Spintronics closer as scientists find new method for generating spin currents

The laser pulse hits nickel (green), exciting the electrons, which move towards the silicon (yellow), causing more spin up electrons (red) to pass into the silicon than spin down ones (blue)

The emerging technology of spintronics, which utilizes the spin of electrons to store and manipulate data, promises devices that are faster and more energy efficient than conventional electronics, but a major obstacle has been how to effectively generate the spin current in the first place. Now, scientists have formulated a new method for quickly creating currents using ultra short laser pulses.

Electronic chips transport data through electrical charges, but spintronics does so via the angular momentum of electrons – or their spin – and since they can "spin up" or "spin down", individual electrons can represent either of the two binary states. We're quickly closing in on the limit to how small we can make current electronics, but spintronics has the potential for much smaller, faster and more energy efficient devices.

We've seen breakthroughs in simultaneous storage and processing, controlling the spin of electrons in motion and real-time detection of the flow, and now getting the spin current flowing in the first place can be added to the list. Initial research efforts in this area focussed on the use of ferromagnetism. This was because a magnetic field is created when many electrons in a metal are spinning in the same way, so using ferromagnets to control how the electrons are spinning seemed like an obvious avenue to pursue.

"There have been attempts to send an electric current through a combination of magnets and semiconductors", says Marco Battiato, one of the researchers on the project. "The idea is to create a flux of electrons with uniform spin, which can then be used for spintronic circuits. But the efficiency of this method is very limited."

Instead, Battiato and fellow TU Wien scientist Karsten Held developed a new method for generating the spin current, which can be performed extremely quickly. In computer simulations, the researchers attached a layer of nickel to silicon, and zapped the nickel with short laser pulses. This excites the electrons in the nickel to move towards the silicon, with some then passing through into it. The key is that spin up electrons can move much more freely in nickel than spin down ones, and so the majority of those that reach the barrier and pass into the silicon are electrons with a spin up current.

In doing this the team has effectively injected silicon with a specific spin current, without creating an electrical charge. The researchers have calculated that the current created is much stronger than those produced through other methods, and can be done extremely quickly – within quadrillionths of a second.

"Spintronics has the potential to become a key technology of the next few decades", says Battiato. "With our spin injection method there is now finally a way to create ultrafast, extremely strong spin currents."

Of course, at this stage the method has only been performed in computer simulations, but the team is currently working to bring it to physical experiments.

The team's paper appears in the journal Physical Review Letters.

Source: TU Wien

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