Spintronics demistified


Spins on a finite-size frustrated lattice:
a Mo72Fe30 supramolecular magnet
Source: ILL

Spintronics might be the future of electronics, but what is it?

In short, it is a way of getting over a fundamental limitation in electronics: that electrons have charge and every time a charge is moved to do something, some of the effort gets turned into heat.

Quite separately from charge, electrons have a property called spin, and it is this property that might one day be exploited in spintronics to move information without losing so much energy.

So, what is spin?

“An electron has charge which is electrical and spin which is magnetic. In spintronics, we would use spin in addition to charge,” Dr Paul Steffens, a neutron scientist studying magnetism at the ILL neutron lab in Grenoble. “Magnetism can come from loop of current: a loop of wire, or an electron circling an atom, or an electron’s magnetic moment. It is not completely absurd to think of spin as a little current loop. It is a picture that can help. Think of it as a tiny magnet which you can manipulate with a magnetic field.”

Spin is already used commercially, in ‘GMR’ read heads within hard disc drives.

“There are many things that can happen with spin. On a large scale, iron staying magnetic is the most useful example. Data storage on a hard disc with information in magnetic regions is on the limit of where quantum effects play a role, where we have to think in terms of quantum mechanics,” said Steffens.

For the purposes of spintronics, the spin of a particular electron can either be ‘spin up’ or ‘spin down’, and the handy thing about spin is that it can move around without capacitively coupling into the surroundings wasting energy.

“If you could just move the spin part of the electron, you would generate less heat, but if you only have one electron, you can’t move spin and not charge,” ILL theoretical physicist Dr Tim Ziman told Electronics Weekly. “If there are many electrons, we can move spin ups in one direction and spin downs in the other direction. Then, to a first approximation, spin is actually moving but there is no net movement of charge because there is no net movement of electrons.”

A first approximation?

“They do exchange charge and spin, but that is not the important thing,” said Ziman.

Charge current and spin current have different characteristics, he went on to explain: Charge current keeps on moving while spin current is at the mercy of ‘spin diffusion’ – the random flipping of spin states – which means net spin flow soon peters out.

Luckily, spin diffusion need not be a barrier to spintronics as, in good conditions, spin can travel over micrometres before it diffuses, which is plenty if spintronics is to follow the ever-smaller model of electronics. This does mean that data has to arrive and leave in electronic form, with suitable converters around the spintronics.

“One of the problems, which is partially solved, is to convert charge current into spin current and then detect spin as electrical polarisation,” said Ziman, whose is seeking ways to make strong spin currents: “I am looking at co-operative scattering, for example spin waves.”

‘Skew scattering’ one way of separating spin up and spin down electrons, said Ziman: “You need to do it efficiently. I don’t think anyone has made a useful circuit yet.”

Methods of both storing and moving information have been proposed and, with the less-than-perfect conversion available, “people have made devices in last five to ten years, and have integrated them into normal electronic circuits”, said Ziman.

Quantum computing

Spintronics is closely related to two other research fields, and developments in any of them can benefit all three.

One is quantum computing. “Devices are feasible. You want quantum interference; it is part of the same technology, at a similar stage, with a similar community working on it,” said Ziman.

The other is manipulating magnetic domain walls with electrical currents. “This is similar to spintronics, it is really part of it, for memory rather than logical operations. There is a lot of similar physics.”

Slightly less related is a push to move NMR – the body scanning technique – into the nano world. “It could revolutionise biology,” said Ziman.

Why do magnetics and spin scientist work at a neutron laboratory?

The answer is that neutrons have spin, but carry no charge.

Just as x-rays can reveal the structure of molecules and crystals – as they did with DNA – neutrons can reveal a material’s spin structure without being confused by its internal charge structure.

In the ILL case, the beam comes from a nuclear reactor and is the most powerful in the world, according to Ziman. It is around 1cm in diameter, and its diffraction is analysed after it has passed through a sample of the material under scrutiny.

“Neutrons are best for looking at the magnetic properties of bulk material. You can’t put a nanowire into a neutron beam and expect a result,” said Ziman. “Neutrons are also good for viewing domain walls, and exotic things like magnetic skyrmions which are the smallest domains and form lattices that could store data.”

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