Coherent propagating spin waves excited in nano-structures by pure spin currents
Recent intense research on the interactions of pure spin current with magnetization resulted in the demonstration of novel nano-devices, where the coherent magnetization oscillations are driven by spin currents created either due to the spin-Hall effect in materials with strong spin-orbit interaction or due to the nonlocal spin injection. Utilization of pure spin currents not accompanied by the flow of electrical charge creates essentially novel opportunities for emerging technologies based on the use of the spin degree of freedom for nanoscale data storage, transmission, and procemssing, such as spintronics and magnonics. In addition to the evident advantages of smaller power dissipation and the possibility to utilize insulating magnetic materials the absence of the requirement of electrical current flow in active magnetic layers makes nano-devices based on pure spin current uniquely flexible resulting in novel device geometries and functionalities. Recently it was shown that pure spin currents can be used to excite coherent magnetization dynamics in magnetic nano-structures. However, in all the demonstrated devices the magnetic auto-oscillations are spatially confined and do not emit propagating spin waves in the outside world, which does not allow the use of these devices as sources of traveling waves for nano-scale magnonic circuits, where spin waves play the role of the signal carrier.
We demonstrate experimentally with applying a micro-focus Brillouin light scattering spectroscopy a magnonic system driven by pure spin currents, where the above conflicting requirements are simultaneously satisfied. This is achieved by utilizing a combination of two dynamically coupled subsystems possessing different characteristics: the active subsystem where the interaction of the pure spin current with the magnetization takes place resulting in the excitation of spatially confined dynamical mode, and the spin-wave guiding subsystem, where the efficient propagation of spin waves is possible. We show that such heterogeneous systems can be implemented by using the nonlocal spin-injection spin-current generation mechanism, which allows one easily to tailor the topography of the active magnetic layer. The demonstrated system shows efficient and controllable excitation of coherent propagating spin waves with large propagation lengths and their directional transmission.
The proposed construction is amendable to modifications and can be used as a building block for complex magnonic integrated circuits.