Spin Pumping Research Articles

The Giant Spin-to-Charge Conversion of the Layered Rashba Material BiTeI.

Rashba spin-orbit coupling (SOC) could facilitate an efficient interconversion between spin and charge currents. Among various systems, BiTeI holds one of the largest Rashba-type spin splittings. Unlike other Rashba systems (e.g., Bi/Ag and Bi2Se3), an experimental investigation of the spin-to-charge interconversion in BiTeI remains to be explored. Through performing an angle-resolved photoemission spectroscopy (ARPES) measurement, such a large Rashba-type spin splitting with a Rashba parameter αR = 3.68 eV Å is directly identified. By studying the spin pumping effect in the BiTeI/NiFe bilayer, we reveal a very large inverse Rashba-Edelstein length λIREE ≈ 1.92 nm of BiTeI at room temperature. Furthermore, the λIREE monotonously increases to 5.00 nm at 60 K, indicating an enhanced Rashba SOC at low temperature. These results suggest that BiTeI films with the giant Rashba SOC are promising for achieving efficient spin-to-charge interconversion, which could be implemented for building low-power-consumption spin-orbitronic devices.

Single-beam integrated hybrid optical pumping spin exchange relaxation free magnetometer for biomedical applications

An ingenious approach to accomplish the high signal strengthen and relatively homogeneous spin polarization has been presented in a hybrid optical pumping spin-exchange-relaxation-free atomic magnetometer only utilizing single-beam configuration. We have experimentally demonstrated an approximately three-fold enhancement of the output signal at the optimal spin polarization by optically pumping the thin vapor due to the same spin evolution behavior of the two different kinds of vapor atoms. Eventually, a measuring sensitivity of 30 fT/Hz1/2 was achieved combined with the homemade differential detection system for attenuating large background offset and suppressing optical power noise. This scheme provides a prospect for the development of ultra-highly sensitive and chip-scale atomic magnetometer for the applications that desire both high signal-to-noise ratio and uniform spin polarization, such as magnetocardiography and magnetoencephalography.

Spin rectification effects in ferromagnetic metal microstrips induced by anisotropic magnetoresistance, planar Hall effect, and anomalous Hall effect

Spin rectification effect (SRE) is an important electrical detection tool employed to probe magnetic dynamics and spin-orbit torques, and it is also closely related to spin pumping measurements. The SRE has multiple physical origins, and the entanglement among them makes it difficult to derive quantitative information. Herein, we present a systematic study of the SRE in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ and Ni microstrips with out-of-plane-dominant microwave magnetic field excitation. The SREs from different physical mechanisms can be quantitatively decomposed via a symmetry consideration. The validity of the method is further supported by angular-dependent analyses. We observe both the anisotropic magnetoresistance induced SRE from the longitudinal induction current, and the planar Hall effect as well as the anomalous Hall effect induced SREs from the transverse induction current. Our work provides a comprehensive understanding of the SREs in ferromagnetic metal microstrips, and it is beneficial for quantitative analyses in microwave-related studies in spintronics.

The enhanced ultrafast demagnetization at low temperature for MBE-grown Co2FeAl film on GaAs

The ultrafast demagnetization of Co2FeAl thin film grown on n-GaAs by molecular beam epitaxy was investigated with the time-resolved magneto-optical Kerr technique, by varying temperature, pump wavelength and fluence. A faster demagnetization with stronger magnetization quenching was observed at the on-resonant excitation of GaAs band-edge at 10 K. In addition, the pump fluence dependent ultrafast demagnetization at 10 K exhibits different behavior from that of the room temperature case. The enhanced spin angular momentum dissipation induced by the spin pumping effect was suggested to contribute to the ultrafast demagnetization for Co2FeAl thin film epitaxied on GaAs especially at low temperature, in addition to the local spin-flip scattering. Our results provide experimental proof for the ultrafast demagnetization facilitated by the spin pumping effect, which is of great importance for understanding the ultrafast spin angular momentum dissipation in ferromagnetic/semiconductor heterojunctions.

Introduction to nuclear spin waves in ferro- and antiferromagnets

Collective nuclear spin excitations, called nuclear spin waves or magnons, are enabled in strongly magnetic materials by the hyperfine coupling of the nuclear and electronic spins in an atom and the exchange interaction between electronic spins of neighboring atoms. Nuclear spin waves attracted the interest of theoretical and experimental researchers worldwide about four to five decades ago and then waned. Very recently, two experimental reports of nuclear spintronic effects in the canted antiferromagnet MnCO3 have shown that spin currents can be generated using nuclear spin states, bridging two quite separate worlds, one of nuclear spin excitations and the other of spintronics. In this Tutorial, we briefly review the basic concepts and properties of nuclear spin waves in ferro- and antiferromagnetic (AF) materials and present a few significant experimental results obtained some time ago with the uniaxial anisotropy AF MnF2 and the cubic anisotropy AF RbMnF3 and compare them with theory. We also briefly present the recent experimental observations of the nuclear spin pumping effect and the nuclear spin Seebeck effect in the canted antiferromagnet MnCO3. Other possible AF candidates for studies of nuclear spintronic effects are discussed.

