Spin Torque Nano-Oscillators

Spin Torque Nano-Oscillators (STNOs) are a new class of nano-scopic microwave oscillators with ultrawide operating frequency ranges, ultrahigh modulation rates, and manufacturing processes compatible with RF CMOS in e.g. the same way as Magnetoresistive Random Access Memory (MRAM). STNOs utilize a number of spintronic and nanomagnetic phenomena for their operation, such as spin transfer torque (STT), giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and various spin wave modes. To the left we show a typical nano-contact STNO. The active layer is a permalloy (NiFe) thin film, typically between 2 and 5 nm thick, in which a spin wave resonance is excited by a high current density injected through a nano-contact, with a typical diameter from 40 to 400 nm. The fixed CoFe layer is substantially thicker, typically about 10 to 20 nm and has two purposes. First, it provides STT to the NiFe layer through reflection of electrons at the Cu/CoFe interface. Electrons that reflect at this interface will be preferentially spin polarized in a direction opposite that of the spin polarization in the CoFe layer and in the typical case of the NiFe and the CoFe experiencing the same large external magnetic field, the spin polarization of the reflected electrons will also be opposite the spin polarization in the NiFe. When these spins impinge back onto the NiFe/Cu interface, they tend to destabilize the magnetization, and if the current density is high enough, the magnetization can be entirely destabilized and begin to auto-oscillate at the lowest energy spin wave mode. This spin wave mode initially grows exponentially in amplitude until a new equilibrium steady auto-oscillating state is reached. This steady state is characterized by a large spin wave amplitude at typically a single frequency set by the applied field strength and the current density. The second purpose of the CoFe layer is now to provide GMR together with the NiFe layer. Since the NiFe magnetization oscillates at a particular frequency, the resistance of the NiFe/Cu/CoFe trialayer also oscillates at this frequency. Consequently, the direct current and the oscillating resistance together created a microwave voltage across the STNO.

For recent good reviews about STNOs please see e.g.:

[1] T. Silva, and W. Rippard, "Developments in nano-oscillators based upon spin-transfer point-contact devices." J. Magn. Magn. Mater. 320, 1260–1271 (2010).

[2] J.-V. Kim, "Spin-Torque Oscillators." Solid State Physics 63, 217–294 (2012).

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