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Posted: Aug 27, 2012
Driving an electron spin vortex 'skyrmion' with a microcurrent
(Nanowerk News) RIKEN, the University of Tokyo, and the National Institute for Materials Science (NIMS) succeeded in forming a skyrmion crystal, in which electron spin is aligned in a vortex shape, in a microdevice using the helimagnet FeGe, and driving the skyrmion crystal with a ultra-low current density less than 1/100,000 that of the current necessary to drive magnetic domain walls in ferromagnets. As a result of this research, it was possible to obtain guidelines for the realization of a technology for manipulating the states of magnetic information media with extremely low power consumption.
This research result ("Skyrmion flow near room temperature in an ultralow current density") was achieved by a team headed by Dr. Xuizhen Yu, a Postdoctoral Researcher in the Strong-Correlation Physics Research Team of the Correlated Electron Research Group of the RIKEN Advanced Science Institute (ASI), Group Director Prof. Yoshinori Tokura of the University of Tokyo Graduate School of Engineering, and Dr. Koji Kimoto, Unit Director of the Surface Physics and Structure Unit, Advanced Key Technologies Division of NIMS.
Fig. :Magnetic structure observed by Lorentz transmission electron microscopy. (a) Helical stripe structure in the zero magnetic field. Dotted lines show the crystal grain boundary. (b) Skymrion crystal formed by applying a 150mT magnetic field perpendicular to the device. (c) Enlarged diagram of the skyrmion crystal. (d) Distribution of magnetization in a single skyrmion. Colors and arrows show the direction of electron spin in the skyrmion.
Magnetic memory devices that use the direction of electron spin, which is the source of magnetism, as digital information have attracted attention as devices with the important features of high speed and non-volatility, etc. In recent years, numerous attempts have been made to manipulate that magnetic information electrically without utilizing a magnetic field. If a current is passed through a ferromagnet, it is possible to move the magnetic domain walls, which are the boundaries between domains where magnetization is upward-oriented and domains with downward orientation (at domain walls, the direction of magnetic spin gradually changes). Therefore, reversal of magnetization becomes possible and information can be written. However, in order to drive the domain walls in this manner, a large current density of at least approximately 105 A/cm2 was necessary. Because this causes large energy loss, in other words, large energy consumption, a method of manipulating magnetic information media with a smaller current density had been desired.
The research team investigated various functional magnetic materials, and in 2010, succeeded in forming and observing a skyrmion crystal by applying a weak magnetic field of less than 200 millitesla (mT) to a thin slice of the helimagnet FeGe at near room temperature. In the present research, the team fabricated microdevices with a length of 165µm, width of 100µm, and thicknesses of 100nm to 30µm using the same FeGe. When a magnetic field of approximately 150mT at temperatures from -23°C to near-room temperature (-3°C) was applied, skymrion crystals in which a stable skyrmion with a diameter of about 70nm was aligned in a triangular lattice shape were observed.
The team succeeded in driving the skymrion crystal with an ultra-low current density (the minimum density is approximately 5A/cm2), which is less than 1/100,000th that required to drive magnetic domain walls in conventional ferromagnets. The fact that the skymrion can be driven with this extremely low current density represents the first step toward the development of low power consumption magnetic memory devices using skymrions as an information medium. Various applications can also be expected in the field of spintronics, which is currently an area of active research as a next-generation electronic technology.