Theoretical analysis of FMR-driven spin pumping current and its properties via the self-consistent harmonic approximation

We applied the self-consistent harmonic approximation (SCHA), combined with coherent states formalism, to study the ferromagnetic resonance (FMR) in a ferromagentic/normal metal junction. Due to the interface interaction, the FMR-generated spin current is injected from the magnetic insulator to the normal metal, the so-called spin pumping. Ordinarily, ferromagnetic models are described by bosonic representation or phenomenological theories; however, in a coherent magnetization state, the SCHA is the more natural choice to treat FMR problems. Over the years, the SCHA has successfully applied to investigate ferro- and antiferromagnetism in a wide range of scenarios. The main point of the SCHA formalism involves the adoption of a quadratic model for which corrections are included through temperature-dependent renormalization parameters. Therefore the SCHA is an efficient method for determining the properties of magnetically ordered phases. Using the SCHA, we obtained the temperature dependence of FMR-driven spin pumping. In addition, we found the spin-mix conductance, the additional damping from the angular momentum injection into the normal metal side, and the magnetic susceptibility. The SCHA outcomes are in remarkable agreement with the results of the literature.

Field-assisted sub-terahertz spin pumping and auto-oscillation in NiO

Spin pumping converting sub-terahertz electromagnetic waves to DC spin currents has recently been demonstrated in antiferromagnets (AFMs) with easy-axis magnetic anisotropy. However, easy-plane AFMs such as NiO, which are easier to prepare experimentally, are considered to be bad candidates for spin pumping because the Néel vector oscillation is linearly polarized, placing a major restriction on the material choice for practical applications. Through a case study of NiO, we show that an applied magnetic field below the spin–flop transition can substantially modify the polarization of the resonance eigenmodes, which enables coherent sub-terahertz spin pumping as strong as that in easy-axis AFMs. In addition, we find that an applied magnetic field can significantly reduce the threshold of Néel vector auto-oscillation triggered by spin-transfer torques. These prominent field-assisted effects can greatly facilitate spintronic device engineering in the sub-terahertz frequency regime.

Spin pumping by higher-order dipole-exchange spin-wave modes

Spin pumping (SP) and inverse spin Hall effect (ISHE) driven by parametrically-excited dipole-exchange spin waves in a yttrium iron garnet film have been systematically investigated. The measured voltage spectrum exhibits a feature of the field-induced transition from parallel pumping to perpendicular pumping because of the inhomogeneous excitation geometry. Thanks to the high precision of the SP-ISHE detection, two sets of fine structures in the voltage spectrum are observed, which can correspond well to two kinds of critical points in the multimode spin-wave spectrum for magnetic films. One is the q = 0 point of each higher-order dispersion branch, and the other is the local minimum due to the interplay between the dipolar and exchange interactions. These fine structures on the voltage spectrum confirm the spin pumping by higher-order dipole-exchange spin-wave modes, and are helpful for probing the multimode spin-wave spectrum.

Excitation and detection of coherent sub-terahertz magnons in ferromagnetic and antiferromagnetic heterostructures

Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent sub-terahertz (THz) magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are also performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject sub-THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using a dynamical phase-field model that considers the coupled dynamics of acoustic waves, spin waves, and EM waves, we show that the emitted EM wave retains the spectral information of all the sub-THz magnon modes and has a sufficiently large amplitude for near-field detection. These predictions indicate that the excitation and detection of sub-THz magnons can be realized in rationally designed FM or AFM thin-film heterostructures via ultrafast optical-pump THz-emission-probe spectroscopy.

Spin Pumping through Different Spin-Orbit Coupling Interfaces in β-W/Interlayer/Co2FeAl Heterostructures.

