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What is giant magnetoresistance?

A key development in ultrathin film magnetism were the independent reports of giant magnetoresistance (GMR) in the late 1980s by groups working at Orsay, France and at Julich, Germany. Giant magnetoresistance is a quantum mechanical phenomenon in which electrons travelling in ultrathin magnetic film multilayer structures experience large scattering according to their spin state which give rise to correspondingly large changes in the electrical resistance (typically greater than 10 %). The simplest structures which exhibit GMR consist of two inequivalent ferromagnetic layers separated by a non-magnetic spacer layer. These ultrathin layers, typically a few nm thick, behaves as 'giant magnetic molecules' in which the quantum mechanical exchange force is strong enough to completely overwhelm dipole (stray magnetic) fields and align the spins within each layer. Thus each layer behaves as though it had a single spin vector or magnetic moment which varies in orientation but not magnitude according to the strength of an applied field. When the layers are magnetically aligned, both spin up and down electrons can propagate through the structure with a low probability of scattering; however, when the layers are antiparallel, at low fields, one spin state is strongly scattered giving rise to a strong increase in the total resistance.

Because of the increased sensitivity of giant magnetoresistance sensors over conventional sensors used as hard disc read heads, GMR sensors can be made smaller than conventional sensors allowing an increase in data storage density. 5 Gb/in2 GMR read heads were demonstrated by Toshiba/Fujitsu in 1995. The GMR head introduced commercially by IBM in 1997 increased the typical hard disc capacity of PCs from around 1 to 20 Gb. Honeywell is currently developing random access memory arrays in which GMR sensors are used as the active elements. Such non-volatile memory could make a major impact on the RAM market in future (currently worth around $100 bn per year).

Magnetoelectronics

The discovery of GMR shows the potential for finding new spin-dependent electron transport phenomena in nanostructures. A new field is now envisioned in which the spin of the electron is exploited in heterostructure devices which combine ferromagnetic films and semiconductor structures. By finding materials which create a large electron spin polarization and by controlling the spin transmission at interfaces it is envisaged that it will be possible to use the electron spin (up and down) rather than the electrons and holes of conventional semiconductor devices in order to create much smaller devices than those currently used. Such structures will also have intrinsic memory due to the non-volatile properties of the ferromagnet and could be used to create devices which can be programmed at very high frequencies (GHz range) during the computational process itself, so allowing the device function to be defined by computer software rather than requiring new microprocessor chips. Future computers which use such devices could be highly efficient and ultrafast since computing then becomes software and not hardware driven.

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