06-12-2012, 01:16 PM
Spintronics and its application
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Spintronics
1. In addition to their mass and electric charge,
electrons have an intrinsic quantity of angular momentum
called spin, almost as if they were tiny spinning balls.
2 Associated with the spin is a magnetic field
like that of a tiny bar magnet lined up with the spin axis.
3 Scientists represent the spin with a vector.
For a sphere spinning "west to east" the vector points "north" or "up.“ It points "down" for the opposite spin.
4 In a magnetic field, electrons with "spin up" and "spin down“ have different energies.
5 In an ordinary electric circuit the spins are oriented
at random and have no effect on current flow.
6 Spintronic devices create spin-polarized currents
and use the spin to control current flow.
Spintronics Logic
The magnetic orientations of electron spin, “spin up” and “spin down” can represent “1” and “0” respectively.
Unlike traditional digital logic, which is based on existence and absence of electrons.
Its advantages over classical charge-based devices
1. Can easily manipulated by externally applied magnetic field
2. Long coherence, or relaxation time
3. Allow devices to be much smaller, consume less electricity and be more powerful in certain types of computation
GMR effect basics
What’s GMR
Giant Magnetoresistive Effect is an effect of very large resistance change in materials comprised of alternating very thin layers of various metallic elements. Usually two layers of ferromagnetic materials sandwiching one layer of non-magnetic material. The total resistance of this material is lowest when the magnetic orientations of the two ferromagnetic layers are aligned, is highest when the orientations are anti-aligned.
Ferrimagnetic minerals have two types of magnetic crystal lattice sites that naturally align antiparallel. The net magnetic moment within the ferrimagnet is due to either a difference in the ionic make up of different crystal sites, or a crystallographic inhomogeneity between different sites. In antiferromagnetic minerals there are again two different magnetic crystallographic sites, however the magnetic moments of the ions at different sites entirely cancels, hence leaving no net magnetic moment. A net magnetic moment can only exist within an antiferromagnet if its individual magnetic ion sites are not entirely antiparallel, this is called imperfect or canted antiferromagnetism. In practice ferrimagnets have strong magnetic properties and moderate coercivities. Imperfect antiferromagnets have weaker magnetic properties but very high coercivities. These differences can be used to detect them in natural materials (Thompson and Oldfield, 1984).
Temperature
Ferromagnetism is a temperature dependent phenomenon. In fact ferromagnetism and paramagnetism are at different ends of a thermal energy / magnetic energy scale. At a low enough temperature paramagnetic behaviour can become ferromagnetic due to the lack of randomising thermal energy. Likewise at a high enough temperature ferromagnets can become paramagnetic due to thermal energy randomising the direction of individual ionic dipoles. The temperature at which ferromagnetism breaks down is called the Neel or Curie Temperature and is dependent on mineral composition. Indeed it is often used to identify minerals (Thompson and Oldfield, 1984).
Chief source of GMR
Spin-dependent scattering of conduction electrons
Electrons scatter little when the magnetic orientation of the material is aligned with the electron spin direction, thus the resistance is low
Electrons scatter most then the magnetic orientation of the material is anti-aligned with the electron spin direction, thus the resistance is high