Professor, Institute for Chemical Research, Kyoto University
It is well known that ferromagnets show the magneto-resistance effect, i.e., the resistivity in ferromagnets depends on the relative angle between the direction of magnetization and the electrical current. Thus, magnetization controls the flow of the electrical current. We explored the possibility of the reversed phenomena, i.e., control of magnetization by electric current.
We demonstrated this concept using nano-magnets (magnetic wire or magnetic disk). A magnetic domain wall (DW) exists between two opposite magnetizations (blue and red arrows) in a magnetic wire (Fig.1 (a)). It was found that the injection of electrical current through the wire successfully displaced the DW (Fig.1 (a)-(c)). This current-induced domain DW motion is considered to be a kind of magnetic excitation by a current that flows through a spin structure with spatial variation (DW). This concept has been tested for another typical noncollinear spin structure: a magnetic vortex. Figure 2 shows a perspective view of the magnetization with a moving vortex structure. The height is proportional to the out-of-plane magnetization component. The rainbow colour indicates the in-plane component as exemplified by the white arrows. On application of the AC current though the magnetic disk, the core starts to make a circular orbital motion around the disk centre, and finally the magnetization of the core flips into the reversed direction.
The electrical control of magnetization described is very efficient and scalable down to the nano world, as it is based on the quantum mechanical interaction between the flowing spins and the localized spins that constitute magnetization. It is therefore a key technology for future spintronic devices. In fact, IBM proposed a novel storage device called “racetrack memory,” whose operation relies on the current-induced DW motion.View PDF