Monday, February 6, 2012

Is there such a thing as "Electric Field Force"?

In class one day, each student wrote down a concept that was fuzzy to them on a note card. The cards got shuffled and redistributed. The note card brought up a question of the electric field force. Guessing that they meant electric field intensity, or simply, electric field, I will explain.

The electric field, E, is a vector quantity that corresponds to how strong the force is per coulomb, on a single charge, q.  The equation then is E = F/q. The electric force, F, if not given, can be calculated by using the equation F =q1 q2 / r2. Naturally there must be two charges for this, q1 and q2. To explain the concept a little better, you have a single particle or source emitting an electric field. The electric field strength depends on how far the electrons (or any other charged object) are from the source particle, and the magnitude of the charge on the source.

Say you want to know how strong that field is at your test particle. The distance between the source of the electric field and your test particle is extremely crucial in finding the electric field value at your test particle. Notice that the source charge is not in the equation E= F/q; it’s because you don’t need it. You will need it if the force is not given, and in that case a distance would be useful as well. The electric field strength at your electron or test charge is the force, on that charge, divided by its charge.

Understanding how to use the electric force equation and the electric field intensity equations are the first building block to being able to work with electric fields.  And, yes, there is an electric field force, when you use "electric field" as a modifier for force.

- Shannon Strutz

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.