Sunday, February 5, 2012

Spintronics: Introduction
- J.D. Patterson & M.M. Patterson (father and daughter)

S. Goudsmit and G.E. Uhlenbeck are given the credit for introducing the concept of electron spin in 1925 when they were trying to interpret certain spectra of alkali atoms.  In 1928, P.A.M. Dirac combined quantum mechanics and special relativity to develop the Dirac equation.  It explained the spin and related magnetic moment of the electron.  In other words, electrons have two degrees of freedom; charge and spin.  Modern electronics is mostly based on controlling electron flow using charge.  Spintronics exploits electron spin (as well as charge) to control the device operation (Bland, et al.).  Spintronic devices control the flow of "spin-current", (a.k.a spin transport).  S.A. Wolf in 1996 coined the name spintronics, short for spin transport electronics (Zutic, et al.).

The magnetic moments allowed an electron; spin-up and spin-down, from Prof. Petr Maly's research page.

Conventional silicon technology, based on electronics, is beginning to approach fundamental size limits (Bland).  It is getting increasingly difficult to follow Moore's "law":  chip density doubles every two years.  Spintronics technology may allow industry to continue to reduce chip density.

The smallness of devices is limited by the heat generated and dissipated, and by quantum mechanical effects not planned for, such as tunneling.  These degrade, and finally limit device operation.  Spintronics has an advantage over electronics: flipping spin is faster and lower-power than pushing an electron.  Spintronic devices may be faster, smaller and require less battery power than purely electronic devices based on charge motion (Bland).

Besides ordinary materials (Spintronics:  Appendix A), low-dimensional semiconductor systems (a.k.a. nanoelectronics) such as quantum wells (Zutic) may allow increased flexibility in manipulating the charge and spin properties of the electrons.  For spintronics devices, basic physics is needed to understand the interaction between the electron's spin and its solid-state environment.

The most widespread application of spintronics is giant magnetoresistance (GMR).  It differs from anisotropic magnetoresistance (AMR) first discovered by Lord Kelvin in 1856 (Sokolov).  Kelvin observed that in Fe there is an increase of resistance when the magnetization is parallel to the current direction, and a decrease of resistance when the magnetization is perpendicular to the current.  This effect comes from spin-orbit coupling.  Spin-orbit coupling is often the way spin can be affected by macroscopic field effects.  AMR produces changes of order 1% or so.
Peter Grunberg and Albert Fert won the 2007 Nobel Prize for Physics for discovering Giant Magnetoresistance (GMR), which produces roughly 10% difference in resistances as above.  It is now routinely used in iPods and also in hard drives in modern computers.  GMR is a term coined to describe the behavior of materials consisting of alternating layers of ferromagnetic and non-magnetic metals deposited on an insulating substrate.

Spintronics may also facilitate new sorts of devices, such as quantum computers (Awschalom, et al., 2002Awschalom, et al., 2007).  In proposed spintronic devices, it is convenient to have a way to change electron spin without using an external magnetic field.  This can be done with internal magnetization, optical devices or resonance methods.  These effects can be found in the literature (Zutic).  Generation of magnetic fields can be energy inefficient.

In summary, spintronic devices offer the possibility of increased speed, lower power consumption, smaller sizes and non-volatility, compared to charge-only-based devices.