Spin pumping has been considered a powerful tool to manipulate the spin current in a ferromagnetic/nonmagnetic (FM/NM) system, where the NM part exhibits large spin-orbit coupling (SOC). In this work, the spin pumping in β-W/Interlayer (IL)/Co2FeAl (CFA) heterostructures grown on Si(100) is systematically investigated with different ILs in which SOC strength ranges from weak to strong. We first measure the spin pumping through the enhancement of effective damping in CFA by varying the thickness of β-W. The damping enhancement in the bilayer of β-W/CFA (without IL) is found to be ∼50% larger than the Gilbert damping in a single CFA layer with a spin diffusion length and spin mixing conductance of 2.12 ± 0.27 nm and 13.17 ± 0.34 nm-2, respectively. Further, the ILs of different SOC strengths such as Al, Mg, Mo, and Ta were inserted at the β-W/CFA interface to probe their impact on damping in β-W/ILs/CFA. The effective damping reduced to 8% and 20% for Al and Mg, respectively, whereas it increased to 66% and 75% with ILs of Mo and Ta, respectively, compared to the β-W/CFA heterostructure. Thus, in the presence of ILs with weak SOC, the spin pumping at the β-W/CFA interface is suppressed, while for the high SOC ILs effective damping increased significantly from its original value of β-W/CFA bilayer using a thin IL. This is further confirmed by performing inverse spin Hall effect measurements. In summary, the transfer of spin angular momentum can be significantly enhanced by choosing a proper ultrathin interface layer. Our study provides a tool to increase the spin current production by inserting an appropriate thin interlayer which is useful in modifying the heterostructure for efficient performance in spintronics devices.

High spin mixing conductance and spin transparency in ion-beam sputtered Ta/Co60Fe20B20 bilayers on Si (100)

Ta/CoFeB heterostructure has been considered as a promising structure for spin torque-based memory applications. Here, a systematic investigation of the magnetic (static and dynamic) properties of ion-beam sputtered Ta/Co60Fe20B20(CFB) bilayer thin films has been done. In the series, the thickness of CFB (tCFB) layer is varied from 2 to 24 nm, keeping the buffer Ta layer thickness fixed at 5 nm as a spin sink reservoir. The structural studies reveal a mixed phase (α+β) growth of Ta layer, while CFB is amorphous, and the interface between them is quite smooth with extremely low interface roughness (<3Å). The saturation magnetization is found to be increased from 957(±30) kA/m to 1097(±19) kA/m with tCFB while the coercivity lies in the range of 0.18-0.75 mT, suggesting the highly soft magnetic nature of the CFB films. Ferromagnetic resonance spectroscopy was performed to study the spin pumping response of the Ta/CFB bilayer thin films. From the variation of effective magnetization with tCFB, surface anisotropy energy of the CFB film is calculated to be 0.24±0.07 erg/cm2. From the effective damping change with tCFB, the effective spin mixing conductance (geff↑↓) and transparency (T) of the Ta/CFB interface are extracted and found to be 11.74±1.14 nm−2 and 60.61±5.88 %, respectively. The origin of large geff↑↓ and T lies at the smooth interface between Ta and CFB. Such a large values of these parameters in Ta/CFB heterostructures are essential requirements for spin device applications.

Quantum scattering theory of spin transfer torque, spin pumping, and fluctuations

Spin transfer torque and spin pumping are central reciprocal phenomena in spintronics. The author demonstrates a way to understand these effects within a quantum out-of-equilibrium path-integral model. At a finite temperature, frequency, or spin accumulation, the magnet also experiences fluctuating torques. In the quantum regime, transport coefficients related to electron shot noise caused by the discrete spin angular momentum of electrons govern the fluctuations at low temperatures. The results open a path for exploring low-temperature magnetization dynamics and spin caloritronics.

Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor δ -TaN

In the course of searching for promising topological materials for applications in future topological electronics, we evaluated spin-orbit torques (SOTs) in high-quality sputtered \ensuremath{\delta}-TaN/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ devices through spin-torque ferromagnetic resonance (ST-FMR) and spin pumping measurements. From the ST-FMR characterization we observed a significant linewidth modulation in the magnetic ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ layer attributed to the charge-to-spin conversion generated from the \ensuremath{\delta}-TaN layer. Remarkably, the spin-torque efficiency determined from ST-FMR and spin pumping measurements is as large as $\mathrm{\ensuremath{\Theta}}=$ 0.034 and 0.031, respectively. These values are over two times larger than for \ensuremath{\alpha}-Ta, but almost five times lower than for \ensuremath{\beta}-Ta, which can be attributed to the low room temperature electrical resistivity $\ensuremath{\sim}74\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in \ensuremath{\delta}-TaN. A large spin diffusion length of at least $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ is estimated, which is comparable to the spin diffusion length in pure Ta. Comprehensive experimental analysis, together with density functional theory calculations, indicates that the origin of the pronounced SOT effect in \ensuremath{\delta}-TaN can be mostly related to a significant contribution from the Berry curvature associated with the presence of a topically nontrivial electronic band structure in the vicinity of the Fermi level (${E}_{\mathrm{F}}$). Through additional detailed theoretical analysis, we also found that an isostructural allotrope of the superconducting \ensuremath{\delta}-TaN phase, the simple hexagonal structure \ensuremath{\theta}-TaN, has larger Berry curvature, and that, together with expected reasonable charge conductivity, it can also be a promising candidate for exploring a generation of spin-orbit torque magnetic random access memory as cheap, temperature stable, and highly efficient spin current source.

