Giant magnetoresistance

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 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.

Discovery

GMR was first discovered in 1988, in Fe/Cr/Fe trilayers by a research team led by 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 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. 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.

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^{[inconsistent]}. 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.^{[citation needed]}

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^[update], 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

• Magnetoresistance
• Colossal magnetoresistance
• Tunnel magnetoresistance
• Stuart Parkin

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

• L. L. Hinchey and D. L. Mills (1986). "Magnetic properties of superlattices formed from ferromagnetic and antiferromagnetic materials". Physical Review B 33 (5): 3329–3343. Bibcode 1986PhRvB..33.3329H. doi:10.1103/PhysRevB.33.3329. 
• P. Grünberg, R. Schreiber, Y. Pang, M. B. Brodsky, and H. Sowers (1986). "Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers". Physical Review Letters 57 (19): 2442–2445. Bibcode 1986PhRvL..57.2442G. doi:10.1103/PhysRevLett.57.2442. PMID 10033726. 
• C. Carbone and S. F. Alvarado (1987). "Antiparallel coupling between Fe layers separated by a Cr interlayer: Dependence of the magnetization on the film thickness". Physical Review B 36 (4): 2433. Bibcode 1987PhRvB..36.2433C. doi:10.1103/PhysRevB.36.2433. 
• M. N. Baibich , J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas (1988). "Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices". Physical Review Letters 61 (21): 2472–2475. Bibcode 1988PhRvL..61.2472B. doi:10.1103/PhysRevLett.61.2472. PMID 10039127. 
• G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn (1989). "Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange". Physical Review B 39 (7): 4828–4830. Bibcode 1989PhRvB..39.4828B. doi:10.1103/PhysRevB.39.4828. 
• A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, S. Zhang, F. E. Spada, F. T. Parker, A. Hutten, and G. Thomas (1992). "Giant magnetoresistance in heterogeneous Cu-Co alloys". Physical Review Letters 68 (25): 3745–3748. Bibcode 1992PhRvL..68.3745B. doi:10.1103/PhysRevLett.68.3745. PMID 10045786. 
• John Q. Xiao, J. Samuel Jiang, and C. L. Chien (1992). "Giant magnetoresistance in nonmultilayer magnetic systems". Physical Review Letters 68 (25): 3749–3752. Bibcode 1992PhRvL..68.3749X. doi:10.1103/PhysRevLett.68.3749. 
• S. Bala Kumar, M. B. A. Jalil, S. G. Tan, and Z. Y. Leong (2006). "Magnetoresistance effects arising from interfacial resistance in a current-perpendicular-to-plane spin-valve trilayer". Phys. Rev. B 74 (18): 184426. Bibcode 2006PhRvB..74r4426K. doi:10.1103/PhysRevB.74.184426. 

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