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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.

Contents 1 Discovery 2 Types of GMR 2.1 Multilayer GMR 2.2 Spin valve GMR 2.3 Pseudo-spin valve 2.4 Granular GMR 3 GMR's relation to TMR 4 Applications 5 References 6 See also 7 External links

[edit] 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.

[edit] Types of GMR

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] References

1. ^ Stoner-Leeds

[edit] See also

• Magnetoresistance
• Colossal magnetoresistance
• Tunnel magnetoresistance

[edit] 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 Scientific backgroundPDF (472 KB)
• Prize Walf