We use microfocus Brillouin light scattering spectroscopy to study the interaction of spin current with magnetic fluctuations in a Permalloy microdisk located on top of a Pt strip carrying an electric current. We show that the fluctuations can be efficiently suppressed or enhanced by different directions of the electric current. Additionally, we find that the effect of spin current on magnetic fluctuations is strongly influenced by nonlinear magnon-magnon interactions. The observed phenomena can be used for controllable reduction of thermal noise in spintronic nanodevices.

Understanding the evolution of spin-orbit torque (SOT) with increasing heavy-metal thickness in ferromagnet/normal metal (FM/NM) bilayers is critical for the development of magnetic memory based on SOT. However, several experiments have revealed an apparent discrepancy between damping enhancement and damping-like SOT regarding their dependence on NM thickness. Here, using linewidth and phase-resolved amplitude analysis of vector network analyzer ferromagnetic resonance (VNA-FMR) measurements, we simultaneously extract damping enhancement and both field-like and damping-like inverse SOT in Ni 80 Fe 20 /Pt bilayers as a function of Pt thickness. By enforcing an interpretation of the data which satisfies Onsager reciprocity, we find that both the damping enhancement and damping-like inverse SOT can be described by a single spin diffusion length (≈ 4 nm), and that we can separate the spin pumping and spin memory loss contributions to the total damping. This analysis indicates that less than 40% of the angular momentum pumped by FMR through the Ni 80 Fe 20 /Pt interface is transported as spin current into the Pt. On account of the spin memory loss and corresponding reduction in total spin current available for spin-charge transduction in the Pt, we determine the Pt spin Hall conductivity (σ SH = (2.36 ± 0.04) × 10 6 Ω −1 m −1 ) and bulk spin Hall angle (θ SH = 0.387 ± 0.008) to be larger than commonly-cited values. These results suggest that Pt can be an extremely useful source of SOT if the FM/NM interface can be engineered to minimize spin loss. Lastly, we find that self-consistent fitting of the damping and SOT data is best achieved by a model with Elliott-Yafet spin relaxation and extrinsic inverse spin Hall effect, such that both the spin diffusion length and spin Hall conductivity are proportional to the Pt charge conductivity. * thomas.silva@nist.gov † Contribution of the National Institute of Standards and Technology; not subject to copyright.

Terahertz near fields of gold metamaterials resonant at a frequency of 0.88 THz allow us to enter an extreme limit of nonperturbative ultrafast terahertz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasistatic interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of 10 19 cm −3 . This process causes bright luminescence at energies up to 0.5 eV above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude-modulation of transmitted few-cycle terahertz transients. Our results pave the way towards highly nonlinear terahertz optics and optoelectronic nanocircuitry with subpicosecond switching times. DOI: 10.1103/PhysRevLett.113.227401 PACS numbers: 78.20.-e, 42.65.Ky, 42.65.Sf, 72.20.Ht Intense, phase-locked light pulses in the terahertz spectral range have opened up an exciting arena for field-sensitive nonlinear optics . For a given peak electric field E, the low carrier frequency ω THz gives rise to a potentially large ponderomotive energyTHz , which quantifies the cycle-averaged quiver energy of a free electron of mass m. This situation promises a new quality of nonperturbative light-matter interaction at the boundary of terahertz optics and highspeed electronics.For frequencies between 0.5 and 3 THz, optical rectification [7][8][9][10] has enabled transients with peak field amplitudes in excess of 1 MV=cm [10]. Using such an electromagnetic pulse as an alternating bias, terahertzdriven carrier multiplication in doped semiconductors [12] and graphene [14] has been demonstrated. Strong terahertz fields have also been used to drive spectacular nonlinear intraband dynamics of quasiparticles, such as field ionization of impurity states [13] or excitons, electronhole recollisions, and high-order sideband generation [11]. In these experiments, the terahertz bias facilitates tunneling of bound electrons through potential energy barriers, which correspond to binding energies between a few meV and several 10 meV [ Fig. 1(a)] [11,13,23].A new limit of nonperturbative nonlinearities is expected if terahertz amplitudes approach atomically strong fields. As depicted in Fig. 1(b . This breakthrough has enabled ultrafast biasing of bulk solids in an unprecedented high-field limit, where a coherent interplay between nonresonant interband polarization and intraband Bloch oscillations generates terahertz high-harmonic radiation [6]. The diffraction limit of focusing, however, has precluded comparably high fields at frequencies as low as 1 THz where yet larger ponderomotive potentials U p combined with photon energies orders of magnitude below electronic interband resonances could pave the way to quasistatic biasing. Custom-tailored metamaterials are a promising concept for overcoming the diffraction limit. Indeed, field enhancement in metamaterials has been exploited to induce a metal-insulator phase transition in VO 2 by a terahertz transient with...

