The condition for sustained gyration of the vortex core is that the spin transfer force counterbalances the damping. It follows from the respective signs of equations 2 and 3 that spin-transfer induced vortex oscillations can be observed only if the vortex core and the polarizer are pointing in the same direction when the current is positive, and if they point in opposite directions when the current is negative. Our MgO based nanojunctions with a vortex configuration are model systems compared to metallic devices to test these theoretical predictions. The reasons are twofold. First the SAF structure of the polarizer provides a uniform and fixed spin polarization. Second the current amplitude is small enough not to bend the polarizer through the Oersted field or induce some dynamics into the SAF. To show that, we plot in Fig.2 the integrated microwave power for I$_{dc}$ = 3.2 mA (top panel)and the frequency for two values of I$_{dc}$ (bottom panel) as a function of the out-of-plane magnetic field H$_{perp}$ . The field is swept from -8.1 to 8.1 kOe and the out-of-plane component of the polarization p$_{z}$ is expected to be proportional to | H$_{perp}$ | . Considering first the variation of the integrated power as a function of the out of plane field in the top panel of Fig.2, we can identify the following successive zones. In zone I, for H$_{perp}$ < - 7.5 kOe, the absolute value of field is large enough to induce a uniform magnetic configuration (no vortex) and the power is negligibly small. In zone II, a vortex turns out and, as the vortex core polarity and the out-of-plane component of the polarization H$_{z}$ are both negative, a vortex gyration is induced and a large microwave power is emitted (about 5 nW at I$_{dc}$ = 3.2 mA). When | H$_{perp}$ | becomes smaller than H$_{c}$ ≈ 4.5 kOe (zone III), the out of plane polarization p$_{z}$ is too small to excite large gyrations and the power is negligible at the scale of the figure. In zone IV, not only | p$_{z}$ | is too small for H$_{perp}$ < H$_{c}$ but, with the inversion of H$_{perp}$ , the vortex core polarity and the polarizer have opposite signs. The condition for vortex gyration is not satisfied and, even for H$_{perp}$ > H$_{c}$ , the power remains very small (max. 270 pW at I$_{dc}$ = 3.2 mA). In zone V that begins at H$_{sw}$ = 5 kOe, the switching of the vortex core aligns p and p$_{z}$ , which induces gyrations and large microwave power. Finally, in zone VI, for H$_{perp}$ > 6.5 kOe, as in zone I, there is no vortex and the power is negligible. The variation of the frequency as a function of the field in the bottom panel of Fig.2 fits with the variation of the power discussed above. For | H$_{perp}$ | > 7.5 kOe (zones I and VI), the magnetic field is strong enough to saturate the magnetization of the NiFe layer for all I$_{dc}$ and a spatially uniform configuration with the magnetization pointing out-of-plane is 5