1. Department of Physics, Keio University, Yokohama, 223-8522, Japan
2. SII NanoTechnology Inc., Matsudo, 270-2222, Japan
3. Department of Earth and Space Science, Osaka University, Osaka, 560-0043, Japan

Correspondence to: Eiji Saitoh¹ Email: eizi@phys.keio.ac.jp

Abstract

A magnetic domain wall (DW) is a spatially localized change of magnetization configuration in a magnet. This topological object has been predicted to behave at low energy as a composite particle with finite mass¹. This particle will couple directly with electric currents as well as magnetic fields, and its manipulation using electric currents^{2, 3, 4, 5, 6, 7, 8} is of particular interest with regard to the development of high-density magnetic memories⁹. The DW mass sets the ultimate operation speed of these devices, but has yet to be determined experimentally. Here we report the direct observation of the dynamics of a single DW in a ferromagnetic nanowire, which demonstrates that such a topological particle has a very small but finite mass of 6.6 10^-23 kg. This measurement was realized by preparing a tunable DW potential in the nanowire, and detecting the resonance motion of the DW induced by an oscillating current. The resonance also allows low-current operation, which is crucial in device applications; a DW displacement of 10 m was induced by a current density of 10¹⁰ A m^-2.

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