Spintronics: a new spin on electronics
Spintronics is the idea of using not just the charge, but also the spin of an electron in technology.
The electron has been a crucial part of technology for nearly two hundred years now. It is a small, negatively-charged particle and we can push streams of electrons through wires to create electricity. We use this to light our homes, make telephone calls and power our industries.
Electronics, rather than electricity, uses electrons for more sophisticated purposes. Electronics doesn’t just move electrons through a wire; it controls them in specific ways so they can perform calculations. We use quantum mechanics, the physics of the very small scale, to make this work. And it is the basis of modern computers, smartphones and tablets.
However, electronics still relies on just using the charge of the electron to make these devices function. Spintronics is the idea of also using the spin of the electron for the calculations. This would effectively add an extra dimension of the kinds of things which are possible with the technology.
What is spin?
Electrons are often described as orbiting the nucleus of an atom, a bit like planets orbiting the sun. Continuing this analogy, we can think of electron spin as a planet spinning on its own axis during the orbit. There are two ways this can happen - clockwise and anti-clockwise. In the case of electrons, these are normally referred to as spin up or spin down states.
However, when we zoom down to the scale of individual electrons, quantum mechanics kicks in, and our intuitions about how the world works break down. An electron isn’t really spinning like a planet. A more technically accurate description of spin is as a combination of the angular momentum (how much something rotates) of an electron and its magnetic moment (the magnetic field it generates).
Effectively, it is just a fundamental property of the electron, like the mass or the charge. It determines the arrangement of electrons in solids and its discovery has led to a much better understanding of many physical and chemical reactions.
How do magnets work?
Electricity and magnetism are two sides of the same coin - an electric current can generate a magnetic field, and vice versa. An electric current is simply a flow of electrons between two points, and the spin of these electrons is what causes magnetism.
All electrons have spin, but not all materials are magnetic. This is because in most materials, there are an equal number of electrons which have spin up and spin down. This means that they cancel each other out. In a handful of elements, like cobalt, nickel and iron, the numbers don’t quite balance, so there is a residual spin and this makes these magnetic.
Opposites attract - so a magnet sticks to a fridge door because the spins of the magnet electrons are up (or like a north pole), while the spins of the fridge electrons are down (or like a south pole), or vice versa. It’s remarkable how strong this force is - it can hold up a magnet onto a fridge against the gravitational force that is pulling it down towards the earth!
So what can we do with spin? Other than decorating our fridges with holiday photos - what are the technological possibilities?
Spin is most commonly used for data storage and there is ongoing research on using spin for making calculations in electronic devices, and for use in medical applications.
There is even speculation that we can use the properties of spin to replicate a human brain...
Spin for data storage
Magnetic hard drives use spin to store data. They work a bit like vinyl record players - there is a circular disk which can rotate, and an arm called a read-write head which comes down over it. The disk is divided into concentric tracks, just like a record, and each track is divided into smaller sectors. The spins of the electrons in each sector can be switched using the read-write head.
A computer uses a code, called binary, to transform any type of data into a string of 1s and 0s. The read-write arm then moves along the disk, selectively switching the spins of each sector to either be up or down, to represent a 1 or a 0. When the data needs to be extracted, the arm can go back to read the string of 1s and 0s, and the computer can convert that back to human-intelligible data.
One of the most useful properties of magnetism is that it is stable. Once something is magnetised, it will stay magnetised without needing an external power source to keep it going. This makes it a good candidate for data storage - even if you switch the computer off, the data stays put. This also means that if the spins can actually be used for calculations, it can provide a low-energy alternative to conventional electronics and this is currently an active topic of research.
Spin for medical applications
As well as being a stable method of data storage, spin can also be used to move things around. Just like a bar magnet can push iron fillings around, the spin of the electron can be used as an actuator, to move things around on a tiny scale on a spintronic chip. This can be useful in a chip which needs to do a number of tasks, for example, a detector
A current topic of research focuses on building a low-cost, early-detection tool for kidney cancer. Certain molecules, or biomarkers, are released during the very early stages of kidney cancer and these are present in urine. Russell Cowburn and his team, from the University of Cambridge, are trying to develop a spintronic chip which can detect these molecules from a sample of urine, like in a pregnancy test.
In order to be an effective detector, the chip needs to see if a biomarker is present, and then also work out exactly which biomarker it is. Different parts of the chip can test this - and we need to move the biomarker from one part of the chip to another. The detector combines the biochemistry needed to test the biomarkers, with spintronics to move the molecules around, effectively creating a mini, portable chemistry lab.
A spintronic brain?
Most computers today rely on the Von Neumann architecture - this is a conceptual design for how to move electrons around to perform calculations. Even though it seems like a computer is doing lots of different things at once, each processor is actually only performing one task at a time.
This is fundamentally different to how the human brain works, where lots of different processes can take place in parallel. This means that replicating a brain with a conventional Von Neumann architecture is impossible as the approach to calculations is completely different.
By adding spin manipulation, as well as charge manipulation, into the picture, non-Von Neumann computational processes can be imagined. This means that more than one process could potentially take place at the same time, using the same electrons. Including spin adds a new dimension to the calculations that are possible.
It may seem like science fiction now, but maybe we can imagine building a spintronic brain one day...