Ultrafast perturbation of magnetic domains by optical pumping in a ferromagnetic multilayer
Dmitriy Zusin, Ezio Iacocca, Loïc Le Guyader, Alexander H. Reid, William F. Schlotter, Tian-Min Liu, Daniel J. Higley, Giacomo Coslovich, Scott F. Wandel, Phoeb… 展开作者
Dmitriy Zusin, Ezio Iacocca, Loïc Le Guyader, Alexander H. Reid, William F. Schlotter, Tian-Min Liu, Daniel J. Higley, Giacomo Coslovich, Scott F. Wandel, Phoebe M. Tengdin, Sheena K. K. Patel, Anatoly Shabalin, Nelson Hua, Stjepan B. Hrkac, Hans T. Nembach, Justin M. Shaw, Sergio A. Montoya, Adam Blonsky, Christian Gentry, Mark A. Hoefer, Margaret M. Murnane, Henry C. Kapteyn, Eric E. Fullerton, Oleg Shpyrko, Hermann A. Dürr, T. J. Silva
Ultrafast optical pumping of spatially nonuniform magnetic textures is known to induce far-from-equilibrium spin transport effects. Here, we use ultrafast x-ray diffraction with unprecedented dynamic range to study the laser-induced dynamics of labyrinth domain networks in ferromagnetic CoFe/Ni multilayers. We detected azimuthally isotropic, odd order, magnetic diffraction rings up to 5th order. The amplitudes of all three diffraction rings quench to different degrees within 1.6 ps. In addition, all three of the detected diffraction rings both broaden by 15% and radially contract by 6% during the quench process. We are able to rigorously quantify a 31% ultrafast broadening of the domain walls via Fourier analysis of the order-dependent quenching of the three detected diffraction rings. The broadening of the diffraction rings is interpreted as a reduction in the domain coherence length, but the shift in the ring radius, while unambiguous in its occurrence, remains unexplained. In particular, we demonstrate that a radial shift explained by domain wall broadening can be ruled out. With the unprecedented dynamic range of our data, our results provide convincing evidence that labyrinth domain structures are spatially perturbed at ultrafast speeds under far-from-equilibrium conditions, albeit the mechanism inducing the perturbations remains yet to be clarified.
We study the magnetization dynamics in thin ferromagnetic films and small ferromagnetic particles in contact with paramagnetic conductors. A moving magnetization vector causes \textquotedblleft pumping\textquotedblright of spins into adjacent nonmagnetic layers. This spin transfer affects the magnetization dynamics similar to the Landau-Lifshitz-Gilbert phenomenology. The additional Gilbert damping is significant for small ferromagnets, when the nonmagnetic layers efficiently relax the injected spins, but the effect is reduced when a spin accumulation build-up in the normal metal opposes the s…
Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of rapid magnetic order recovery in materials with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify both localisation and coalescence regimes, whereby localised magnetic textures nucleate and subsequently evolve in accordance with a power law fo…
We report the magnetic response of Co/Pt multilayers to picosecond electrical heating. Using photoconductive Auston switches, we generate electrical pulses with 5.5 picosecond duration and hundreds of pico-Joules to pass through Co/Pt multilayers. The electrical pulse heats the electrons in the Co/Pt multilayers and causes an ultrafast reduction in the magnetic moment. A comparison between optical and electrically induced demagnetization of the Co/Pt multilayers reveals significantly different dynamics for optical vs. electrical heating. We attribute the disparate dynamics to the dependence of…
Layered van-der-Waals materials with hexagonal symmetry offer an extra degree of freedom to their electrons, the so called valley index or valley pseudospin. This quantity behaves conceptually like the electron spin and the term valleytronics has been coined. In this context, the group of semiconducting transition-metal dichalcogenides (TMDC) are particularly appealing, due to large spin-orbit interactions and a direct bandgap at the K points of the hexagonal Brillouin zone. In this work, we present investigations of excitonic transitions in mono- and multilayer WSe$_2$ and MoSe$_2$ materials …
We measured spin transport in nonferromagnetic (NM) metallic multilayers from the contribution to damping due to spin pumping from a ferromagnetic Co90Fe10 thin film. The multilayer stack consisted of NM1/NM2/Co90Fe10(2 nm)/NM2/NM3 with varying NM materials and thicknesses. Using conventional theory for one dimensional diffusive spin transport in metals, we show that the effective damping due to spin pumping can be strongly affected by the spin transport properties of each NM in the multilayer, which permits the use of damping measurements to accurately determine the spin transport properties …
The critical current of the spin transfer torque-driven magnetization dynamics was studied by taking into account both spin pumping and the finite penetration depth of the transverse spin current. We successfully reproduced the recent experimental results obtained by Chen et al. [Phys. Rev. B {\bf 74}, 144408 (2006)] and found that the critical current remains finite even in the zero-thickness limit of the free layer. We showed that the remaining value of the critical current is determined mainly by spin pumping.
