Nanowire Spin-Hall oscillator stationary vs non-stationary dynamics

It has been demonstrated that the transfer torque from spin orbit coupling can also act as negative damping by compensating the natural damping losses due to the Gilbert damping, in this scenario it can give rise either to magnetization switching and persistent magnetization precession in devices composed by heavy metal/ferromagnet bilayer.[1][2][3][4] This category of devices is very interesting from technological point of view being those structures simpler than the standard spintransfer torque oscillators. At the origin of the spinorbit torques there are at least two key mechanisms: Rashba and Spin-Hall effects.[1][2] Both terms give rise to a Slonczewski-like and field-like torque term. The Rashba terms are proportional to the current flowing into the ferromagnets and to the Rashba coefficient, while the spin-Hall terms are proportional to the current flowing into the heavy metal and to the spin-Hall angle.[5] [6] Here, we studied the dynamical properties of the self oscillations in response to an in-plane current of a system composed by a bilayer of Pt(8nm)/Py(5nm) of dimension 2000x200nm2. The computation of the spatial distribution of the current density shows that the 95% of the current flows in the Pt layer (we considered conductivities of 5.1x107 (m)−1 and 6.4x106 (m)−1 Platinum, and Permalloy respectively). With this in mind, we considered only the effect of the spin-Hall effect. Previous works considered the dynamical properties of spin-Hall oscillator where the current were injected in the center of the ferromagnet via two Gold contacts.[3][4] Differently from that devices where for in-plane fields the modes were localized in a reduced region of the ferromagnets as showed by micro-focus Brillouin light scattering technique,[3] here for some field and current ranges, we are able to excite a uniform mode of the magnetization dynamics in the whole ferromagnet. Our results are based on micromagnetic simulations performed with a “state of the art ” micromagnetic solver.[7][8] Together to the standard micromagnetic contributions, we have included the Oersted field due to the current flowing into the Pt layer.