In
research published in Science Advances ("Nonreciprocal surface acoustic
wave propagation via magneto-rotation coupling"), a group led by
scientists from the RIKEN Center for Emergent Matter Science (CEMS) have used a
principle, “magneto-rotation coupling,” to suppress the transmission of sound
waves on the surface of a film in one direction while allowing them to travel
in the other.
This could
lead to the development of “acoustic rectifiers”—devices that allow waves to
propagate preferentially in one direction, with potential applications in
communications technology.
Devices known as “rectifiers,” are extremely important for our technological civilization. The best known are electronic diodes, which are used to convert AC into DC electricity, essentially making electrification possible.
Schematics
of the magneto-rotation coupling. Courtesy: Mari Ishida, RIKEN CEMS.
In the
current study, the group examined the movement of acoustic surface
waves—movements of sound like the propagation of earthquakes over the surface
of the earth—in a magnetic film. It is known that there is a interplay between
the surface acoustic waves and “spin waves”—disturbances in magnetic fields
within the material that can move through the material.
Acoustic
surface waves can excite spin waves in two different ways. One, magneto-elastic
coupling, is very well documented. However, a second, magneto-rotation
coupling, was proposed more than forty years ago by Sadamichi Maekawa, one of
the authors of the current study, but was not experimentally verified until
now.
In the
current study, the authors found that the two mechanisms occur at the same time
but under different intensities. They found that when the magnetization of the
magnetic specimen is rotating in the same direction of the surface acoustic
waves, the energy of the acoustic surface waves is more efficiently transferred
to the spin waves, increasing the rotation of the magnetization.
In fact,
the researchers were able to identify a configuration of unidirectional
coupling where only the energy of surface acoustic waves in one direction could
be transferred to the rotation of the magnetization.
They also noticed that this “rectification” effect was more pronounced when the magnetic material contained magnetic anisotropy, meaning there was a preferred direction of internal magnetization even before the application of an external magnetic field.
Nonreciprocal
propagation of acousto-magnetic waves in Ta/CoFeB/MgO.
Courtesy: Mari Ishida,
RIKEN CEMS.
According
to Mingran Xu of RIKEN CEMS, the first author of the paper, “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.”
Jorge
Puebla, also of RIKEN CEMS, says, “We hope that we can use this work to create
an “acoustic diode” equivalent to the electronic diodes that are so important.
We could relatively easily make a device where the acoustic energy is
efficiently transfer 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.”