In retrospect, it seems surprising that, although spin and charge are two of the most fundamental properties of electrons, the advantage that could be gained from combining them in a consumer device was only realized in the 1990s, when IBM introduced a new type of hard-disk drive that would revolutionize data storage.
Crucial to this technological revolution was an earlier discovery made by the groups of Albert Fert and Peter Grünberg — for which the two won the 2007 Nobel Prize in Physics. In 1988, they had observed a large change in the electrical resistance of thin metal layers as a function of an external magnetic field. Although bulk magnetoresistive effects had been known for more than a century (discovered by William Thomson, later to become Lord Kelvin), they were usually only moderate. However, in the studies led by Fert and Grünberg, the effect was much more pronounced. From the outset, these thin magnetic multilayer structures showed magnetoresistance of up to 50%; the phenomenon was dubbed 'giant' magnetoresistance (GMR).
GMR is based on the spin-dependent scattering of electrons travelling across metallic thin films. In its most basic realization, a GMR device consists of two thin magnetic metal films, separated by a non-magnetic metal. If the magnetic layers have a different magnetic orientation with respect to each other, the electrons scatter strongly in the trilayer structure and the electrical resistance is high. However, once the magnetic orientation of the magnetic layers is aligned using an external magnetic field, electrons with spins antiparallel to that direction scatter much less, and move more easily between the magnetic and non-magnetic layers — hence, the electrical resistance is low.
This groundbreaking discovery quickly led to the use of GMR to miniaturize the recording heads of hard-disk drives. IBM had already, in 1991, developed a hard drive based on the smaller bulk magnetoresistance effect; in 1997, thanks largely to the efforts of Stuart Parkin and colleagues in the IBM laboratories, the first hard drives based on GMR were commercialized.
More recently, a new magnetoresistive device has been incorporated into hard-disk drives — the magnetic tunnel junction. Magnetic tunnel junctions (introduced in 1975 by Michel Jullière) are similar in structure to the GMR trilayers, except that the metallic non-magnetic layer is replaced by an insulating layer. However, it was only in 1995, following advances in techniques for growing materials, that Jagadeesh Moodera and colleagues, and Terunobu Miyazaki and Nobuki Tezuka, were able to realize tunnel junctions with practical magnetoresistance. Further work by Parkin et al. and by Shinji Yuasa and colleagues, in 2004, proved that satisfactory room-temperature operation could be achieved when the barrier layer was made of magnesium oxide.
The advance made in reducing the size of read and write heads has raised the hope that such spin-electronic effects might also be used for solid-state data storage. Indeed, the tunnel-junction structure is a useful template for such magnetoresistive random-access memory (MRAM) devices, as the two possible relative orientations of the magnetic layers could be interpreted as 'bits' in a storage device. Moreover, new concepts for devices are evolving continually (see also Milestone 20) — for example, as proposed by Parkin, using the thin domain walls that separate the regions of different magnetic orientation to store data in a single nanostructure, such as a narrow wire.
