Topological Skyrmion Dynamics Driven By Spin-Transfer Torque - Presentazione orale - Conferenza internazionale

Very recently, an important class of magnetic solitons called magnetic skyrmions has been widely studied [1,2]. The stability of these topological defects has been analyzed in the presence of intrinsic dissipation both as a function of Dzyaloshinskii–Moriya interaction (DMI) and of a spin-polarized current (SPC). Here, we show that in a spin-valve consisting of Pt(5 nm)/Co(0.6 nm)/Cu (4 nm)/CoPt(4 nm) a perpendicular spin current, in the presence of DMI [3] and strong perpendicular anisotropy, induces the rotation of the spins from the hedgehog-like to the vortex-like texture in the topological droplet state and excites low-frequency topological modes. The SPC is injected locally in the Co giving rise to the local excitation of the magnetization. The FL has a square cross section of 400 x 400 nm2, while the point contact has a diameter of 70 nm. Both FL and polarizer have out-of-plane magnetic state at zero bias field. The “topological droplet” state (integer skyrmion number) arises and its dynamical response, namely a “topological mode” (TM), can be excited and sustained by a SPC. The low-frequency TM is linked to a continual conversion from hedgehog-like (Néel skyrmion) to vortex-like (Bloch skyrmion) state, preserving the topology represented by the skyrmion number S = -1. The topological character of these spin-wave excitations results from the synchronized dynamics between the 360o rotation of the spin of the outer droplet domain and the expansion/shrinking of the droplet core. A quantitative description of topological droplet modes is given according to an analytical model based on the linearization of the equations of motion including intrinsic positive Gilbert damping and negative damping related to the spin-transfer torque. The predictions on the TM dynamics of the analytical model are quantitatively enforced by the ones obtained according to numerical simulations. The micromagnetic parameters are typical of Co: saturation magnetization Ms=900 kA/m, perpendicular anisotropy constant ku=1.10 MJ/m3, exchange constant A=20 pJ/m, DMI parameter 0<D<3.0 mJ/m2 and magnetic damping αG=0.1. The frequencies show red-shift behavior as a function of the current and the analytical calculations are in good accordance with micromagnetic predictions. The theoretical results could open the route for experiments based on the giant magnetoresistance effect able to detect the topological modes and for the design of a further generation of nanoscale microwave oscillators. References [1] Fert, V. Cros, and J. Sampaio, Nat. Nanotech. 8, 152, (2013). [2] J. Iwasaki, M. Mochizuki, and N. Nagaosa, Nat. Comm. 4, 1463, (2013). [3] T. Moriya, Phys. Rev. Lett. 4, 228, (1960). I. Dzyaloshinskii, J. Phys. Chem. Solids 4, 241, (1958).