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.

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We investigate the vortex excitations induced by a spin-polarized current in a magnetic nanopillar by means of micromagnetic simulations and analytical calculations. Damped motion, stationary vortex rotation and the switching of the vortex core are successively observed for increasing values of the current. We demonstrate that even for small amplitude of the vortex motion, the analytical description based the classical Thiele approach can yield quantitatively and qualitatively unsound results. We suggest and validate a new analytical technique based on the calculation of the energy dissipation.PACS numbers: 75.40.Gb, A magnetic vortex is a curling magnetization distribution, with the magnetization pointing perpendicular to the plane within the nanometer size vortex core. This unique magnetic object has attracted much attention recently because of the fundamental interest to specific properties of such a nanoscale spin structure. Gyrotropic modes of vortices in magnetic nanocylinders have been intensively studied theoretically [1] and experimentally [2]. Apart from their fundamental relevance, the unique properties of the vortices are of considerable practical interest for new applications to memory and microwave technologies. In this view, the reversal of the magnetization within the vortex core by magnetic field and spinpolarized current has been thoroughly studied [3,4,5,6]. More recently, sub-GHz dynamics of magnetic vortices induced by the spin transfer effect observed in nanopillars and nanocontacts [7,8,9] have raised a strong interest. Indeed, the associated microwave emissions in such vortex-based Spin-Transfer Nano-Oscillators (STNOs) occur at low current densities, without external magnetic field, together with high powers and narrow linewidths (< 1 MHz) comparatively to single-domain STOs.Traditionally, the analytical description of the vortex gyrotropic motion is based on the general approach for a translational motion of a magnetic soliton in an infinite media developed by A. Thiele [10]. This calculation consists in a convolution of the Landau-Lifshitz-Gilbert (LLG) equation with the magnetization distribution under a specific condition of a translational motion of the magnetization pattern. Eventually a single equation (often referred to as the Thiele equation) for the vortex core position X can be derived. The approach developed by Thiele to build his equation has been used for a long time to derive equations of vortex motion in many magnetic systems. In particular, it is often used to describe analytically the vortex oscillations induced by spin current [11,12,13,14] in magnetic nanodiscs, in the 'current perpendicular to the plane' (CPP) or 'current in the plane' (CIP) configurations. Vortex dynamics in magnetic submicron discs can not be considered as translational due to a strong deformation of the vortex structure by the edges [15]. Guslienko et al. demonstrated that this deformation should be taken into account in the calculation of the system energy [1]. However we show here that the impac...

We investigate experimentally and analytically the impact of thermal noise on the sustained gyrotropic mode of vortex magnetization in spin transfer nano-oscillators and its consequence on the linewidth broadening due to the different nonlinear contributions. Performing some time domain measurements, we are able to extract separately the phase noise and the amplitude noise at room temperature for several values of dc current and perpendicular field. For a theoretical description of the experiments, we extend the general model of nonlinear auto-oscillators to the case of vortex core dynamics and provide some analytical expressions of the response-to-noise of the system as the coupling coefficient between the phase and the amplitude of the vortex core dynamics due to the nonlinearities. From the analysis of our experimental results, we demonstrate the major role of the amplitude-to-phase noise conversion on the linewidth broadening, and propose some solutions to improve the spectral coherence of vortex based spin transfer nano-oscillators.

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We report the first intensity correlation measured with star light since Hanbury Brown and Twiss' historical experiments. The photon bunching g (2) (τ, r = 0), obtained in the photon counting regime, was measured for 3 bright stars, α Boo, α CMi, and β Gem. The light was collected at the focal plane of a 1 m optical telescope, was transported by a multi-mode optical fiber, split into two avalanche photodiodes and digitally correlated in real-time. For total exposure times of a few hours, we obtained contrast values around 2 × 10 −3 , in agreement with the expectation for chaotic sources, given the optical and electronic bandwidths of our setup. Comparing our results with the measurement of Hanbury Brown et al. on α CMi, we argue for the timely opportunity to extend our experiments to measuring the spatial correlation function over existing and/or foreseen arrays of optical telescopes diluted over several kilometers. This would enable µas long-baseline interferometry in the optical, especially in the visible wavelengths with a limiting magnitude of 10.

We investigate experimentally the synchronization of a vortex based spin transfer oscillator to an external rf current whose frequency is at multiple integers, as well as half integer, of the oscillator frequency. Through a theoretical study of the locking process, we highlight both the crucial role of the symmetries of the spin torques acting on the magnetic vortex and the nonlinear properties of the oscillator on the phase locking process. Through the achievement of a perfect injection locking state, we report a record phase noise reduction down to -90dBc/Hz at 1 kHz offset frequency. The phase noise of these nanoscale oscillators is demonstrating as being low and controllable which is of significant importance for real applications using spin transfer devices.In the last decade, large expectations have been anticipated on how the rich spin transfer physics will give birth to a new generation of multifunctional spintronic devices [1]. The tunable response of spin torque devices has been predicted to play a crucial role in several domains such as radio frequency [2] or magnonic [3] nanoscale and low energy cost devices for ICTs as well as neuroinspired memory devices [4]. For all of these potential applications, and notably for the corresponding microwave applications, it is essential to identify the mechanisms leading to a fine control of the phase of these spin torque devices. Indeed, it has been often emphasized that their nonlinear behavior gives a unique opportunity to tune their radiofrequency properties [5][6][7] but at the cost of large phase noise, not compatible with targeted applications [1,2]. In order to tackle these issues, a solution is to rely either on their synchronization to a reference external signal [8][9][10][11] or to achieve mutual synchronization [12,13] in arrays of spin torque nano-oscillators (STNOs). However, in all the reported studies made on the locking regime of STNOs, the phase noise, often measured through the estimation of the spectral linewidth measured with a spectrum analyzer, remains large, typically in the kHz range. This feature reveals that phase slips associated with the large thermal energy lead to a loss of synchronization [9,10,14] and have a detrimental and non-controllable impact on the expected behavior of STNOs.In this letter, we investigate the mechanism leading to a perfect phase locking of a double vortex based STNO to an external rf current with a F s frequency at f 0 /2, f 0 and 2f 0 , where f 0 is the frequency of our STNO. Indeed, thanks to their large intrinsic coherence compared to other types of STNOs [7,15,16], we succeed to elucidate the strong correlation between the oscillator parameters and the locking process through a thorough experimental study combining time domain measurements and analytical developments. This allows understanding of the locking range characteristics [8,17,18] as well as the high phase coherence in the locked regime [19][20][21]. Our results demonstrate the specific spin transfer locking process of our vortex based STNO, a...