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Ni80Fe20/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion.

The spin–orbit interaction enables interconversion between a charge current and a spin current. It is usually believed that in a nonmagnetic metal (NM) or at a NM/ferromagnetic metal (FM) bilayer interface, the symmetry of spin–orbit effects requires that the spin current, charge current, and spin orientation are all orthogonal to each other. Here we demonstrate the presence of spin–orbit effects near the NM/FM interface that exhibit a very different symmetry, hereafter referred to as spin-rotation symmetry, from the conventional spin Hall effect while the spin polarization is rotating about the magnetization. These results imply that a perpendicularly polarized spin current can be generated with an in-plane charge current simply by use of a FM/NM bilayer with magnetization collinear to the charge current. The ability to generate a spin current with arbitrary polarization using typical magnetic materials will benefit the development of magnetic memories.

We demonstrate experimentally that the characteristics of the ferromagnetic resonance in a microscopic magnetic system based on a Permalloy/Cu/Pt multilayer can be varied over a wide range by the spin Hall effect. Specifically, by applying a dc current through the Pt strip, we achieve a reduction of the effective damping constant in Permalloy by a factor of two below its standard value. We show that this reduction is not significantly affected by the Joule heating effects. We also find that, apart from influencing the damping, the spin Hall effect results in the amplification or suppression of the coherent magnetization dynamics.

We report ultra-low intrinsic magnetic damping in Co 25 Fe 75 heterostructures, reaching the low 10 −4 regime at room temperature. By using a broadband ferromagnetic resonance technique in out-of-plane geometry, we extracted the dynamic magnetic properties of several Co 25 Fe 75 -based heterostructures with varying ferromagnetic layer thickness. By measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26 nm thick sample to be α 0 3.18 × 10 −4 . Furthermore, using Brillouin light scattering microscopy we measured spin-wave propagation lengths of up to (21 ± 1) µm in a 26 nm thick Co 25 Fe 75 heterostructure at room temperature, which is in excellent agreement with the measured damping.Itinerant ferromagnets (FM) are advantageous for spintronic and magnonic devices. They benefit from, e.g., large magnetoresistive effects and current-induced spinorbit torques 1 . In many magneto-resistive technologies (e.g., anisotropic magnetoresistance, giant magnetoresistance, tunnel magnetoresistance) electronic conductivity is indispensable. Moreover, due to high saturation magnetization in metallic FMs, spin-wave (SW) group velocities are in general significantly higher than in insulating ferrimagnets 2-5 . High saturation magnetizations in general ease detection. Nevertheless, itinerant FMs typically have considerable magnetic damping 6,7 . This is unfavorable for many applications. For example, low damping is crucial for oscillators based on spin transfer torques and spin orbit torques as well as for achieving large spin-wave propagation lengths (SWPL) 8-10 . The need for thin film materials with low magnetic damping has triggered the interest in the insulating ferrimagnet yttrium-iron garnet (Y 3 Fe 5 O 12 , YIG) 11-13 . Although for YIG, very small total (Gilbert) damping parameters in the order of α G ≈ 10 −5 , and large SWPLs of a few tens of micrometers (up to ∼ 25 µm) in thin films (∼ 20 nm) have been reported 5,13,14 , its insulating properties and requirement for crystalline growth are challenges for large scale magnonic applications.Schoen et al. recently observed ultra-low intrinsic magnetic damping in Co 25 Fe 75 (CoFe) metallic thin films (α 0 = (5 ± 1.8) × 10 −4 ) 15 , and Krner et al. reported PLs of 5 µm − 8 µm in CoFe using time resolved scanning magneto-optical Kerr microscopy 4 . This motivated our study on sputter-deposited CoFe-based thin film heterostructures. We use broadband ferromagnetic resonance (BB-FMR) spectroscopy 16 in outa) Electronic of-plane (OOP) geometry and Brillouin light scattering (BLS) microscopy 17 and find intrinsic damping parameters in the lower 10 −4 regime as well as SWPLs of more than 20 µm. The damping is therefore comparable to YIG/heavy metal (HM) heterostructures 18 and the SWPL is comparable to that of state-of-the-art YIG thin films 5,13 . Thin film CoFe is a promising candidate for all-metal magnonic devices, as it combines low magnetic damping with good electrical conductivity and large saturation magnetization, while enabling easy fab...

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