Spintronics - recent developments

Let's wrap up our discussion of spintronics with a look at some recent developments in the field: creating spin-valve devices in graphene.

Graphene is a relatively new wonder-material that we'll discuss later this semester. Graphene comes in sheets made of single layers of carbon atoms: 

Image credit: http://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Graphen.jpg/800px-Graphen.jpg

Graphene is extremely strong and extremely conductive of both electric current and heat.

A spin-valve device is multiple conducting materials stacked in layers whose combined resistivity changes drastically depending on whether their magnetizations are parallel or antiparallel. (Sound familiar?) In other words, this device permits or prevents current passing through with a simple switch of the magnetic field, just like a faucet permits or prevents water passing through with the turn of a handle.

In http://arxiv.org/pdf/1407.1439.pdf, Fu et al discuss the creation of spin-valve devices with graphene using chemical vapor deposition, which assembles nanoscopic devices one layer at a time. Have a look to see spin valves in action!

Spintronics - Making it work!

This week, we're learning about spintronics - manipulating the spins of individual electrons to store information. Unfortunately, electron spins tend to "reset" (lose the spin orientation that we set up) after only a hundred picoseconds (a picosecond = 10^-12 seconds), which is too short to be read by our computer processors. (Remember: period = 1/frequency! So, for example, a 1 gigahertz processor would need the spins to have a lifetime of at least 1/(10^9 Hz) = 10^-9 seconds.) 

Fortunately, in 2012, IBM announced they had successfully synchronized the spins of clusters of electrons, increasing the spin lifetime by a factor of 30.

Here's an example of their data (with time increasing as you move up the vertical axis), showing that the spins remain coherent for just over a nanosecond - enough time to be useful to a 1 gigahertz processor!

http://www.computerworld.com/common/images/site/features/2012/06/Spintronics%20photo1.jpeg

Spintronics - some important concepts

Last time, we introduced the concept of spintronics - manipulating spins of individual electrons for applications in memory storage and quantum computing. Today, we take a more detailed look at the physics concepts and material properties involved in spintronics. Science published a great review article about this topic in 2001, available at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA516289. 

Here are some concepts discussed in the article that you might like some additional resources to learn about:

Giant Magnetoresistance (GMR): Magnetoresistance (giant or small) is when a material's electrical resistance increases when the current runs parallel to an applied magnetic field. Magnetoresistance can therefore be used to interface with magnetic storage devices, but ordinary-sized magnetoresistance isn't strong enough to do the job. GMR, on the other hand, employs quantum mechanical concepts to introduce a large change in resistance, such that the resistance change can be used to read information stored in magnetic memory.

Ferromagnets: In elementary school, you might have learned to call these "permanent magnets"--materials that retain their magnetization even when there's no external magnetic field to keep all the spins in the same direction. But ferromagnets do have a weakness; if you heat them beyond their Curie temperature, they'll lose their magnetic ordering! So, when we design magnetic storage devices, it's important to know how hot they can get!

Semiconductors: On first pass, a semiconducting material is a pretty straightforward concept: It has a resistivity somewhere between the high resistivity of an insulator (letting no electrons through) and the low resistivity of a metal (letting all the electrons through). The reason a semiconductor behaves this way, though, is that its electronic band structure (the configuration of quantum states that the electrons are allowed to be in) has a small gap that can be easily manipulated:

Image credit:
 http://upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Band_gap_comparison.svg/350px-Band_gap_comparison.svg.png

If you found the article above interesting, check out these even deeper (i.e., lengthier) reviews at http://arxiv.org/pdf/cond-mat/0405528.pdf, http://arxiv.org/ftp/arxiv/papers/0711/0711.1461.pdf, and http://arxiv.org/pdf/0801.0145v1.pdf. 

Spintronics! Learn from the leaders of the field

This week, we'll take a look at the emerging field of spintronics, which offers many technological revolutions through manipulating the spins of electrons.

Spin is a fundamental property of matter, just like mass or charge. In the equations, this property behaves as if the particles were spinning like a top, so that's why we call it spin. Spin has the important property that it produces a magnetic field. In most materials, electrons' (and protons') spins are randomly oriented, so they all tend to cancel out. If you align those spins, though, you could create a powerful magnet. If you align local clusters of the spins, you can use encode information!

Spintronics is likely already being used in your computer's hard drive and refining the technology can lead to novel applications like quantum computers. 

In 2009, Physics World shared a series of video interviews with some of the leaders in the field. Have a look at these this week, and over the next few days we'll take a look at some of the breakthroughs and applications of spintronics!