In this work we focus on magnetic relaxation in Mn$_{80}$Ir$_{20}$(12 nm)/ Cu(6 nm)/ Py($d_\mathrm{F}$) antiferromagnet/Cu/ferromagnet (AFM/Cu/FM) multilayers with different thickness of the ferromagnetic permalloy layer. An effective FM-AFM interaction mediated via the conduction electrons in the nonmagnetic Cu spacer -- the spin-pumping effect -- is detected as an increase in the linewidth of the ferromagnetic resonance (FMR) spectra and a shift of the resonant magnetic field. We further find experimentally that the spin-pumping-induced contribution to the linewidth is inversely proportional…
We study coherent nonlinear dynamics of carriers under ultrafast interband excitation of an intrinsic graphene. The Rabi oscillations of response appear with increasing of pumping intensity. The photoexcited distribution is calculated versus time and energy taking into account the effects of energy relaxation and dephasing. Spectral and temporal dependencies of the response on a probe radiation (transmission and reflection coefficients) are considered for different pumping intensities and the Rabi oscillations versus time and intensity are analyzed.
The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 cm2/Vs at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for MoS2, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier…
In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has been achieved without the need of any symmetry breaking magnetic field. This phenomenon is often referred to as all optical switching (AOS). However, so far, GdFeCo remains the only material system where…
Symmetry is a powerful concept in physics, but its applicability to far-from-equilibrium states is still being understood. Recent attention has focused on how far-from-equilibrium states lead to spontaneous symmetry breaking. Conversely, ultrafast optical pumping can be used to drastically change the energy landscape and quench the magnetic order parameter in magnetic systems. Here, we find a distinct symmetry-dependent ultrafast behaviour by use of ultrafast x-ray scattering from magnetic patterns with varying degrees of isotropic and anisotropic symmetry. After pumping with an optical laser,…
Probing Interfacial Spin Dynamics and Temperature Dependent Asymmetry in Spin Pumping Across Ni80Fe20/Cu/Cr1.12Te2 Interfaces
Department of Physics, Indian Institute of Technology Kanpur（印度坎普尔理工学院物理系）；Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University（南洋理工大学国立教育学院自然科学与科学教育）
We investigate the cross-interactions in a two-component polariton quantum fluid coherently driven by two independent pumping lasers tuned at different energies and momenta. We show that both the hysteresis cycles and the ON/OFF threshold of one polariton signal can be entirely controlled by a second polariton fluid. Furthermore, we study the ultrafast switching dynamics of a driven polariton state, demonstrating the ability to control the polariton population with an external laser pulse, in less than a few picoseconds.
Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.
Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn
The highly efficient, easy-to-implement, long-ranged and non-destructive way to realize active pumping has been still a great challenge. Here, using molecular dynamics simulations, terahertz electromagnetic wave (TEW) is firstly employed to stimulate an active pump for water transportation by biasedly irradiated in a (6,6) single-walled carbon nanotube (SWCNT) under no external pressure gradient. It is found that an ultrafast conductivity (up to 9.5 /ns) through the pump around a characteristic frequency of 14 THz. The excellent pumping ability is attributed to the resonance coupling between t…
Transient optical heating provides an efficient way to trigger phase transitions in naturally occurring media through ultrashort laser pulse irradiation. A similar approach could be used to induce topological phase transitions in the photonic response of suitably engineered artificial structures known as metamaterials. Here, we predict a topological transition in the isofrequency dispersion contours of a layered graphene metamaterial under optical pumping. We show that the contour topology transforms from elliptic to hyperbolic within a subpicosecond timescale by exploiting the extraordinary p…
Based on spin-dependent transport theory and thermodynamics, we develop a generalized theory of the Joule heating in the presence of a spin current. Along with the conventional Joule heating consisting of an electric current and electrochemical potential, it is found that the spin current and spin accumulation give an additional dissipation because the spin-dependent scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a change of entropy. The theory is applied to investigate the dissipation due to pure spin-current generated by spin pumping across a ferromagnetic/nonmagneti…
We report the experimental observation of collective picosecond magnetization dynamics in [Co/Pd]8 multilayers with perpendicular magnetic anisotropy. The precession frequency shows large and systematic variation from about 5 GHz to about 90 GHz with the decrease in the Co layer thickness from 1.0 nm to 0.22 nm due to the linear increase in the perpendicular magnetic anisotropy. The damping coefficient 'alpha' is found to be inversely proportional to the Co layer thickness and a linear relation between the perpendicular magnetic anisotropy and 'alpha' is established. We discuss the possible re…
The field of THz spintronics is a novel direction in the research field of spintronics that combines magnetism with optical physics and ultrafast photonics. The experimental scheme of the field involves the use of femtosecond laser pulses to trigger ultrafast spin and charge dynamics in bilayers composed of ferromagnetic (FM) and non-magnetic (NM) thin films where the NM layer features a strong spin-orbit coupling. The key technological and scientific challenges of THz spintronic emitters is to increase their intensity and frequency bandwidth. To achieve this the control of the source of the r…
Using a multilayer structure containing (cobalt detector layer)/(copper spacer)/(Permalloy source layer), we show experimentally how the non-reorientable spin-orbit torque generated by the Permalloy source layer (the component of spin-orbit torque that does not change when the Permalloy magnetization is rotated) can be measured using spin-torque ferromagnetic resonance (ST-FMR) with lineshape analysis. We find that dynamic spin pumping between the magnetic layers exerts torques on the magnetic layers as large or larger than the spin-orbit torques, so that if dynamic spin pumping is neglected t…
In magnetic trilayer systems, spin pumping is generally addressed as a reciprocal mechanism characterized by one unique spin mixing conductance common to both interfaces. However, this assumption is questionable in cases where different types of interfaces are present in the material. Here, we present a general theory for analyzing spin pumping in cases with more than one unique interface. The theory is applied to analyze layer-resolved ferromagnetic resonance experiments on the trilayer system Ni$_{20}$Fe$_{80}$/Ru/Fe$_{49}$Co$_{49}$V$_2$ where the Ru spacer thickness is varied to tune the in…
Coherent manipulation of quantum bits (qubits) on time scales much shorter than the coherence time is a key prerequisite for quantum information processing. Electron spins in quantum dots (QDs) are particularly attractive for implementations of qubits. Efficient optical methods for initialization and readout of spins have been developed in recent years. Spin coherence times in the microsecond range have been demonstrated, so that spin control by picosecond optical pulses would be highly desirable. Then a large number of spin rotations could be performed while coherence is maintained. A major r…
We demonstrate the tuneablity of the ultrafast energy flow in magnetic/non-magnetic bilayer structures by changing the wavelength of the optical excitation. This is achieved by an advanced description of the temperature based $μ$T-model that explicitly considers the wavelength- and layer-dependent absorption profile within multilayer structures. For the exemplary case of a Ni/Au bilayer, our simulations predict that the energy flow from Ni to Au is reversed when changing the wavelength of the excitation from the infrared to the ultraviolet spectral range. These predictions are fully supported …
Spin pumping in nonmagnetic/ferromagnetic metal multilayers is studied both theoretically and experimentally. We show that the line widths of the ferromagnetic resonance (FMR) spectrum depend on the thickness of the ferromagnetic metal layers, which must not be in resonance with the oscillating magnetic field. We also show that the penetration depths of the transverse spin current in ferromagnetic metals can be determined by analyzing the line widths of their FMR spectra. The obtained penetration depths in NiFe, CoFe and CoFeB were 3.7 [nm], 2.5 [nm] and 12.0 [nm], respectively.
