The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from a DC-current-driven Pt layer and polarized along the hard AFM anisotropy axis excites a non-uniform in time precession of magnetizations sublattices in the AFM, due to the presence of a weak easy-plane AFM anisotropy. The frequency of the AFM oscillations varies in the range of 0.1–2.0 THz with the driving current in the Pt layer from 108 A/cm2 to 109 A/cm2. The THz-frequency signal from the AFM with the amplitude exceeding 1 V/cm is picked up by the inverse spin-Hall effect in Pt. The operation of a room-temperature AFM THz-frequency oscillator is similar to that of a cryogenic JJ oscillator, with the energy of the easy-plane magnetic anisotropy playing the role of the Josephson energy.

Spin Hall nano-oscillators (SHNOs) utilize pure spin currents to drive local regions of magnetic films and nanostructures into auto-oscillating precession. If such regions are placed in close proximity to each other they can interact and sometimes mutually synchronize, in pairs or in short linear chains. Here we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from 2 x 2 to 8 x 8 nano-constrictions, observed both electrically and using micro-Brillouin Light Scattering microscopy. The signal quality factor, Q = f /∆f , increases linearly with number of mutually synchronized nanoconstrictions (N ), reaching 170,000 in the largest arrays. While the microwave peak power first increases as N 2 , it eventually levels off, indicating a non-zero relative phase shift between nano-constrictions. Our demonstration will enable the use of SHNO arrays in two-dimensional oscillator networks for highquality microwave signal generation and neuromorphic computing. arXiv:1812.09630v1 [cond-mat.mes-hall]

It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to π/2, there is an optimum AFM thickness for which the output spin current reaches a maximum, that can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Neel temperature of the AFM layer.Progress in modern spintronics critically depends on finding novel media that can serve as effective conduits of spin angular momentum over large distances with minimum losses [1][2][3]. The mechanism of spin transfer is reasonably well-understood in ferromagnetic (FM) metals [4,5] and insulators [3,4,[6][7][8][9], but there are only very few theoretical papers describing spin current in antiferromagnets (AFM) (see, e.g., [10]).The recent experiments [11][12][13] have demonstrated that a thin layer of a dielectric AFM (NiO, CoO) could effectively conduct spin current. The transfer of spin current was studied in the FM/AFM/Pt trilayer structure (see Fig. 1). The FM layer driven in ferromagnetic resonance (FMR) excited spin current in a thin layer of AFM, which was detected in the adjacent Pt film using the inverse spin Hall effect (ISHE). It was also found in [13] that the spin current through the AFM depends on the ambient temperature and goes through a maximum near the Neel temperature T N . The most intriguing feature of the experiments was the fact that for a certain optimum thickness of the AFM layer (∼ 5 nm) the detected spin current had a maximum [11,12], which could be even higher than in the absence of the AFM spacer [12]. The spin current transfer in the reversed geometry, when the spin current flows from the Pt layer driven by DC current through the AFM spacer into a microwave-driven FM material has been reported recently in [14].The experiments [11-14] posed a fundamental question of the mechanism of the apparently rather effective spin current transfer through an AFM dielectric. A possible mechanism of the spin transfer through an easy-axis AFM has been recently proposed in [10]. However, this uniaxial model can not explain the non-monotonous dependence of the transmitted spin current on the AFM layer thickness and the apparent "amplification" of the spin current seen in the experiments [11,12] performed with the bi-axial NiO AFM layer [15].In this Letter, we propose a possible mechanism of spin current transfer through anisotropic AFM dielectrics, which may explain all the peculiarities of the experiments [ 11,12,14]. Namely, we show that the spin current can be effectively carried b...

Magnonics addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operation in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors that covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with Boolean digital data, unconventional approaches like neuromorphic computing, and the progress towards magnon-based quantum computing. The article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of current challenges and the outlook of further development for each research direction.

Spin-orbit torque (SOT) can drive sustained spin wave (SW) auto-oscillations in a class of emerging microwave devices known as spin Hall nano-oscillators (SHNOs), which have highly non-linear properties governing robust mutual synchronization at frequencies directly amenable to high-speed neuromorphic computing. However, all demonstrations have relied on localized SW modes interacting through dipolar coupling and/or direct exchange. As nanomagnonics requires propagating SWs for data transfer, and additional computational functionality can be achieved using SW interference, SOT driven propagating SWs would be highly advantageous. Here, we demonstrate how perpendicular magnetic anisotropy can raise the frequency of SOT driven auto-oscillations in magnetic nano-constrictions well above the SW gap, resulting in the efficient generation of field and current tunable propagating SWs. Our demonstration greatly extends the functionality and design freedom of SHNOs enabling long range SOT driven SW propagation for nanomagnonics, SW logic, and neuromorphic computing, directly compatible with CMOS technology.