Researchers change direction of magnetic field with light without heating

Researchers at Radboud University have shown that in the future it should be possible to adjust the spins in iron atoms with light in such a way that the magnetic poles can be reversed. If this is done in a controlled manner, it will save heat production with magnetic storage.

The discovery was made by the same group of researchers who already demonstrated in 2007 that fast laser light pulses can reverse magnets through very local, temporary heating. Johan Mentink, one of the authors of the new article that appeared in Nature Communications on Wednesday, explains that the material used at that time actually consisted of two magnets, not one: an alloy of gadolinium-iron-cobalt. Gadolinium is then one magnetic material and the iron-cobalt alloy the other.

“Normally, those materials are so strongly connected that you can see them like a magnet. With the laser pulse, the materials could be ‘confused’. The internal forces in the magnet then ensure that the system reaches a new ground state by ourselves. Then we thought: if we cannot directly address these internal forces, then heating is unnecessary.”

The latter seems indeed to be possible: directly with light the internal forces, the so-called quantum-mechanical exchange interactions, which are responsible for magnetism itself, change. Laser pulses are still used, but they do not heat the material now used for research, because this transparent material, iron oxide, does not absorb the laser and therefore does not heat it up. “We now have a handle with light to directly influence that exchange interaction or those forces that are the origin of magnetism. What we want to do in the future is change those forces so much that the magnet gets completely confused. , which makes polarity reversal possible, just like with rapid heating,” says Mentink.

One of the reasons Mentink cites where fundamental research may one day find practical use is in magnetic storage. Mentink: “It has already been shown that switching with flash costs more energy than switching with light. If you calculated how much energy it takes to write 1 bit, that would already be less for laser-induced switching than some flash. Now it is about 1 nanojoule per bit with flash. With us it would even cost 10 femtojoule, although there are very big caveats: with flash nothing rotates and in a hard disk it does. the 10 and 100 nanojoules per bit.”

Still, research has been going on for some time into other ways of influencing magnetism in hard drives. Mentink says that Seagate has been working on heat assisted magnetic recording for some time. A laser pulse is used to heat the material and make writing with an external magnetic field easier. “We then say: we only need a laser pulse on the writing head and not a small coil.”

With the increasing surface density of hard disks, current technology is reaching the so-called superparamagnetic limit, which means that an increasingly larger magnetic field is required. “If you then switch with a fundamentally different technique, as in our case with the laser, there is no longer any need for heat and magnetic field.”

Iron oxide crystal in which red indicates the spins of iron ions and blue the spins of oxygen ions. Green are the electrons in their orbit that are responsible for the exchange interaction. They trap the iron spins in their position. A light pulse breaks the electron orbits, causing spins to move.