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Nanomagnetism at NIST: Spin Transfer Torques
Magnetic multilayers that exhibit giant magnetoresistance for current flowing perpendicular to the interfaces can also exhibit an effect called spin transfer. When the magnetizations of two adjacent layers are not parallel to each other, the current, which has been spin polarized by passing through one layer, can exert a torque on the next layer. When the current is high enough, it can cause the magnetization to switch from one orientation to another or even to precess in steady state. These devices are small enough that it is difficult to measure the magnetization directly, but it is possible to detect the configuration electrically through the Giant Magnetoresistance effect. The ability to change the magnetization and detect those changes electrically suggests a number of possible applications.
At NIST, this effect is being explored for use as the basis of current controlled oscillators. In these devices, the spin transfer torque causes one of the layers to precess at high frequencies. The precession can have sharply defined frequencies, which can be varied by changing the current or the external field. These properties suggest a number of applications in integrated circuits. Spin transfer torques are also being explored as the basis for switching the magnetic bits in Magnetic Random Access Memory (MRAM). NIST is supporting a TIP program to develop this capability.
NIST has a project to develop the theory of spin transfer torques. Work in this project considers many aspects of these effects, ranging from the quantum mechanical behavior that gives rise to it, to the transport of spins through the devices, and finally to the resulting magnetization dynamics. These theoretical efforts complement additional efforts to simulate experimental devices.