In the US work, the researchers used a customized "magnetic domain wall," a nanometer-sized barrier between two neighboring magnetic structures. They layered a pattern of cobalt/nickel nanofilms -- each a few atoms thick -- with certain desirable magnetic properties that can handle a high volume of spin waves. Then they placed the wall in the middle of a magnetic material with a special lattice structure, and incorporated the system into a circuit.
On one side of the circuit, the researchers excited constant spin waves in the material. As the wave passes through the wall, its magnons immediately spin in the opposite direction: Magnons in the first region spin north, while those in the second region -- past the wall -- spin south. This causes the dramatic shift in the wave's phase (angle) and slight decrease in magnitude (power).
In experiments, the researchers placed a separate antenna on the opposite side of the circuit, that detects and transmits an output signal. Results indicated that, at its output state, the phase of the input wave flipped 180 degrees. The wave's magnitude -- measured from highest to lowest peak -- had also decreased by a significant amount. The team then discovered a mutual interaction between spin wave and domain wall that enabled them to efficiently toggle between two states. Without the domain wall, the circuit would be uniformly magnetized; with the domain wall, the circuit has a split, modulated wave.
By controlling the spin wave, they found they could control the position of the domain wall. This relies on a phenomenon called spin-transfer torque where spinning electrons jolt a magnetic material to flip its magnetic orientation. Boosting the power of injected spin waves induced a certain spin of the magnons, drawing the wall toward the boosted wave source. In doing so, the wall gets jammed under the antenna -- effectively making it unable to modulate waves and ensuring uniform magnetization in this state.
Using a magnetic microscope, they showed that this method causes a micrometer-size shift in the wall, which is enough to position it anywhere along the material block. Notably, the mechanism of magnon spin-transfer torque was proposed, but not demonstrated, a few years ago. "There was good reason to think this would happen," Liu says. "But our experiments prove what will actually occur under these conditions."