This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential.

Magnetic Droplet: When a solitary wave travels atop the surface of a fluid, its shape generally changes with time, with some of its components traveling at velocities slightly different than others. In nonlinear media, this spreading effect may be countered by a slimming effect stemming from the non-linearity, which generates an object with perfectly preserved shape, called a soliton. Solitons have been observed in fluids, granular media, and other systems. Mohseni et al. (p. 1295 ) detected a dissipative soliton (one that also balances gain and dissipation) in a magnetic system, in the form of a magnetic droplet consisting of a core of spins pointing opposite to the external magnetic field. The droplet exhibited peculiar dynamics and could be controlled by electric current.

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

We study the impact of the magnetic field angle on the oscillation frequency of a nanocontact spin torque oscillator (STO) in magnetic fields up to 2.1 T. A model based on a single nonlinear, nonpropagating spin wave mode is found to explain the experimental data. We observe oscillation frequencies as high as 46 GHz in high magnetic fields applied normal to the film plane, and we are able to extrapolate the maximum expected operating frequency to beyond 65 GHz for in-plane magnetic fields. The STO signal remains surprisingly strong at these conditions, which opens up for possible millimeter-wave applications.

Spin–torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100 MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin–torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin–torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top–down lithography processes is not readily available to many groups, attempts to synchronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin–torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin–torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

We study the current-controlled modulation of a nanocontact spin-torque oscillator. Three principally different cases of frequency nonlinearity ($d^{2}f/d{I}_{\text{dc}}^2$ being zero, positive, and negative) are investigated. Standard nonlinear frequency-modulation theory is able to accurately describe the frequency shifts during modulation. However, the power of the modulated sidebands only agrees with calculations based on a recent theory of combined nonlinear frequency and amplitude modulation.