Spin Pumping Effect Research Articles

Effect of magnon decays on parametrically pumped magnons

We investigate the influence of magnon decays on the non-equilibrium dynamics of parametrically excited magnons in the magnetic insulator yttrium-iron garnet (YIG). Our investigations are motivated by a recent experiment by Noack et al. [Phys. Status Solidi B 256, 1900121 (2019)] where an enhancement of the spin pumping effect in YIG was observed near the magnetic field strength where magnon decays via confluence of magnons becomes kinematically possible. To explain the experimental findings, we have derived and solved kinetic equations for the non-equilibrium magnon distribution. The effect of magnon decays is taken into account microscopically via collision integrals derived from interaction vertices involving three powers of magnon operators. Our results agree quantitatively with the experimental data.

Chiral Emission of Exchange Spin Waves by Magnetic Skyrmions.

Spin waves or their quanta magnons raise the prospect to act as information carriers in the absence of Joule heating. The challenge to excite spin waves with nanoscale wavelengths free of nanolithography becomes a critical bottleneck for the application of nanomagnonics. Magnetic skyrmions are chiral magnetic textures at the nanoscale. In this work, short-wavelength exchange spin waves are demonstrated to be chirally emitted in a low damping magnetic insulating thin film by magnetic skyrmions. The spin-wave chirality originates from the chiral spin pumping effect and is determined by the cross product of the magnetization orientation and the film normal direction. The Halbach effect explains the enhancement or attenuation of the spin-wave amplitude with a reversed sign of the Dyzaloshinskii-Moriya interaction. Controllable spin-wave propagation is demonstrated by rotating a moderate applied field. Our findings are key for building compact low-power nanomagnonic devices based on intrinsic nanoscale magnetic textures.

Interfacial chemical states and recoverable spin pumping in YIG/Pt

Ion etching is an essential step in the processing of spintronic devices. In this work, we investigated the role of argon ion (Ar+) bombardment in the spin pumping and inverse spin Hall effect in the Y3Fe5O12/Pt (YIG/Pt) heterostructure. The inverse spin Hall voltage is found to reduce by an order of two when the argon ion bombardment is employed on the YIG surface before the deposition of the Pt layer. This giant inhibition of spin injection efficiency is undesirable. In this work, we propose an experimental technique for its recovery via a chemical route. The interface property and chemical state were identified by transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. We found that the argon ion bombardment on the YIG surface leads to an increase in the ratio of Fe2+ ions in the YIG/Pt interface region. Moreover, the interface magnetic moment reduces in the presence of Fe2+ ions, which resulted in the decrease in spin injection efficiency. A strong oxidizing solution (a mixture of concentrated H2SO4 and 30% H2O2 of 1:1 volume ratio) was used to recover the valence of iron and subsequently the interface magnetic moment. Our results are helpful for the understanding of the importance of interface properties and the optimization of spintronic device processing technology.

Parametron on magnetic dot: Stable and stochastic operation

Magnetization dynamics induced by parametric excitation in a magnetic dot has been investigated by using ac spin pumping and inverse spin-Hall effects. An Ising-like pair of states with different precession phases was found to be stabilized in a controllable way under the excitation. The result shows that the dot can be used as a parametron-bit carrier. Upon increasing the excitation power, stochastic transition between the states was observed, and the occurrence probability of each state can be tuned by means of additional microwaves, opening an application to probabilistic bit operation.

Spin injection characteristics of Py/graphene/Pt by gigahertz and terahertz magnetization dynamics driven by femtosecond laser pulse

Spin transport characteristics of graphene have been extensively studied so far. The spin transport along the c-axis is however reported by rather limited number of papers. We have studied spin transport characteristics through graphene along the c-axis with permalloy(Py)/graphene(Gr)/Pt by gigahertz (GHz) and terahertz (THz) magnetization dynamics driven by femtosecond laser pulses. The relatively simple sample structure does not require electrodes on the sample. The graphene layer was prepared by chemical vapor deposition and transferred on Pt film. The quality of the graphene layer was characterized by Raman microscopy. Time-resolved magneto-optical Kerr effect is used to characterize gigahertz magnetization dynamics. Magnetization precession is clearly observed both for Pt/Py and Pt/Gr/Py. The Gilbert damping constant of Pt/Py was 0.015, indicating a spin pumping effect from Py to Pt. The Gilbert damping constant of Pt/Gr/Py was found to be 0.011, indicating that the graphene layer blocks spin injection. We also performed the measurement of THz emission for Pt/Py and Pt/Gr/Py. While a THz emission is clearly observed for Pt/Py, a substantial reduction of THz emission is observed for Pt/Gr/Py. With these two different experiments, and highly anisotropic resistivity of graphite, we conclude that the vertical spin transport is strongly suppressed by the graphene layer.

