Spintronics

Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome. All spintronic devices act according to the simple scheme: (1) information is stored (written) into spins as a particular spin orientation (up or down), (2) the spins, being attached to mobile electrons, carry the information along a wire, and (3) the information is read at a terminal. Spin orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femtoseconds during which electron momentum decays), which makes spintronic devices particularly attractive for memory storage and magnetic sensors applications, and, potentially for quantum computing where electron spin would represent a bit (called qubit) of information.

Conventional use of electron state within a semiconductor is a purely binary proposition, where an electron's state represents only 0 or 1, and a range of eight bits can represent every number between 0 and 255, but only one number at a time. Spintronics quantum bits (known as qubits) exploit the "spin up" and "spin down" states as superpositions of 0 or 1, possessing the ability to represent every number between 0 and 255 simultaneously.

Spintronics is likely to have radical implications in the field of mass-storage devices; recently (in 2002) IBM scientists announced that they could compress massive amounts of data into a small area, at approximately one trillion bits per square inch (1.5 Gbit/mm²) or roughly 1 TB on a single sided 3.5" diameter disc.

Less radical spintronic devices would allow one to put a pair of signals through a single wire, by using spin polarised electrons and producing a different signal on spin up electrons to the spin down. This has the effect of doubling the bandwidth of the cable.

In order to make a spintronic device, the primary requirement is to have a system that can generate a current of spin polarised electrons, and a system that is sensitive to the spin polarisation of the electrons. The radical devices would also have a unit in between these that does some processing to the current of electrons, dependent on the spin states.

The simplest method of generating a spin polarised current is to pass the current through a ferromagnetic material. In order for this to work, the ferromagnetic must be a single crystal, so that it filters the electrons in a uniform manner. Similarly, putting a ferromagnetic filter in front of a device such as a transistor will make it a spin sensitive detector. If the two magnetic fields are aligned, then a current is passed, whereas if opposed then the resistance of the whole system is higher. This effect is known as giant magnetoresistance (GMR).

It can be claimed that the most successful spintronic device to date is the spin valve. This is a device, utilising a layered structure of magnetic materials, which displays a huge sensitivity to magnetic fields. When in the presence of a magnetic field, it allows all electrons through, but in the absence of such a field, it only lets electrons with some spins through. This shows sensitivity to very weak fields, and has been used as the transducer in computer hard disk drive heads since about 2001, becoming common in 2002.