Giant magnetoresistive effect Founding results of Fert et al . The Giant Magnetoresistance Effect ( GMR ) is a quantum mechanical effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic metal layers . The effect manifests itself as a significant decrease in resistance from the zero-field state , when the magnetization of adjacent ferromagnetic layers are antiparallel due to a weak anti-ferromagnetic coupling between layers , to a lower level of resistance when the magnetization of the adjacent layers align due to an applied external field . The spin 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 . Discovery Magnetoresistive effects due to what was only later recognized to be Giant Magnetoresistance , were probably first seen by Sato in Co/Ag multilayers 1987 ( H. Sato et al . `` Galvanomagnetic properties of Ag/Co layered metallic films '' , Superlattices and Microstructures , Vol. 4 , No. 1 , 1988 ) . However , the true genesis of the effect involved studies of Fe/Cr/Fe trilayers in 1988 by a research team led by Peter Grünberg of the Jülich Research Centre , who owns the patent , and of Fe/Cr multilayers by the group of Albert Fert of the University of Paris-Sud , who first saw the large effect in multilayers that led to its naming , coined the name , and first correctly explained the underlying physics . Types of GMR 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 several 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 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 spin-valve . Spin-valve GMR is the configuration that is most industrially useful , and is the configuration used in hard-drives . Granular GMR 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 counterparts . Applications As stated above , GMR has been used extensively in the read heads in modern hard drives . Another application of the GMR effect is in non-volatile , magnetic random access memory ( MRAM ) . References Magnetic properties of superlattices formed from ferromagnetic and antiferromagnetic materials , L. L. Hinchey and D. L. Mills , Physical Review B , vol. 33 , no. 5 , pp 3329 , March 1986 . Layered Magnetic Structures : Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers , P. Grünberg , R. Schreiber , Y. Pang , M. B. Brodsky , and H. Sowers , Physical Review Letters , vol. 57 , no. 19 , pp 2442 , November , 1986 . Antiparallel coupling between Fe layers separated by a Cr interlayer : Dependence of the magnetization on the film thickness , C. Carbone and S. F. Alvarado , Physical Review B , vol. 36 , no. 4 , pp 2433 , August 1987 . Giant Magnetoresistance of ( 001 ) Fe/ ( 001 ) Cr Magnetic Superlattices , M. N. Baibich , J. M. Broto , A. Fert , F. Nguyen Van Dau , F. Petroff , P. Eitenne , g . Creuzet , A. Friederich , and J. Chazelas , Physical Review Letters , vol. 61 , no. 21 , pp. 2472 , November 1988 . See also Magnetoresistance Colossal magnetoresistance Categories : Condensed matter physics | Electric and magnetic fields in matter | Quantum electronics | Spintronics In other languages : Deutsch | Español | Français | 日本語 | Polski | Српски / Srpski 