We experimentally demonstrate magnetic nano-oscillators driven by pure spin current produced by the spin Hall effect in a bow tie-shaped nanoconstriction. These devices exhibit single-mode auto-oscillation and generate highly-coherent electronic microwave signals with a significant power and the spectral linewidth as low as 6.2 MHz at room temperature. The proposed simple and flexible device geometry is amenable to straightforward implementation of advanced spintronic structures such as chains of mutually coupled spin-Hall nano-oscillators.

Recently, a novel type of spin-torque nano-oscillators driven by pure spin current generated via the spin Hall effect was demonstrated. Here we report the study of the effects of external microwave signals on these oscillators. Our results show that they can be efficiently synchronized by applying a microwave signal at approximately twice the frequency of the auto-oscillation, which opens additional possibilities for the development of novel spintronic devices. We find that the synchronization exhibits a threshold determined by magnetic fluctuations pumped above their thermal level by the spin current, and is significantly influenced by the nonlinear self-localized nature of the auto-oscillatory mode.

Nonlocal spin injection has been recognized as an efficient mechanism for creation of pure spin currents not tied to the electrical charge transfer. Here we demonstrate experimentally that it can induce coherent magnetization dynamics, which can be utilized for the implementation of novel microwave nano-sources for spintronic and magnonic applications. We show that such sources exhibit a small oscillation linewidth and are tunable over a wide frequency range by the static magnetic field. Spatially resolved measurements of the dynamical magnetization indicate a relatively large oscillation area, resulting in a high stability of the oscillation with respect to thermal fluctuations. We propose a simple quasilinear dynamical model that reproduces well the oscillation characteristics.

We utilize a nanoscale magnetic spin-valve structure to demonstrate that current-induced magnetization fluctuations at cryogenic temperatures result predominantly from the quantum fluctuations enhanced by the spin transfer effect. The demonstrated spin transfer due to quantum magnetization fluctuations is distinguished from the previously established current-induced effects by a non-smooth piecewise-linear dependence of the fluctuation intensity on current. It can be driven not only by the directional flows of spin-polarized electrons, but also by their thermal motion and by scattering of unpolarized electrons. This effect is expected to remain non-negligible even at room temperature, and entails a ubiquitous inelastic contribution to spin-polarizing properties of magnetic interfaces.Spin transfer [1][2][3] -the transfer of angular momentum from spin-polarized electrical current to magnetic materials -has been extensively researched as an efficient mechanism for the electronic manipulation of the static and dynamic states in nanomagnetic systems, advancing our understanding of nanomagnetism and electronic transport, and enabling the development of energy-efficient magnetic nanodevices [3][4][5][6][7][8][9][10][11][12][13][14][15]. This effect can be understood based on the argument of spin angular momentum conservation for spin-polarized electrons, scattered by a ferromagnet whose magnetization M is not aligned with the direction of polarization. The component of the electron spin transverse to M becomes absorbed, exerting a torque on the magnetization termed the spin transfer torque (STT). In nanomagnetic devices such as spin valve nanopillars [ Fig. 1(a)], STT can enhance thermal fluctuations of magnetization [ Fig. 1(b)], resulting in its reversal [5,16] or auto-oscillation [6], which can be utilized in memory, microwave generation, and spin-wave logic [17,18]. The approximation for the magnetization as a thermally fluctuating classical vector M provides an excellent description for the quasi-uniform magnetization dynamics [19]. However, the short-wavelength dynamical modes of the magnetization whose frequency extends into the THz range [20] become frozen out at low temperatures, and the effects of spin transfer on them cannot be described in terms of the enhancement or suppression of thermal fluctuations. Short-wavelength modes are not readily accessible to the common electronic spectroscopy and magneto-optical techniques, and their role in spin transfer remains largely unexplored.Here, we introduce a frequency non-selective, magnetoelectronic measurement approach allowing us to demonstrate that at low temperatures the current-dependent magnetization fluctuations arise predominantly from the enhancement of quantum fluctuations by spin transfer. The observed effect is analogous to the well-studied spontaneous emission of a photon by a two-level system, also caused by quantum fluctuations, which occurs even when there are no photons to stimulate the emission. In the studied magnetic system, the role of photons...

We utilize nanoscale spin valves with Pt spacer layers to characterize spin scattering in Pt. Analysis of the spin lifetime determined from our measurements indicates that the extrinsic Elliot-Yafet spin scattering is dominant at room temperature, while the intrinsic Dyakonov-Perel mechanism dominates at cryogenic temperatures. The significance of the latter is supported by the suppression of spin relaxation in Pt layers interfaced with a ferromagnet, likely caused by the competition between the effective exchange and spin-orbit fields.The interplay between electron's motion and its spin due to the spin-orbit interaction (SOI) opens unprecedented opportunities for the control of both spin and orbital degrees of freedom [1][2][3][4][5]. For instance, the spin Hall effect (SHE) results in generation of pure spin current flowing transverse to charge current [6], enabling electronic control of static and dynamic states of magnetization in metallic and insulating nanomagnets [7][8][9]. Extensive recent studies of materials that exhibit large SOI, including Pt, Ta, W, topological insulators, and alloys such as CuBi, have focused on identifying the intrinsic and extrinsic mechanisms controlling SOI, and characterizing the relevant parameters including the spin-orbit scattering rates, the spin Hall angle, and the effective spin-orbit field [10][11][12][13][14][15][16][17]. Another relevant parameter is the spin diffusion length λ, defined as the length scale over which the spin polarization relaxes away from the external source, which is determined mostly by the spin scattering due to SOI. It is also the length scale for spin current generation via the SHE, and is thus directly related to material's performance in spin-Hall applications.Pt is one of the most extensively studied spin-orbit materials, thanks to the large SOI effects [10,18,19], relatively low resistivity that minimizes Joule heating and current shunting in heterostructures, and low reactivity. A variety of approaches have been utilized to determine the parameters relevant to SOI in Pt such as the spin Hall angle and λ [10,14,18,[20][21][22]. Nevertheless, the values and the mechanisms controlling these parameters are still debated. In particular, the reported values of the spin Hall angle in Pt range from 0.004 to over 0.1 [10,14,23], and those of λ range from less than 1 nm to over 10 nm [10,14,18,[20][21][22][24][25][26]. Such a large spread of the reported characteristics makes it challenging to establish the dominant contributions to spin-orbit effects and the mechanisms controlling them.One of the main difficulties in analyzing SOI is posed by the interplay between the interfacial and bulk effects. For instance, measurements of spin current generated by SHE are inevitably affected by the spin relaxation at the Pt interfaces, and by its generation via the interfacial Rashba effect [27]. Indeed, the apparent spin Hall angle has been shown to depend on the transparency of the interfaces [23]. Measurements of λ based on the spin absorption efficiency [21...