magnetoelectronic Materials and Devices, 2003". http://web.phys.tue.nl/fileadmin/tn/de_faculteit/capaciteitsgroepen/FM/FNA/Students_Education/Lectures_Courses/Coehoorn_Lecture-Notes-SVs-Part1-final.pdf.  100615 web.phys.tue.nl

External links


Simple English


Giant magnetoresistance (GMR) is a very small magnetic effect found in thin layers of iron and other material. It is used to read and write information in hard drives.

The GMR effect can be measured when a magnet is used to change the flow of electricity. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.

Contents

Discovery

GMR was discovered in layers iron, chrome and ferrite by Peter Grünbergs research team of the Jülich Research Centre (Germany) in 1988. Peter Grünberg owns a patent for the technology. It was also discovered by Albert Ferts research group of the University of Paris-Sud (France) in layers ferrite and chrome. The Fert group were first to see what they thought of as a large effect which is why they gave the name "Giant". The Fert group were also first to explain the correct physics of GMR. The discovery was the beginning of the science spintronics. Grünberg and Fert have been given prizes and awards, including the 2007 Nobel Prize in Physics, for this discovery and other spintronics work.

Types of GMR

Multilayer GMR

In multilayer GMR, two or more magnetic layers are separated by a very thin (about 1 nm) non-magnetic (insulating) layer. Ferrite, a form of iron, is a magnetic layer and chrome is an insulating layer. At certain thicknesses, the strength of magnetism between the layers becomes easy to measure and adjust. The strength of electrical current between the layers can change by up to 10%.

The GMR effect was first observed in stacks of 10 or more layers.

Spin valve GMR

In spin valve GMR, two magnetic layers are separated by a thin (~3 nm) non-magnetic (insulating) layer. It is possible to measure and adjust the strength of magnetism between these layers.

It is hoped that research into spinning electrons will improve spin valves.

Materials used in spin valves are copper and an alloy of nickel and iron.

Spin valve GMR is the most useful sort for hard drives and is tested carefully to meet industry standards.

Granular GMR

Granular GMR is an effect found in copper containing grains of cobalt. It is not possible to control the strength of granular GMR in the same manner as Multilayer GMR.

Use of GMR

GMR is used in modern hard drives and magnetic sensors. Another use of the GMR effect is in magnetoresistive random access memory (MRAM). GMR has begun a new science of electronics called spintronics.

References

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