Spin-orbit torque oscillators are nano-scale sources of microwave radiation. Their output power and frequency are tunable by changes in applied direct current and magnetic field. They are attractive for practical applications such as microwave assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless communication. However, practical application of spin-orbit torque oscillators will require significant increases in both the microwave emission power and efficiency.
In their simplest form, spin-orbit torque oscillators can be made from bilayers of heavy metals and ferromagnetic materials. Owing to spin-orbit interaction in heavy metals, an applied in-plane direct electric current leads to an orthogonal spin current. This spin current can then create a torque on the magnetization of the ferromagnetic material which can counteract the natural magnetic damping, and above a certain threshold spin current density, induce magnetization dynamics. The threshold spin current densities are achievable in nanoscale systems, which are typically achieved by etching bilayer films into regions with nanowires or nano-constrictions. Owing to magnetoresistance effects, the dynamic magnetization leads to dynamic resistance oscillations. The result of the applied direct electric current and dynamic resistance is a voltage emission in the gigahertz frequency regime.
The typical bilayer spin-orbit torque oscillator relies on the relatively weak anisotropic magnetoresistance for conversion of magnetization dynamics to microwave emission, which typically does not exceed a few pico-Watts. Spin transfer toque oscillators based on magnetic tunnel junctions exploit the large tunneling magnetoresistance to lead to much larger microwave emission; however, the fabrication of these devices is a formidable task requiring multiple lithography and etching steps and electric current must flow perpendicular-to-plane. Our paper explores the question: Can we increase the output power of the spin-orbit torque nano-oscillator while maintaining its structural simplicity and ease of fabrication?
Our approach towards a solution to this issue is to add a second ferromagnetic layer to the standard spin-orbit torque bilayer. The second ferromagnetic layer is exchange biased to an additional antiferromagnetic layer such that its magnetization is pinned in a desired direction. A thin non-magnetic, low spin-orbit spacer metal layer separates the pinned ferromagnetic layer and the original free magnetic layer. This configuration allows us to use current-in-plane giant magnetoresistance, which is typically much larger than anisotropic magnetoresistance (hence the term giant), to convert magnetization dynamics to microwave emission. We find this leads to a significant amplification of the output power while remaining a simple-to-fabricate structure.
Furthermore, the most common spin-orbit torque oscillators exploit heavy metal layers where the dominant spin-orbit torque is provided by the spin Hall effect, which we use in our case. The spin Hall effect on it own has an angular dependence such that the efficiency of anti-damping of the magnetization is maximized when the free layer magnetization is in-plane and perpendicular to the electric current flow direction. In the case of spin-orbit torque oscillators relying on anisotropic magnetoresistance, this poses an issue: the efficiency of anisotropic magnetoresistance conversion is maximized at angles 45 degrees from this direction. This fact results in an optimization trade-off where spin-orbit torque efficiency and magnetoresistance conversion efficiency are both compromised. However, in our new structure with the additional giant magnetoresistance layers, both the spin-orbit efficiency and magnetoresistance conversion efficiency are identically maximized at the same angle. Therefore, we find that the spin-orbit torque oscillator with giant magnetoresistance readout both increases maximum output power and efficiency.
For more information, please see our recent publication in Communications Physics: Spin–orbit torque nano-oscillator with giant magnetoresistance readout