We determined the spin-transport properties of Pd and Pt thin films by measuring the increase in ferromagnetic resonance damping due to spin-pumping in ferromagnetic (FM)-nonferromagnetic metal (NM) multilayers with varying NM thicknesses. The increase in damping with NM thickness depends strongly on both the spin- and charge-transport properties of the NM, as modeled by diffusion equations that include both momentum- and spin-scattering parameters. We use the analytical solution to the spin-diffusion equations to obtain spin-diffusion lengths for Pt and Pd. By measuring the dependence of cond…
The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetisation reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetisation of bulk nickel, in tandem with the expected demagnetisation. As …
We demonstrate a strong tunability of the spin-pumping contribution to magnetic damping in a thin-film ferromagnetic free layer interfaced with a synthetic ferrimagnet (SFM), acting as a spin-sink, via a thin Cu-spacer. The effect strongly depends on the magnetic state of the SFM, a trilayer structure composed of two Fe layers coupled via indirect exchange mediated by a Cr spacer. With increasing Cr thickness, the SFM state undergoes a transition from an antiparallel via a non-collinear to a parallel configuration. We can explain the corresponding non-monotonous dependence of spin relaxation i…
Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring i…
The richness in both the dispersion and energy of antiferromagnetic magnons has spurred the magnetism community to consider antiferromagnets for future spintronic/magnonic applications. However, the excitation and control of antiferromagnetic magnons remains challenging, especially when compared to ferromagnetic counterparts. A middle ground is found with synthetic antiferromagnet metamaterials, where acoustic and optical magnons exist at GHz frequencies. In these materials, the magnon energy spectrum can be tuned by static symmetry-breaking external fields or dipolar interactions hybridizing …
Ultrafast optical excitation of magnons in 2D antiferromagnetic semiconductors via spin torque mediated by unbound electron-hole pairs and excitons: Signatures in magnonic charge pumping
Department of Physics and Astronomy, University of Delaware（特拉华大学物理与天文学系）
The total angular momentum of a close system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has extinguished. Such conservation, however, is never satisfied in practice in any real-time first principles description of the demagnetization process. Furthermore, there is a growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here we critically analys…
Recent development of the magnetic material engineering led to achievement of the systems with a high interfacial Dzyaloshinskii-Moriya interaction (DMI). As a result, the formation of non-collinear magnetic soliton states or nonreciprocal spin wave dynamics is achievable. Typically used materials are based on bi-layers Heavy Metal/Ferromagnet, e.g., Pt/Co. These layers are characterized not only by a strong DMI, but also by the spin pumping effect and the resulting relatively large damping. Here, we show that the considerable interfacial DMI can be also present in bi-layers based on Ru/Co, ch…
We report the observation of a feedback process between the nuclear spins in a single charged quantum dot and its trion transition, driven by a periodic sequence of optical pulses. The pulse sequence intersperses off-resonant ultrafast pulses for coherent electron-spin rotation and resonant narrow-band optical pumping. The feedback manifests as a hysteretic triangle-like pattern in the free-induction-decay of the single spin. We present a simple, quasi-analytic numerical model to describe this observation, indicating that the feedback process results from the countering effects of optical nucl…
Spin-orbitronics and single pulse all-optical switching (AOS) of magnetization are two major successes of the rapidly advancing field of nanomagnetism in recent years, with high potential for enabling novel, fast and energy-efficient memory and logic platforms. Fast current-induced domain wall motion (CIDWM) and single shot AOS have been individually demonstrated in different ferrimagnetic alloys. However, the stringent requirement for their composition control makes these alloys challenging materials for wafer-scale production. Here, we simultaneously demonstrate fast CIDWM and energy efficie…
We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L$_3$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.
