Topological modes driven by spin-transfer torque - Presentazione poster - Conferenza nazionale

It is shown 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 the Dzyaloshinskii–Moriya interaction [1,2] 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 negative current (J < 0) flows from the free layer (Co) to the fixed (nanocontact, CoPt) layer. 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 [3]. The analytical frequency of the topological droplet mode is expressed as the solution of a second-order algebraic equation written in terms of the magnetic parameters including the intrinsic and extrinsic damping. It is found a red-shift behaviour of the topological mode frequencies as a function of the current density above its threshold value. The analytical calculations are in very good accordance with micromagnetic simulations. The analysis of the symmetry properties of the linearized equations of motion demonstrates the nonreciprocal role of the spin polarized current on the topological mode dynamics. [1] T. Moriya Phys. Rev. Lett. 4 (1960) 228 [2] I. Dzyaloshinskii J. Phys. Chem. Solids 4 (1958) 241 [3] R. Zivieri et al. “Topological skyrmion dynamics driven by spin-transfer torque” submitted Figure