Methods : the efficiency of the spin transfer and the microwave power. 11 The magnetic stacks grown by sputtering in a CANON ANELVA chamber contain a synthetic antiferromagnet (SAF) and a free layer of NiFe separated by a thin MgO insulating barrier : PtMn 15/ CoFe 2.5 / Ru 0.85 / CoFeB 3 / MgO 1.075 / NiFe 15 /Ru 10 (nm). Details of the growth and fabrication process have been presented elsewhere [35]. The RA product is 1.3 Ω. µm 2 for the parallel magnetization configuration. At room temperature, the TMR ratio is 14 % under a bias current I$_{dc}$ of 1.6 mA. The topology of the top NiFe layer has been investigated by means of Atomic Force Microscopy (see Fig.5 (b)). We find that the surface roughness is mainly due a spacial distribution of the thickness of the poly- crystal NiFe layer. Indeed, the surface of the underlying MgO layer was observed to be much smoother than the NiFe layer. Prior to the investigation of transport and microwave properties, a specific study has been performed by high-resolution spin-SEM images (SEMPA) obtained at zero magnetic field to characterize the actual magnetic structures on arrays of unconnected circular MTJs with different diameters d (see Additional materials). For d = 72 nm, the remanent magnetic state is a single-domain state for all the nanodots. By increasing the diameter to d = 125 nm, a few dots remain in single domain state whereas for most of them, the direction of the magnetic moments is changing gradually in-plane leading to a contrast varying between black and white. For larger diameters, the most favorable configuration is eventually a single vortex state, in particular for the diameter of d = 170 nm chosen for this study. The microwave response associated to the current induced vortex dynamics is studied by applying the magnetic field H$_{perp}$ out of plane and then by sweeping the dc current I$_{dc}$ from 0 to 3.2 mA. At each current value, microwave measurements up to 1.5 GHz are recorded on a spectrum analyzer after 32 dB amplification. The background noise, measured at zero dc current, is subtracted to the power spectra. In our convention, a positive current is defined as electrons flowing from the NiFe magnetic layer to SAF layer. For the micromagnetic simulations, we use our finite-difference micromagnetic code SPIN PM. The simulated NiFe disk has a diameter of 170 nm, with a thickness of 15 nm. The mesh cell size is set to 2 × 2 × 3.75 nm $^{3}$. We took the following magnetic parameters: α = 0.01 for the Gilbert damping, M NiFe s = 9.0 kOe for the magnetization of the NiFe layer, M SAF s = 10.3 kOe for the effective magnetization of the polarizer (extracted from the