Inverse Spin Hall Effect enhancement in Pt layer doped by Ge

We systematic studied the spin pumping (SP) and inverse spin Hall effects (ISHE) in yttrium iron garnet Y3Fe5O12 (YIG)/PtGe bilayer using a coplanar waveguide based broadband ferromagnetic resonance setup. More efficient spin injection occurs at the YIG/PtGe interface. The spin Hall angle of Pt (4.4%) and PtGe (9%) are obtained from the metal layer thickness dependent inverse spin Hall voltage measurement. PtGe films are promising materials with large spin Hall angles for future spintronics applications.

Impact of interfacial chemical state on spin pumping and inverse spin Hall effect in YIG/Pt hybrids

The interface between ferromagnetic (FM) and nonmagnetic (NM) metal plays a fundamental role in the generation, transmission, detection of spin current, and spin-based devices. The impact of physical properties on the interface such as roughness, crystallization conditions, and even impedance matching has been widely investigated. In this work, the effects of interfacial chemical state at the interface of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ and Pt (YIG/Pt) are systematically investigated through spin injection. Prior to Pt deposition, the spin mixing conductance and inverse spin Hall voltage of YIG/Pt hybrids increased up to 300%, using surface chemical processing on YIG. The interfacial chemical state is identified from x-ray photoelectron spectroscopy spectra and peak decomposition. Both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations appear at the interface and higher content of ${\mathrm{Fe}}^{3+}$ enhances the interface magnetic moment which consequently plays a crucial role in spin injection efficiency and inverse spin Hall voltage. In order to prevent the influence of magnetic proximity effect of Pt layers close to the FM/NM interface, a Cu spacer is inserted. It is found that the improved spin injection efficiency due to iron cations still exists in the YIG/Cu/Pt structure, which confirms the universal influence of the interfacial chemical state on spin pumping efficiency. This discovery has potential implications for spin generation, detection, and even optimization of spintronic devices.

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator Bi2Se3.

Bi2Se3 is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λIREE). We report an λIREE of ∼0.36 nm, which is the highest ever observed in Bi2Se3. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated Bi2Se3/CoFeB bilayer. The Bi2Se3 thickness dependence of λIREE, perpendicular surface anisotropy (KS), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of KS. The SCCE is found to be high when the value of KS is small.

Finite-frequency spin susceptibility and spin pumping in superconductors with spin-orbit relaxation

Static spin susceptibility of superconductors with spin-orbit relaxation has been calculated in the seminal work of A.A. Abrikosov and L.P. Gor'kov [Sov. Phys. JETP, {\bf 15}, 752 (1962)]. Surprisingly the generalization of this result to finite frequencies has not been done despite being quite important for the modern topic of superconducting spintronics. The present paper fills this gap by deriving the analytical expression for spin susceptibility. The time-dependent spin response is shown to be captured by the quasiclassical Eilenberger equation with collision integrals corresponding to the ordinary and spin-orbit scattering. Using the developed formalism we study the linear spin pumping effect between the ferromagnet and the adjacent superconducting film. The consequences for understanding recent experiments demonstrating the modification of Gilbert damping by the superconducting correlations are discussed.

Anomalous Hall effect in 3d/5d multilayers mediated by interface scattering and nonlocal spin conductivity

We evidenced unconventionnal Anomalous Hall Effects (AHE) in 3d/5d (Co0.2nm/Ni0.6nm)N multilayers grown on a thin Pt layer or thin Au:W alloy. The inversion observed on AHE originates from the opposite sign of the spin-orbit coupling of Pt compared to Ni. Via advanced simulations methods for the description of the spin-current profiles based on the spin-dependent Boltzmann formalism, we extracted the spin Hall angle (SHA) of Pt and (Co/Ni) as well as the relevant transport parameters. The extracted SHA for Pt, +20%, is opposite to the one of (Co/Ni), giving rise to an effective AHE inversion for thin (Co/Ni) multilayers (N < 17). The spin Hall angle in Pt is found to be larger than the one previously measured in combined spin-pumping inverse spin-Hall effect experiments in a geometry of current perpendicular to plane. Whereas magnetic proximity effects cannot explain the effect, spin-current leakage and anisotropic electron scattering at Pt/(Co,Ni) interfaces fit the experiments.

