  In a ferromagnetic nanodisk, the magnetization tends to swirl around in the
plane of the disk and can point either up or down at the center of this
magnetic vortex. This binary state can be useful for information storage. It is
demonstrated that a single nanosecond current pulse can switch the core
polarity. This method also provides the precise control of the core direction,
which constitutes fundamental technology for realizing a vortex core memory.
