Spin transfer torques (STTs) [1] control magnetisation by electric currents, enabling a range of spintronic applications [2]. STTs are in general employed for switching the magnetisation from one local energy minima to the other or in entering auto-oscillation, but so far only these two have been mainly studied as non-linear dynamics driven by STTs. Inspired by the Kapitza pendulum [3] where a high potential energy state can be stabilised by dynamical injection of mechanical force, here we study the spintronic analogue of such a state achieved by nonequilibrium excitation of STTs.To achieve the same potential landscape with only one global potential minimum, and to maximise the efficiency of current-driven STTs, we made a dedicated CoFeB thin film layer with a characteristic property of the nearly-isotropic magnet. We use STTs generated by spin-Hall effect in the adjacent W layer to de-stabilise the energy minimum state achieved by an external field and observed the experimental signature of stabilising the state at the energy maxima, i.e. the moment pointing at the direction opposite to the external field, when the STT drive is strong enough to compensate the material damping. One such observation is the sign reversal of ferromagnetic resonance voltage amplitude when increasing the STT drive [4].We will discuss more technical details of these measurements, quantitative analysis and results by stochastic Landau-Lifshitz-Gilbert equation. The discovery of a nano-scale rigid pendulum with dynamical stabilisation and controllable stochasticity is an ideal platform for studying dynamical systems with promising functionalities for probabilistic computing applications [5] and anti-magnonics [6].

References:

[1] Slonczewski, Magn. Magn. Mater. 159, L1–L7, (1996); Berger, Phys. Rev. B 54, 9353 (1996).
[2] Ralph et al., J. Magn. Magn. Mater. 320, 1190 (2008).
[3] Kapitza, Sov. Phys. JETP 21, 588 (1951).
[4] Kurebayashi et al., submitted.
[5] Chowdhury et al. IEEE Journal on Exploratory Solid-State Computational Devices Circuits 9,1–11 (2023).
[6] Harms et al., AIP Advances 14, 025303 (2024).