Spin-polarized current can excite the magnetization of a ferromagnet through the transfer of spin angular momentum to the local spin system. This pure spin-related transport phenomenon leads to alluring possibilities for the achievement of a nanometer scale, complementary metal oxide semiconductor-compatible, tunable microwave generator that operates at low bias for future wireless communication applications. Microwave emission generated by the persistent motion of magnetic vortices induced by a spin-transfer effect seems to be a unique manner to reach appropriate spectral linewidth. However, in metallic systems, in which such vortex oscillations have been observed, the resulting microwave power is much too small. In this study, we present experimental evidence of spin-transfer-induced vortex precession in MgObased magnetic tunnel junctions, with an emitted power that is at least one order of magnitude stronger and with similar spectral quality. More importantly and in contrast to other spintransfer excitations, the thorough comparison between experimental results and analytical predictions provides a clear textbook illustration of the mechanism of spin-transfer-induced vortex precession.

Synchronized spin-valve oscillators may lead to nanosized microwave generators that do not require discrete elements such as capacitors or inductors. Uniformly magnetized oscillators have been synchronized, but offer low power. Gyrating magnetic vortices offer greater power, but vortex synchronization has yet to be demonstrated. Here we find that vortices can interact with each other through the mediation of antivortices, leading to synchronization when they are closely spaced. The synchronization does not require a magnetic field, making the system attractive for electronic device integration. Also, because each vortex is a topological soliton, this work presents a model experimental system for the study of interacting solitons.

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The

We analytically study the impact of an electrical connection of spin transfer nano-oscillators (STNOs) on their synchronization. We demonstrate that the phase dynamics of coupled STNO arrays can be described in the framework of the Kuramoto model. The conditions for successful synchronization of an assembly of STNOs are formulated. Synchronizing an assembly of STNOs appears to be the only solution to make the breakthrough on the emitted output power toward frequency synthesizers. In these potential devices, a large number of STNOs will have to be electrically connected, whatever the coupling mechanisms between oscillators.

We demonstrate the strong impact of the oscillator nonlinearity on the line broadening by studying spintransfer-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.

We experimentally investigated current-driven oscillation in fully epitaxial Fe͑001͒/MgO͑001͒/Fe͑001͒ magnetic tunnel junctions ͑MTJs͒ to pave the way for a better understanding of why the linewidth ͑a few hundred MHz͒ of microwave oscillation in spin-torque nano-oscillators ͑STNOs͒ based on textured MTJs is much larger than that ͑smaller than 10 MHz͒ in STNOs based on current-perpendicular-to-plane giantmagnetoresistance junctions. The epitaxial Fe/MgO/Fe STNO is a model system for studying the physics of spin-transfer torque because it has a well-defined single-crystal barrier and electrode layers with atomically flat interfaces. In the Fe/MgO/Fe STNOs, clear spin-torque-induced switching and spin-torque-induced precession were observed in epitaxial MTJs. When the initial magnetic alignment was antiparallel and the bias current exceeded the threshold current, a state in which the spin-torque compensates for the damping, the STNOs showed a rapid increase in the peak intensity, a redshift of the peak frequency, and a minimum linewidth, all clear evidence of spin-torque-induced precession above the threshold current. The minimum linewidth of the STNOs was 200 MHz, which is comparable to that of textured CoFeB/MgO/CoFeB MTJs. This indicates that the origin of the large linewidth cannot be attributed to structural inhomogeneity in textured MTJs. When the initial magnetic alignment was parallel, the microwave spectrum showed a single peak, which has rarely been observed in textured MTJs without application of a perpendicular magnetic field. The mechanism of the single-peak oscillation can be explained by taking account of the induced perpendicular magnetic anisotropy in the 3-nm-thick Fe͑001͒ free layer grown on the MgO͑001͒ barrier layer.

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We observe both dc voltage rectification and frequency conversion that occur when a reference microwave current is injected to a MgO based magnetic tunnel junction (MTJ). The rectification that is spin-transfer torque dependent is observed when the frequency of the input microwave current coincides with the resonance frequency of the magnetization of the active layer. In addition, we demonstrate that frequency conversion is the result of amplitude modulation between the reference signal and the resistance of the MTJ that is fluctuating at the resonance frequency of the magnetization of the active layer. 75.40.Gb