Magnetoresistive devices are currently used as read heads in hard drives. The context of the miniaturization of magnetic sensors to accommodate the increase in storage density implies an increase of the magnetic noise of thermal origin. The use of a sustained oscillation by the current would increase the combined magnetic susceptibility and its playback speed. The present study aims to clarify the different behavior of the magnetization oscillations induced by spin transfer in the range of tens of gigahertz, in magnetic tunnel junctions with MgO barrier. We present in a first part, a detailed study of the modes of oscillations in different layers of the magnetic system. The identification of modes and can "read" data encoded magnetic fields by measuring the frequency of the oscillator. In the second part of this thesis, we present timing measurements of these oscillations. We analyze the noise amplitude and phase of a sustained oscillation and we provide the first experimental measurement of the coefficient of nonlinearity of frequency. The time characteristics extracts are significant agility and device nonlinearity specific to spin transfer oscillators induces an inhomogeneous broadening of the spectral line.