We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator; N-normal-metal) multilayers of nanoscale thickness attached to two normal metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the "sample," and realistic three-dimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit who…
Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ~20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate …
We report on room temperature ferromagnetic resonance (FMR) studies of [$t$ Co$|2t$ Ni]$\times$N sputtered films, where $0.1 \leq t \leq 0.6$ nm. Two series of films were investigated: films with same number of Co$|$Ni bilayer repeats (N=12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N=1.2/$t$). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field norm…
Optical dual-pulse pumping actively creates quantum-mechanical superposition of the electronic and phononic states in a bulk solid. We here made transient reflectivity measurements in an n-GaAs using a pair of relative-phase-locked femtosecond pulses and found characteristic interference fringes. This is a result of quantum-path interference peculiar to the dual-pulse excitation as indicated by theoretical calculation. Our observation reveals that the pathway of coherent phonon generation in the n-GaAs is impulsive stimulated Raman scattering at the displaced potential due to the surface-charg…
The quest to improve density, speed and energy efficiency of magnetic memory storage has led to exploration of new ways of optically manipulating magnetism at the ultrafast time scale, in particular in ferrimagnetic alloys. While all-optical magnetization switching is well-established on the femtosecond timescale, lateral nanoscale confinement and thus potential significant reduction of the size of the magnetic element remains an outstanding challenge. Here we employ resonant electromagnetic energy-funneling plasmon nanoantennas to influence the demagnetization dynamics of a ferrimagnetic TbCo…
We demonstrate the generation of high order harmonics in carrier pumping from precessing ferromagnetic or antiferromagnetic orders, excited via magnetic resonance, in the presence of topological antiferromagnetic textures. This results in an enhancement of the carrier dynamics by orders of magnitude. Interestingly, the generation process occurs in an intrinsic manner, and is solely governed by the interplay between the s-d exchange coupling underlying the noncollinear antiferromagnetic order and the dynamical s-d exchange parameter of the magnetic drive. Therefore, the relativistic spin-orbit …
Spintronic structures are extensively investigated for their spin orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular the intrinsic inverse spin Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are inves…
We monitor the dynamics of hot carriers in InSe by means of two photons photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with the $\bar M$ valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab-initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\bar M$ valley could travel through a multilayer fl…
Advances in silicon photonics have resulted in rapidly increasing complexity of integrated circuits. New methods are desirable that allow direct characterization of individual optical components in-situ, without the need for additional fabrication steps or test structures. Here, we present a new device-level method for characterization of photonic chips based on a highly localized modulation in the device using pulsed laser excitation. Optical pumping perturbs the refractive index of silicon, providing a spatially and temporally localized modulation in the transmitted light enabling time- and …
The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time, , and intrinsic Gilbert damping, . In previous attempts to establish the relationship between and , the rare-earth doping of a permalloy film with two different demagnetization mechanism is not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between and by means of time-resolved magneto-optical Kerr effect (TRMOKE) via adju…
We advance all optical spin noise spectroscopy (SNS) in semiconductors to detection bandwidths of several hundred gigahertz by employing an ingenious scheme of pulse trains from ultrafast laser oscillators as an optical probe. The ultrafast SNS technique avoids the need for optical pumping and enables nearly perturbation free measurements of extremely short spin dephasing times. We employ the technique to highly n-doped bulk GaAs where magnetic field dependent measurements show unexpected large g-factor fluctuations. Calculations suggest that such large g-factor fluctuations do not necessarily…
Spin pumping in Yttrium-iron-garnet (YIG)/nonmagnetic-metal (NM) layer systems under ferromagnetic resonance (FMR) conditions is a popular method of generating spin current in the NM layer. A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications. It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer. Here, by combining microwave absorption and DC-voltage measurements on YIG/Pt and YIG/NM1/NM2 (NM1=Cu or Al, NM2=Pt or Ta), we unambiguously showed that spin current i…
