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A Strange New Magnetoelectric Effect Has Been Discovered in a Symmetrical Crystal

20 SEPTEMBER 2020

Magnetism and electricity are linked together in many weird and wonderful ways throughout science, including the fascinating magnetoelectric effect noticeable in some crystals – where the electrical properties of a crystal can be influenced by a magnetic field, and vice versa.

 

Now things have gotten even weirder, because scientists have discovered a brand new magnetoelectric effect in a symmetrical crystal – and it shouldn't be possible.

The effect was found in a specific type of crystal called a langasite, which is made up of lanthanum, gallium, silicon and oxygen, plus holmium atoms.

Importantly, this particular crystal has a symmetrical structure, which is thought to rule out the possibility of a link between magnetism and electricity.

"Whether the electrical and magnetic properties of a crystal are coupled or not depends on the crystal's internal symmetry," says physicist Andrei Pimenov, from the Vienna University of Technology (TU Wien) in Austria.

"If the crystal has a high degree of symmetry, for example, if one side of the crystal is exactly the mirror image of the other side, then for theoretical reasons there can be no magnetoelectric effect."

In this case it was different: not only was the symmetrical crystal able to produce a magnetoelectric effect, it was a type of effect not seen before.

The scientists say that while the symmetry was retained in a geometrical sense, the magnetism of the holmium atoms broke the symmetry, enabling an effect that veered into the realm of quantum physics.

 

This break meant polarisation was possible, where the positive and negative charges in the crystal become slightly displaced.

This is easily done via an electric field, but with langasite it could be done with a magnetic field too, and the key turned out to be the strength of the magnetic field.

"The crystal structure is so symmetrical that it should actually not allow any magnetoelectric effect," says Pimenov. "And in the case of weak magnetic fields there is indeed no coupling whatsoever with the electrical properties of the crystal."

"But if we increase the strength of the magnetic field, something remarkable happens: the holmium atoms change their quantum state and gain a magnetic moment. This breaks the internal symmetry of the crystal."

While langasite showed a linear relationship between polarisation and magnetic field strength, which is normal, the relationship between polarisation and the direction of the magnetic field wasn't normal at all – it was strongly non-linear.

That's the brand new aspect, that just a small change in the rotation of the magnetic field could create a big change in the electrical polarisation effect.

 

The next step for the researchers is to see whether this newly discovered effect works in the opposite direction too, changing the magnetic properties with an electrical field.

This might seem like a lot of high-end physics – and it is – but there are real-world applications in terms of saving and storing computer data. The magnetoelectric effect is also important for various types of sensor technologies.

"In magnetic memories such as computer hard disks, magnetic fields are needed today," says Pimenov.

"They are generated with magnetic coils, which requires a relatively large amount of energy and time. If there were a direct way to switch the magnetic properties of a solid-state memory with an electric field, this would be a breakthrough."

The research has been published in NPJ Quantum Materials.