Strong interface-induced spin-charge conversion in YIG/Cr heterostructures

Here, we have investigated the spin pumping effect of Y3Fe5O12 (YIG)/Cu (tCu nm)/Cr heterostructures at room temperature with the thickness of the Cu interlayer varying from 0.4 nm to 5.0 nm. A huge charge signal Ic = 0.239 μA is observed in a YIG/Cr bilayer with direct contact, whereas Ic drops dramatically by two orders of magnitude when thin Cu interlayers down to 0.4 nm are inserted between YIG and Cr. Meanwhile, the injected spin current Js stays almost invariant for all the heterostructures. The effective spin Hall angle “θSH” of the YIG/Cr interface is found to be three orders of magnitude larger than the spin Hall angle θSH of the bulk Cr layer in YIG/Cu/Cr. The huge spin-charge conversion efficiency at the YIG/Cr interface is attributed to the inverse Rashba–Edelstein effect. Our experimental results demonstrate the dominant role of the interfacial effect in the spin-charge conversion process of the YIG/Cr heterostructures.

Exchange stiffness and damping constants of spin waves in CoFeB films

Exchange stiffness and damping of spin waves in a soft ferromagnet of CoFeB are investigated using a time-resolved pump-probe technique. A pump pulse excites coherent spin waves in CoFeB via optical spin-orbit torque from Pt, and a probe pulse detects the phase of the spin waves via magneto-optic Kerr effect. From the frequency of spin waves, exchange stiffness is determined to be 13 pJ m−1, which is nearly independent of the CoFeB thickness from 6 to 16 nm and the annealing temperature of 400 °C. Damping constant of spin waves is determined from the relaxation of spin waves. Importantly, the spin wave damping increases with decreasing the CoFeB thickness, suggesting the spin pumping effect at the interface between CoFeB and Pt. The thickness dependence of damping becomes stronger after annealing at 400 °C, and the spin wave damping of >0.08 is obtained at a CoFeB thickness of 6.3 nm. Such large damping of spin waves would have an important effect on the operation of the CoFeB-based memory devices.

Coherent spin waves driven by optical spin-orbit torque

Phase-coherent spin waves can be generated by magnetic field pulse, spin current pulse, or optical pulse. Here we use optical spin-orbit torque, originating from the conversion of an optical pulse into a spin-polarized current pulse, to excite spin waves in the frequency range from GHz to THz. We investigate the frequency, amplitude, and damping of the spin waves of Co thin films. From the frequency analysis, we determine the stiffness of Co spin waves to be $5\phantom{\rule{0.16em}{0ex}}\mathrm{meV}\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{2}$. From the amplitude analysis, we show that the Co layer acts as a cavity for spin waves. From the damping analysis, we observe that the damping enhancement due to the spin pumping effect is about two times larger in spin waves than in uniform precession.

Underlayer material dependent symmetric and asymmetric behavior of voltage-controlled magnetic anisotropy in CoFeB films

Voltage-controlled magnetic anisotropy (VCMA), observed at the interfaces of ultrathin ferromagnetic metallic films and oxide layer, has proven to be a useful tool for the development of all-electric field controlled spintronics devices. Here, we have studied the symmetric and asymmetric behavior of VCMA in CoFeB/MgO heterostructures, grown on different underlayer materials, by measuring ferromagnetic resonance using spin pumping and inverse spin Hall effect technique. We observe symmetric behavior of VCMA in CoFeB films with Ta underlayer, whereas a systematic transformation from symmetric to asymmetric behavior of VCMA with decreasing CoFeB thickness is observed for Pt underlayer. We speculate that the increased interfacial roughness, defects and strain of ultrathin CoFeB films with Pt buffer layer probably leads to the complicated band structure at CoFeB/MgO interface resulting in asymmetric behavior of VCMA. The observed symmetric behavior of VCMA in control samples justifies the role of interfacial roughness, defects and discards the role of oxide overlayer on the observed asymmetric behavior of VCMA in ultrathin CoFeB films.

Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity

Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent non-magnetic layers via the spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analog of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04 ± 0.02 times larger SAW power absorption in a device with cavity than in the case of no acoustic cavity. We confirmed up to 2.96 ± 0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect at the interface between Cu and Bi2O3. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon–phonon dynamics.

