Two oscillation states in free/hard bilayered nano-pillars

Applied Physics Letters, Volume 118, Issue 18, May 2021. The magnetization oscillation driven by spin transfer torque (STT) in a nano-pillar composed of a large in-plane anisotropy fixed layer and a soft free layer is studied. It is found that instead of frequency continuously changing with current as most nano-oscillators do, this kind of nano-oscillator can only oscillate in two stable states with specific frequencies. In each state, the frequency is almost invariant with current density. The oscillation state could be easily manipulated by the magnetization state of the free layer or an applied pulse magnetic field as the working current density is lower than a critical value (Jc). The critical current density and the frequency difference of the two states can be tuned by the saturation magnetization (Ms) of the two layers and the anisotropy constant K of fixed layer. Phase-locked oscillation is obtained in a two-nanopillar system, suggesting that it may be possible to amplify the oscillation signal by building an array of this kind of nanopillar system. This kind of STT-based nano-oscillator may have various applications in the field of spintronics.