Researchers Formulate Principles for the Development of an ‘Acoustic Diode’

New research has used the principle of magneto-rotation coupling to stop the transmission of sound waves on the surface of a film in one direction, but at the same time allowing them to travel in the other.

The study was published Science Advances and was led by a group of scientists from the RIKEN Center for Emergent Matter Science (CEMS). This result leads to the development of acoustic rectifiers – which are devices that enable waves to spread preferentially in one direction, with probable applications in communications technology.

How Acoustic Waves and Spin Waves Interact

Devices known as rectifiers are highly crucial in technology development. The best known are electronic diodes, which are employed in using to transform AC into DC electricity, practically making electrification possible.

In the current study, the group analyzed the movement of acoustic surface waves, which are movements of sound, such as the propagation of earthquakes the surface of our planet, but in a magnetic field. There is an interaction between the surface acoustic waves and spin waves, more precisely, disturbances in magnetic fields within the material that can relocate through the material.

Acoustic surface waves are able to simulate spin waves in two different ways: first via magneto-elastic coupling, which is very well studied. However, a second way, magneto-rotation coupling, was suggested over 40 years ago by Sadamichi Maekawa, one of the authors of the current research, but was not experimentally analyzed until now.

In the new study, the team of researchers found that the two systems take place at the same time but under different intensities. They discovered that when the magnetization of the magnetic specimen is spinning in the same direction of the surface acoustic waves, the energy of the acoustic surface waves is more effectively to the spin waves, enhancing the rotation of the magnetization.

Creating an Acoustic Diode Based on the Discoveries

As a matter of fact, the scientists could also identify a configuration of unidirectional coupling where only the energy of surface acoustic waves in one way could be shifted to the rotation of the magnetization.

The team of researchers also discovered that this rectification effect was more intense when the magnetic material showed magnetic anisotropy, which means there was a favorite direction of internal magnetization even before the intervention of an external magnetic field.

Mingran Xu of RIKEN CEMS, the first author of the paper, explained: “It was very exciting to show that the phenomenon of magneto-rotation coupling actually takes place and that it can be used to completely suppress the movement of acoustic energy in one direction.”

Now, researchers hope to use these new discoveries and work to create an ‘acoustic diode’ similar to the electronic diodes that are so crucial now.

Jorge Puebla, also of RIKEN CEMS, also said: “We could relatively easily make a device where the acoustic energy is efficiently transferred in one direction but blocked in the other. This is happening at microwave frequencies, which is the range of interest for 5G communication technology, so surface acoustic waves may be an interesting candidate for this technology.”

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