We investigate experimentally the synchronization of vortex based spin transfer nano-oscillators to an external rf current whose frequency is at multiple integers, as well as at an integer fraction, of the oscillator frequency. Through a theoretical study of the locking mechanism, we highlight the crucial role of both the symmetries of the spin torques and the nonlinear properties of the oscillator in understanding the phase locking mechanism. In the locking regime, we report a phase noise reduction down to −90 dBc/Hz at 1 kHz offset frequency.
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A comparative study of spin-transfer-induced excitation of the gyrotropic motion of a vortex core with either uniform or vortex spin polarizers has been performed. The microwave output voltage associated with the vortex dynamics, detected in both cases, displays a strong reduction of phase fluctuations in the case of the vortex polarizer, with a decrease of the peak linewidth by one order of magnitude down to 200 kHz at zero field.
The influence of dynamic coupling in between magnetic layers of a standard spin torque nano-oscillator composed of a synthetic antiferromagnet (SyF) as a polarizer and an in-plane magnetized free layer has been investigated. Experiments on spin valve nanopillars reveal non-continuous features such as kinks in the frequency field dependence that cannot be explained without such interactions.
Spin-torque oscillators (STOs) are devices that allow for the excitation of a variety of magnetodynamical modes at the nanoscale. Depending on both external conditions and intrinsic magnetic properties, STOs can exhibit regimes of mode hopping and even mode coexistence. Whereas mode hopping has been extensively studied in STOs patterned as nanopillars, coexistence has been only recently observed for localized modes in nanocontact STOs (NC-STOs), where the current is confined to flow through a NC fabricated on an extended pseudo spin valve.
Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored.
By investigating thoroughly the tunable behavior of coupled modes, we highlight how it provides a means to tune the properties of spin-transfer nano-oscillators. We first demonstrate that the main features of the microwave signal associated with coupled vortex dynamics, i.e., frequency, spectral coherence, critical current, and mode localization, depend drastically on the relative vortex core polarities. Second, we report a large reduction of the nonlinear linewidth broadening obtained by changing the effective damping through the control of the core configuration.
For practical applications of spin torque nano-oscillators (STNO), one of the most critical characteristics is the speed at which an STNO responds to variations of external control parameters, such as current or/and field. Theory predicts that this speed is limited by the amplitude relaxation rate Gp that determines the timescale over which the amplitude fluctuations are damped out. In this study, this limit is verified experimentally by analyzing the amplitude and frequency noise spectra of the output voltage signal when modulating an STNO by a microwave current.
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 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.
We have investigated the microwave characteristics of a spin transfer nano-oscillator (STNO) based on coupledvortices as a function of the perpendicular magnetic field H⊥. Interestingly, we find that our vortex-basedoscillator is quasi-isochronous independently of H⊥ and for a dc current ranging between 18 and 25 mA. Itmeans that the severe nonlinear broadening usually observed in STNOs can be suppressed on a broad rangeof bias.
The synchronization dynamics of dipolarly coupled vortex based Spin-Torque Nano Oscillators were investigated analytically and numerically for different pillar diameters. The critical interpillar distances are identified for which synchronization occurs as a function of their diameter mismatch. A numerical phase diagram is obtained showing the transition between unsynchronized and synchronized states and compared it to analytical predictions using the Thiele approach.