Manipulation of Coupling and Magnon Transport in Magnetic Metal-Insulator Hybrid Structures

Ferromagnetic metals and insulators are widely used for generation, control and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on Si substrate, we studied the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We found that within this structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both the vibrating-sample magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an ON/OFF ratio of ~130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin Seebeck effect experiments. Thanks to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could potentially find applications in magnon-based logic and memory devices.

Large Spin Hall Angle and Spin-Mixing Conductance in the Highly Resistive Antiferromagnet Mn2Au

Antiferromagnetic (AFM) materials recently have shown interest in the research in spintronics due to its zero stray magnetic field, high anisotropy, and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn attention because it exhibits unique properties and its Neel temperature is very high. Here, we report spin pumping and inverse spin Hall effect investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance. We found large spin Hall angle {\theta}_SH = 0.22

Large spin Hall angle in nonmagnetic PtSn alloy films at room temperature

High efficient nonmagnetic materials for generating and detecting spin currents are extremely sought after for prospective spintronic devices. We investigate the spin pumping and inverse spin Hall effects (ISHE) in Y3Fe5O12 (YIG)/ Pt1-xSnx (x = 0.06, 0.11, 0.17, 0.23, 0.28) bilayer using a coplanar waveguide based broadband ferromagnetic resonance setup. We find that the measured effective spin mixing conductance geff↑↓ and spin Hall angle θSH strongly correlate with the Sn content. A large spin Hall angle θSH = 0.089 ± 0.001 is obtained in a Pt0.77Sn0.23 alloy film while θSH = 0.055 ± 0.001 in pure Pt thin film under the same conditions. This enhancement of spin Hall angle is due to skew scattering from the Sn impurities resulting from spin-orbit interaction. Meanwhile, the spin Hall (SH) resistivity of the spin current induced by Sn impurities in Pt has been revealed. The SH resistivity of Pt1-xSnx remain pretty low values (0.17–1.1 × 10−9 Ω⋅m), which are promising for construct high-speed spintronic devices exploiting the spin Hall effect.

Spin pumping and laser modulated inverse spin Hall effect in yttrium iron garnet/germanium heterojunctions

In this work, undoped semiconductors, germanium (Ge) and germanium tin (GeSn), were grown on ferrimagnetic insulator yttrium iron garnet (YIG) thin films using ultra-high vacuum molecular beam epitaxy. The crystallinity of the structure was determined from x-ray diffraction and high-resolution transmission electron microscopy combined with energy dispersive x-ray spectroscopy. Both spin pumping and inverse spin Hall effects (ISHEs) of YIG/Ge and YIG/GeSn heterojunctions have been investigated with the help of broadband ferromagnetic resonance (FMR). We observe that the spin mixing conductances of YIG/Ge (60 nm) and YIG/GeSn (60 nm) are 5.4 × 1018 m−2 and 7.2 × 1018 m−2, respectively, responsible for giant spin current injection. Furthermore, it is found that spin pumping injects giant spin current from ferrimagnetic YIG into the Ge semiconductor. The infrared laser modulated ISHE was examined using heavy metal platinum as a spin current collector. Also, it has been noted that the variation in the power of laser irradiation significantly changed the ISHE voltage of YIG/Ge/Pt spin junctions, saturated magnetization, FMR linewidth, and Gilbert damping parameter of YIG, which could be attributed to the laser-induced thermal effect. The outcomes from this study are promising for the development of Ge-based spintronic and magnonic devices.

The abnormal damping behavior due to the combination between spin pumping and spin back flow in Ni80Fe20/Rut bilayers

The spin pumping effect of the Ni80Fe20/Rut bilayers was studied by ferromagnetic resonance method. A shutter shift technique was adopted to prepare the wedge-like Ru caplayer on the Ni80Fe20 film with homogeneous thickness and damping constant. Thanks to this special preparation method, the very tiny variation of damping constant due to the spin pumping at the Ni80Fe20/Rut interface was precisely measured and extracted from that of Ni80Fe20 films. Comparing with the conventional monotonically approaching saturation curve of damping constant vs. non-magnetic Ru layer thickness tRu, an additional damping peak was observed. The abnormal damping-tRu curves can be understood as the combined action of spin pumping at Ni80Fe20/Rut interface and spin back flow due to the reflection at Ru surface and Ru/Ni80Fe20 interface.