Spin-Hall Nano-oscillator: a study based on the synchronization

In these last years, experimental and theoretical studies of bilayer composed by a heavy metal film coupled with a thin ferromagnet have opened a route on the development of a more efficient category of spintronic devices. In those samples, the spin-orbit interaction in the Pt/Py bilayer drove the magnetic state to precessional states when the magnetic losses were fully compensated by the torque due to the spin-Hall effect generated by the current flowing in the heavy metal. Here, we study a different geometry to excite propagating spin-waves and get the synchronization between two spin-hall oscillators (see Fig. 1). The structure has dimensions 1.5 μm × 3.0 μm with rectangular shape. The used system is composed by two contacts placed in the center of the device 400nm away from each other, while the two pairs of tips are 100nm and 200nm away for the two spin-Hall oscillators SHO-1 and SHO-2 respectively (see Fig.1a). The Gold (Au) triangular contacts (thickness of 150 nm) are positioned above the bilayer composed by CoFeB(1nm)/ Pt(8nm) (see Fig. 1b). The current is injected (along the x-direction) via the two Gold contacts deposited over the ferromagnet (CoFeB). When the current is injected only through a contact, localized modes and propagating spin waves have been observed when the magnetic field was applied in-plane or out of plane, respectively. With this in mind, the device described above has been studied to obtain the synchronization between two spin-Hall oscillators when the current is injected through the two contacts at once. We have performed a systematic study based on micromagnetic simulations to understand the origin of the excited modes and the dynamical behavior as a function of field amplitude and current. Fig. 2a summarizes the oscillation frequency of the excited mode as a function of the current, for different external fields (H=200, 400, 600, and 800mT) applied out of plane. Those results are referred to the current regi- n where the two oscillators are synchronized. Fig. 2b shows the snapshots of the magnetization for the points A, B, C and D as displayed in Fig. 2a. As the field is reduced, the wavelength of the excited spin-waves increases. In fact, while for H=800mT a clear identification of the two sources of spin-waves can be observed, at H=200mT the wavelength of the propagating modes becomes comparable with the distance between the two oscillators giving rise to an excitation pattern where the two contacts cannot be identified anymore (the excited spin-waves exhibit an elliptical polarization). Our results show how spin-Hall nanoscillators can find application as high tunable spin-wave emitters for magnonic applications where spin waves are used for transmission and processing information on nanoscale. In the full paper, the possibility to obtain synchronized bullet modes by applying in-plane magnetic field will be also demonstrated.