Micromagnetic Simulations of Spin Waves propagation by SOT in a Bi-YIG waveguide

Abstract

In the last decades, the investigation of microwave spin waves (SW) in thin magnetic films has been an attractive field of research due to their very short wavelength reaching down to nanometers at GHz frequencies. SW also have a rich dispersion relation, that depends on their propagation direction with respect to a magnetic field. Hence, SW -based devices are promising candidates for microwave information processing, and eventually for overcoming the limitations encountered with CMOS-devices such as power consumption. In magnonic devices, a key challenge is to achieve long propagation distances of the spin waves. This requires to work with magnetic materials with small magnetic damping such as Yttrium Iron Garnet (YIG).(YIG), has by far the lowest magnetic damping which allows the spin waves propagation to be spread over millimeter-scale distances making it a reference material for spin-wave dynamics studies. Kajiwara and al. [1] confirmed that an electric current injection in a YIG film can be converted into spin-waves. In fact, when placing a non-magnetic metal with a high spin orbit coupling such as platinum on top of a magnetic insulator such as YIG, flow of charge current in the metal leads to its conversion into a pure spin current (via the Spin Hall Effect), which is then injected in the magnetic insulator where it adds up as an additional torque on the YIG’s magnetization. The above-mentioned mechanism of STT generation originating from pure spin-current obtained through the spin-orbit interaction of a heavy metal is called spin orbit torque (SOT).Recent studies [2], [3] with Platinum stripes placed on top of a 20 nm thick YIG waveguide (Pt/YIG) showed that SOT could generate a full compensation of the damping, leading to auto-oscillations of the magnetization above a critical injected current density. When excited with a microwave field, spin waves propagation length was increased by a factor of 10 in the bilayer. However, the possibility to achieve an amplification of propagating spin-waves was not observed yet, due to the onset of nonlinear dissipative processes above the critical current. More recently, materials having perpendicular magnetic anisotropy have demonstrated a full damping compensation along with SW emission, such behavior is accomplished by substituting Bismuth in YIG [4].Here, we report on a study of SW propagation driven by SOT in a Pt/Bi-YIG waveguide where SW are detected using micro-focused Brillouin Light Scattering spectroscopy (BLS). The 20 nm thick and 21 µm long Bi-YIG waveguide is covered with 7 nm of Platinum. The experiments show the lossless propagation of a pulse of spin-waves excited by an rf-field generated by an antenna placed on top of the waveguide, under zero or positive damping during the course of 200 ns. Here, we show the full scale micromagnetic simulations done to emulate this experiment. The result shows a verification of the lossless propagation of SWs as seen in the experiment. By including a realistic Gilbert damping parameter and the Slonczewski torque to the Bi-YIG structure to compensate the magnetic losses, the results reproduce not only the steady state dynamics but also transitional dynamics when the rf-excitation is turned on and off at the ns time-scale.In summary, the inclusion of STT in the micromagnetic simulations showed an accordance with the experimental results, which verifies the ability to obtain a lossless propagation of SWs in the Bi-YIG waveguide and their non-suppression even after a hundred of nanoseconds after cutting off the excitation field. **