2 The spin transfer torque due to the interaction between a spin polarized current and a magnetization [1, 2] has led in the last decade to the emergence of a number of interesting novel effects such as current induced magnetization reversal [3, 4] or self-sustained magneti- zation oscillations [5, 6]. The observation of spin transfer induced magnetization precession brings some promising possibilities for designing a new type of nanoscale microwave oscil- lators, the so-called spin transfer nano oscillators (STNOs), capable to compete in terms of high frequency caracteristics with currently used millimeter-scale microwave synthetizer. One of the tantalizing of the STNOs is that they are tunable over a wide frequency range by varying the applied dc current or magnetic field. In most experiments up to now [5, 6], the current-induced excitations were quasi uniform precessions in purely metallic structures, which leads to a much too small generation of microwave power for the majority of ap- plications. Various solutions have been recently proposed, for example by synchronizing an assembly of STNOs [7–11] or by using MgO based Magnetic Tunnel Junctions (MTJs) [12, 13] that deliver much larger power because of the larger magnetoresistance [14–16]. However the spectral linewidths with MgO MTJs are still very large ( ≈ 100 MHz) for stan- dard excitations of the free layer in MTJs due to the chaotization of the magnetic system induced by the spin transfer torque and/or the high non-linearity [17]. An alternative approach using the current driven motion of a magnetic vortex as source of microwave power has been recently studied in metallic nanopillars [21]. A very interesting mode is the circular motion of the vortex core around its equilibrium position [22]. This low energy mode gives rise to very small linewidths (less than 1 MHz) in the sub-Gigahertz range but generates only a very small integrated power in all metallic devices (about 0.01 nW). In the present paper, we show that large microwave powers and narrow linewidths can be obtained simultaneously by spin transfer induced vortex motion in MgO MTJs. The studied devices are circular shape nanopillars with a diameter d = 170 nm pat- terned from the whole magnetic stack : synthetic antiferromagnet / MgO 1.075 nm/ NiFe 15 nm/capping. For the chosen parameters of NiFe thickness and dot diameter, we have checked that the most favorable configuration is a single vortex state (see Methods). In these nano-MTJs, the vortex dynamics is converted into a microwave signal thanks to the large magnetoresistive ratio of the Tunnel Magnetoresistance effect (TMR). This enable us to detect not only large sustained vortex oscillations induced by spin transfer but also low signal associated to the ferromagnetic resonance of the thermally excited vortex. Such reso-