Spin-transfer Torque Induced Magnetization Dynamics
A spin-polarized current entering into a ferromagnetic material exerts a torque on the magnetization by transferring spin angular momentum from the current to the ferromagnet, if the polarization direction differs from the quantization (magnetization) direction in the ferromagnet. This so-called spin-transfer torque gives rise to current-driven magnetization dynamics with unprecedented properties, such as the switching of the magnetization without applying an external field. This current-induced magnetization switching mechanism provides a smart alternative to magnetization switching by induction and is applied in magnetic random access memories (MRAM) for the writing process. Spin-transfer torques can also excite persistent large-angle precessions of the magnetization with frequencies in the GHz range, which are not accessible using magnetic field excitation alone. Persistent current-driven precessions are the basis for the so-called spin-transfer oscillators that are envisaged for applications in communication technology.