Insight on the effect of SOT on propagating magnons: Spin-wave amplification and frequency dependent efficiency

Abstract

Spin wave (SW) based computing i.e. magnonics relies on propagating SW as information carriers. Being quasiparticles, magnons (the SW quanta) have a fine lifetime characterizing the exponential decay of the magnons population. Consequently, up to now, all magnonic devices operate within a short time-window and on short propagation distances. Finding a SW amplification paradigm is hence a prerequisite for the development of magnonics as a credible CMOS alternative. Two schemes are currently envisioned for SW amplification: Parametric pumping1 and spin-orbit-torque amplification (SOT)2. Here we will discuss SOT amplification in BiYIG/Pt bilayer3 (Fig.1). Using micro Brillouin light scattering spectroscopy (µ-BLS) on a 500 nm wide Pt/BiYIG waveguides, we take advantage of the strong BLS signal of BiYIG (nearly two orders of magnitude the one of YIG3) to extensively study the influence of SOT on coherent propagating magnon modes.By passing current in the Pt layer, the spin accumulation resulting from the spin-Hall effect induces a positive torque that compensates the Gilbert damping of BiYIG when the magnetic field orientation is perpendicular to the waveguide axis. At subcritical values, the current has a linear effect that lowers the effective damping, thus significantly increasing the decay length of the propagating magnons, similar to what was observed previously in a YIG/Pt waveguide4. However, by increasing the excitation frequency of these propagating magnons for a constant external field, we observe a gradual but strong decrease of the modulation efficiency of their lifetime in our BiYIG/Pt waveguide.Once the current in the Pt layer reaches a threshold value, the vanishing effective damping allows for the onset of the auto-oscillation regime5 where the thermal magnons get incoherently amplified and the magnetization auto-oscillates in the main uniform mode. However, coherent propagating magnons strongly interact with the auto-oscillations6, preventing the observation of their propagation at continuous supercritical current. To circumvent these non-linear interactions, we study the auto-oscillation transient regime for the first time by performing time-resolved µ-BLS measurements. When the SOT is switched on, the uniform auto-oscillation grows exponentially, in agreement with the Slavin & Tiberkevitch non-linear auto-oscillator model7 in the linear regime limit. The onset of non-linearities only occurs when the auto-oscillation reaches its stable orbital, after few hundreds of nanoseconds typically. Using a rf and DC pulse scheme, we propagate rf-excited magnons before the onset of non-linearities at supercritical currents and observe the amplification of propagating magnons (Fig. 2-a). Similarly to what was observed in the continuous regime at subcritical currents, the modulation efficiency of the propagating magnons lifetime strongly decreases with frequency (Fig. 2-b). This experiment sheds new light on the influence of SOT on propagating magnons and pave the way toward a magnonic spin-wave amplifier using DC current. **