Magnetization reversal driven by low dimensional chaos in a nanoscale ferromagnet*

Energy-efficient magnetization switching is an essential problem in the realization of practical nonvolatile magnetic storage [1] and magnetic neuromorphic computing [2]. In the past two decades, several efficient methods of magnetic switching were demonstrated including spin torque, magneto-electric, and microwave-assisted switching mechanisms. In this talk, we experimentally demonstrate that low dimensional magnetic chaos [3] induced by alternating spin torque can strongly increase the rate of thermally-activated magnetic switching of the free layer in a magnetic tunnel junction (MTJ)[4]. This mechanism exhibits a well-pronounced threshold character in spin torque amplitude and its efficiency increases with decreasing spin torque frequency. We present analytical and numerical calculations that quantitatively explain these experimental findings and reveal the crucial role played by low dimensional magnetic chaos near saddle equilibria in enhancement of the switching rate. This work shows that ac spin torque driven chaos can facilitate thermally-assisted switching of magnetization in a MTJ [5] and provides a new path towards improved energy efficiency of spin torque memory based on thermally stable MTJs. Furthermore, MTJs with superparamagnetic free layers are attractive for neuromorphic computing. Our results show that low dimensional can be used to tune the switching rate of such systems, and therefore, may lead to computing schemes that simultaneously harness stochasticity and deterministic chaos.

[1] S. Ikeda, et al, IEEE Trans. Electron Devices 54, 991 (2007).
[2] N. Locatelli, V. Cros, and J. Grollier, Nature Mater 13, 11 (2014).
[3] C. Serpico, et al, J.of Appli. Phys. 117, 17B719 (2015).
[4] E. A. Montoya, et al, Nat. Commun. 10, 543 (2019).
[5] J.-G. Zhu, X. Zhu, and Y. Tang, IEEE Trans. Magn. 44, 125 (2008).