Insulating antiferromagnetic spintronics materials could bring cooler computing

Computers that produce far less heat yet compute at higher frequencies than today’s machines are possible with spin waves, signals that propagate due to magnetic fields, not current, and one research team has sent spin waves far enough to make a circuit viable.

Today, computers use electrons and the electron’s charge carries them through the circuits of silicon. This generates heat and more computation produces even more heat. An alternative to this is the transport of spins, the movement of a charge around an electron’s axis in a particular direction. With a variable magnetic field, a charge’s spin direction can be controlled, and this determination of the spin can flow laterally from electron to electron in a material’s atomic structure, like a signal, a one or a zero. An important characteristic of this phenomena, for its eventual practical use, is that it can occur in room temperatures.

“In principle it should work really well also at room temperature. The thing is the material we have been using for this first experiment has some phase transition at -10 degrees Celsius,” explains Dr Romain Le Brun, a physicist in Johannes Gutenberg University’s Institute of Physics in Mainz in Western Germany. “What we will do soon is to dope the material with some other atoms and then we can expand the operation from minus 10 to maybe 40 degrees Celsius.”

An electrical current in a platinum wire (left) creates a magnetic wave in the antiferromagnetic iron oxide (red and blue waves). This is measured as a voltage in a second platinum wire (right). The red and blue arrows represent the antiferromagnetic order of the iron oxide.
Credit: Joel Cramer

Le Brun and his colleagues used a single crystal of haematite, a common antiferromagnetic iron oxide. The substance his team expects to dope the iron oxide with has not been decided upon, but selenium and titanium are candidates. The Johannes Gutenberg University Mainz researchers were able to send a spin wave signal 80 microns, which Le Brun said, “for most circuits it is perfectly sufficient”. Previously, researchers had only sent spin waves a distance of nanometres.

A spin wave is also known as a magnon wave. Electronics based on spin waves are known as spintronics. The next stage in the work of Le Brun and his team is to improve the efficiency and amplitude of the magnon wave signal. “We will put our electrode, to inject the spin current, directly on the [antiferromagnetic] material, and we will treat the surface to improve this surface and have a better injection of spin,” explains Le Brun. “We didn’t try to optimise it until now.” The size of the crystal will also be reduced with a slimmer structure only 25 to 50 nanometres in thickness.

While the magnetic control of the charge spin of a material’s electrons will power the computation, for the time being, a conventional charged current does still have a role. “We still use a charged current to generate this spin current and then it propagates without charge,” Le Brun said.

A simple circuit is one element of a computer, but all important is the transistor for a spintronic computer. “I would like to try to develop some kind of transistor based on this [magnon wave] effect,” he added. He pointed out that along a circuit of 80 microns there could be thousands of such transistors. By reducing the amount of heat produced, components can continue to become smaller and a greater density of these information processors become possible. While today’s microchips operate at gigahertz, spintronics could work at terahertz, a 1,000 times faster.

As well as physicists at Johannes Gutenberg University Mainz, theorists from Utrecht University in the Netherlands and the Center for Quantum Spintronics at the Norwegian University of Science and Technology were also part of the team.