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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. There is a variety of effects that can be called magnetoresistance, some of them occurring in bulk non-magnetic metals and semiconductors (e.g. geometrical magnetoresistance, Shubnikov de Haas oscillations or the common positive magnetoresistance in metals〔A.B. Pippard: Magnetoresistance in Metals, Cambridge University Press (1989)〕), others in magnetic metals (e.g. negative magnetoresistance in ferromagnets or AMR) and since 1980's magnetoresistive effects in multicomponent or multilayer systems (e.g. magnetic tunnel junctions) gained importance (GMR, TMR, EMR). The first magnetoresistive effect was discovered by William Thomson (better known as Lord Kelvin) in 1851, but he was unable to lower the electrical resistance of anything by more than 5%. Nowadays, systems are known (e.g. semimetals or concentric ring EMR structures) where magnetic field can change resistance by orders of magnitude. As the resistance may depend on magnetic field through various mechanisms, it is useful to separately consider situations where it depends on magnetic field directly (e.g. geometric magnetoresistance, multiband magnetoresistance) and those where it does so indirectly through magnetisation (e.g. AMR, TMR).
==Discovery==
William Thomson (Lord Kelvin) first discovered ordinary magnetoresistance in 1856. He experimented with pieces of iron and discovered that the resistance increases when the current is in the same direction as the magnetic force and decreases when the current is at 90° to the magnetic force. He then did the same experiment with nickel and found that it was affected in the same way but the magnitude of the effect was greater. This effect is referred to as anisotropic magnetoresistance (AMR).
In 2007, Albert Fert and Peter Grünberg were jointly awarded the Nobel Prize for the discovery of Giant Magnetoresistance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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