Origin of the spectral linewidth in nonlinear spin-transfer oscillators based on MgO tunnel junctions

We demonstrate the strong impact of the oscillator nonlinearity on the line broadening by studying spin-transfer-induced microwave emission in MgO-based tunnel junctions as a function of both the injected dc current and the temperature. In addition, we give clear evidences that the intrinsic noise is not dominated by thermal fluctuations but rather by the chaotization of the magnetic system induced by the spin transfer torque. A consequence is that the spectral linewidth is almost not reduced in decreasing the temperature.