A four-stroke engine for atoms

If you switch a bit in a computer’s memory and then switch back again, you have restored it to its original state. There are only two states that can be called “0 and 1”.

However, an astonishing 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 that has to be switched back and forth not once, but twice until the original state is restored. During this double switching on and off process, the spin of gadolinium atoms makes one complete revolution. This is reminiscent of a crankshaft, in which an up and down movement is converted into a circular movement.

This new phenomenon opens up interesting possibilities in materials physics. Such systems can even store information. The strange atomic switch has now been presented 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 traveling through a metal or ions whose position has been shifted.

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

However, there are also materials in which electrical and magnetic phenomena are very closely linked. Prof. dr. Andrei Pimenov and his team from the Institute of Solid State Physics at TU Wien are researching such materials. “We exposed a special material of gadolinium, manganese and oxygen to a magnetic field and measured how its electrical polarization changed in the process,” says Andrei Pimenov. “We wanted to analyze how the electrical properties of the material can be changed by magnetism. And surprisingly, we encountered a completely unforeseen behavior.”

Back to the beginning in four steps

In the beginning, the material is electrically polarized – positively charged on one side, negative on the other. Then you turn on a strong magnetic field – and the polarization changes very little. If you then switch off the magnetic field again, a dramatic change becomes visible: suddenly the polarization reverses: the side that was previously positively charged is now negatively charged and vice versa.

Now you can go through the same process a second time: you switch on the magnetic field again and the electrical polarization remains roughly constant. When you turn off the magnetic field, the polarization reverses again and thus returns to its original state.

“This is extremely remarkable,” says Andrei Pimenov. “We do four different steps, each time the material changes its internal properties, but only twice does the polarization change, so you don’t reach the initial state until after the fourth step.”

Four-stroke engine for gadolinium

A closer look reveals that the gadolinium atoms are responsible for this behavior: they change their direction of rotation at each of the four steps, by 90 degrees each time. “In a sense, it’s a four-stroke for atoms,” says Andrei Pimenov. “Even in a four-stroke engine, it takes four steps to return to the initial state – and the cylinder moves up and down twice in the process. In our case, the magnetic field moves up and down twice before the original condition is restored and the spin of the gadolinium atoms points back to 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 bit of information for “0” or “1.” But the effect is also particularly interesting for sensor technology: for example, you could make a counter for magnetic pulses. The effect provides important new inputs for theoretical research: it is another example of a so-called “topological effect”, a class of material effects that has attracted a lot of attention in solid state physics for years and should enable the development of new materials.