The spin transfer torque (STT) effect can transfer angular momentum between conduction electrons and the magnetization in a ferromagnet. The STT effect was predicted by John Slonscewski and Luc Berger, for which they are receiving the 2013 Buckley Prize form the American Physical. There is considerable interest in the STT effect, as it can be used to switch the magnetization in magnetic random access memories or can be used to generate tunable, coherent oscillations in the range of GHz to tens of GHz. These latter devices, spin torque ocillators (STOs), are typically realized either in magnetic tunnel junctions (MTJs), or metallic nano-contact junctions. These both consist of two thin ferromagnetic layers, and in one the magnetization is free to move in the plane. In an MTJ, the layers are separated by a thin insulator (typically MgO), and in a metallic nanocontact by a normal metal (typically Cu).
The STOs are useful for microwave applications due to their attractive combination of very large frequency tuning ranges efficient spin-wave emission in magnonic devices, very high modulation rates, sub-micron footprints, and straightforward integration with semiconductor technology using the same processes as Magnetoresistive Random Access Memory.
In this Colloquium, I will discuss the underlying physics of the STT effect, and also discuss the dynamics of the driven spin torque oscillators. The presence of several modes in them leads to interesting effects but also limits the coherence of the oscillators.
Read More