Quantum inductors hold promise for microscale electronics: Page 2 of 3

October 13, 2020 //By Rich Pell
Quantum inductors hold promise for microscale electronics
Researchers in Japan say they have demonstrated an inductance of quantum-mechanical origin, generated by the emergent electric field induced by current-driven dynamics of spin helices in a magnet.
change of the current density. Because this density would be inversely proportional to the cross-sectional area of the device, the emergent inductance would increase with decreasing area, in sharp contrast to the situation in ordinary inductors.

The researchers exploited this idea using a micrometer-scale magnet made of Gd 3Ru4Al12 (Gd, gadolinium; Ru, ruthenium; Al, aluminum) that contains various non-collinear spin structures, such as helical, conical, and fan-shaped structures. They selected this material because it has a weak magnetic anisotropy (directional dependence of magnetic properties), and because its spin structures have a short pitch (spatial periodicity). Spins can move relatively freely under a weak magnetic anisotropy, and the emergent inductance is inversely proportional to the pitch length9.

The researchers investigated the emergent inductance of their inductor using a technique called lock-in detection, where they controlled the spin-structure state of the device by altering the temperature and strength of an applied magnetic field, and carried out measurements on different states. They also varied the length, width, and thickness of the device, to confirm reproducibility and exclude the possibility that the observed signal was caused by external factors, such as the presence of contact electrodes.

Most strikingly, say the researchers, they observed a large emergent inductance (approximately –400 nanohenries) - comparable to that of a conventional inductor - for a device of about one-millionth the volume of such an inductor. By changing the spin-structure state of the device, the authors clarified the correspondence between the emergent inductance and the non-collinearity and dynamics of the spin structures. This correspondence is well explained by the previously mentioned mechanism for emergent inductance.

For example, say the researchers, they discovered that the current-driven dynamics of the helical spin structures are responsible for the large emergent inductance. By contrast, the fan-shaped structures yield a much lower inductance because their local angular variations are much smaller than are those of the other structures. Moreover, they

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