A four-stroke engine for atoms


If you change a computer memory a bit and then put it back again, you have restored the original state. There are only two states that can be called “0 and 1”.

However, an amazing effect has now been discovered at TU Wien (Vienna): In a crystal based on oxides of gadolinium and manganese, an atomic switch has been found which must be switched back and forth not once, but twice, until the original state is reached again. During this double process of switching on and off, the spin of the gadolinium atoms completes a complete rotation. It is reminiscent of a crankshaft, in which an up and down motion is converted into a circular motion.

This new phenomenon opens interesting possibilities in materials physics, even information could be stored with such systems. The strange atomic switch has just been featured in the scientific journal Nature.

Coupling of electrical and magnetic properties

Normally, a distinction is made between the electrical and magnetic properties of materials. Electrical properties are based on the fact that charge carriers move – for example electrons moving through a metal or ions shifting in position.

Magnetic properties, on the other hand, are closely related to the spin of atoms – the intrinsic angular momentum of the particle, which can point in a very specific direction, much like the Earth’s spin axis points in a very specific direction. .

However, there are also materials in which the electrical and magnetic phenomena are very tightly coupled. Professor Andrei Pimenov and his team from the Institute of Solid State Physics at TU Wien are studying these materials. “We exposed a special material composed of gadolinium, manganese and oxygen to a magnetic field and measured how its electrical polarization changed during the process,” explains Andrei Pimenov. “We wanted to analyze how the electrical properties of the material can be changed by magnetism. And surprisingly, we came across a completely unexpected behavior.”

Back to the beginning in four steps

At first, the material is electrically polarized – on one side it is positively charged, on the other side negatively charged. Then you turn on a strong magnetic field — and the polarization changes very little. However, if you then turn off the magnetic field again, a dramatic change becomes apparent: suddenly the polarization reverses: the side that was previously positively charged is now negatively charged, and vice versa.

Now you can repeat the same process a second time: you activate the magnetic field again and the electrical polarization remains roughly constant. If you switch off the magnetic field, the polarization reverses again and thus returns to its original state.

“It’s extremely remarkable,” says Andrei Pimenov. “We do four different steps, each time the material changes its internal properties, but only twice the polarization changes, so you only reach the initial state after the fourth step.”

Four stroke engine for gadolinium

Closer examination shows that the gadolinium atoms are responsible for this behavior: they change spin direction at each of the four steps, each time by 90 degrees. “In a sense, it’s a four-stroke engine for atoms,” says Andrei Pimenov. “Also in a four-stroke engine, it takes four steps to return to the initial state – and the cylinder rises and falls twice in the process. In our case, the magnetic field rises and falls twice before the initial state .the state is restored and the spin of the gadolinium atoms points again in the original direction.”

Theoretically, such materials could be used to store information: a system with four possible states would have a storage capacity of two bits per switch, instead of the usual information bit for “0” or “1”. But the effect is also particularly interesting for sensor technology: for example, a magnetic pulse counter could be produced. The effect provides important new input for theoretical research: it is another example of a “topological effect”, a class of material effects which has received much attention in solid-state physics for years and should enable the development of new materials.

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