FIG. 7: frequency as a function of the out-of-plane magnetic field for Idc= 1, 1.6, 2.2, 2.6, and 3.2 mA. The field is swept from negative to positive values. The combined action of the spin transfer torque and the Oersted field tends to force the magnetization of the vortex tail to remain in the plane before the vortex is annihilated and therefore restrains the Kittle mode range to higher values of the applied magnetic field. FIG. 8: integrated power as a function of the out-of-plane magnetic field. The field is swept from negative to positive values. Hc is the critical field at which microwave oscillations occur for Idc = -5 mA, Hsw is the field at which the vortex core reverses. Acknowledgments : 14