Rectification of the spin Seebeck current in noncollinear antiferromagnets

In the absence of an external magnetic field and a spin-polarized charge current, an antiferromagnetic system supports two degenerate magnon modes. An applied thermal bias activates the magnetic dynamics, leading to a magnon flow from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate bands contribute to the magnon current but the orientations of the magnetic moments underlying the magnons are opposite in different bands. Therefore, while the magnon current is nonzero, the net spin current is zero. To obtain a nonzero net spin current, one needs to apply either a magnetic field or a spin-polarized charge current that lifts the bands' degeneracy. Here, attaching a thermal contact to one edge of a helical nanowire, we study three different magnonic spin currents: (i) the exchange, and (ii) Dzyaloshinskii--Moriya spin currents flowing along the helical nanowire, and (iii) magnonic spin current pumped into the adjacent normal-metal layer. We find that the combination of Dzyaloshinskii--Moriya interaction and external magnetic field substantially enhances the spin current compared to the current generated solely through a magnetic field. Due to nonreciprocal magnons and magnon dichroism effect, the Dzyaloshinskii--Moriya and exchange spin currents show left-right propagation asymmetry, with $20%$ of current rectification. The spin pumping current shows a slight asymmetry only in the case of a strong Dzyaloshinskii--Moriya interaction. The observed effects are explained in terms of the magnon dispersion relations and the magnon Doppler effect.

Néel vector driven spin current in a van der Waals antiferromagnetic insulator ( CrCl3 )/heavy metal (Pt) bilayer

The widely distributed GHz to THz spin dynamics of antiferromagnets, combined with the recent demonstration of spin pumping, underscores the potential of dynamic antiferromagnetic devices. Here, we report antiferromagnetic spin pumping from a van der Waals antiferromagnetic insulator ${\text{CrCl}}_{3}$. Spin pumping driven by the N\'eel vector and the net magnetization has been verified, by adjusting the angle between two sublattice magnetizations from ${180}^{\ensuremath{\circ}}$ to ${0}^{\ensuremath{\circ}}$ while measuring the spin pumping voltages. An enhancement of acoustic mode spin current while decreasing the angle is observed, which proves the spin current is driven by the net magnetization. However, the optical mode spin current increases first and then decreases with the decreasing angle, which proves the optical mode spin current is driven by the N\'eel vector. These findings reveal the underlying contributions of $\mathbit{n}$ in antiferromagnets, and its distinctive role in antiferromagnetic spin pumping.

Coupling the Higgs mode and ferromagnetic resonance in spin-split superconductors with Rashba spin-orbit coupling

We show that the Higgs mode of superconductors can couple with spin dynamics in the presence of a static spin-splitting field and Rashba spin-orbit coupling. The Higgs-spin coupling dramatically modifies the spin susceptibility near the superconducting critical temperature and consequently enhances the spin pumping effect in a ferromagnetic insulator/superconductor bilayer system. We show that this effect can be detected by measuring the magnon transmission rate and the magnon-induced voltage generated by the inverse spin Hall effect.

Interplay of spin current and magnetization in a topological-insulator/magnetic-insulator bilayer structure

The interaction between accumulated spins on the surface of a heavy metal (HM) and the magnetization of an adjacent magnetic material leads to various spin phenomena, such as spin-orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low charge-to-spin conversion efficiency of the HM. Authors of recent studies have suggested that topological insulators (TIs) are promising candidates for device applications due to their potentially higher charge-to-spin conversion efficiency. Here, we report a multifaceted study of a bilayer structure consisting of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}\mathrm{O}$ (YIG) and demonstrate an approach based on angle-dependent magnetoresistance (ADMR) measurements to determine the effective charge-to-spin conversion efficiency in TIs. Our ferromagnetic resonance measurements demonstrate efficient spin pumping from YIG to ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which is further confirmed by detection of an electromotive force generated in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ via spin-to-charge conversion. Our ADMR measurements show that the interfacial spin diffusion can significantly affect the charge transport in a way like the SHMR effect and provide an estimate of the charge-to-spin conversion efficiency in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ of $\ensuremath{\sim}0.1--0.4$. Neglecting to account for the large out-of-plane magnetoresistance of the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ results in a fivefold overestimate of the charge-to-spin conversion efficiency.

Micromagnetically integrated numerical model of spin pumping based on spin diffusion

We present a nanoscopic numerical model of spin pumping, complete with spin-torque effects, as an extension of a spin-diffusion solver. The model is fully integrated in a micromagnetics finite-element framework. It is shown that this model can recreate analytical solutions for standard problems. Furthermore, the Gilbert damping of a propagating magnon can be properly modeled. The model can be used to examine spin-pumping effects, and those caused by it, e.g., inverse spin Hall, on a nanoscopic scale.

Theoretical approach and experimental investigation of spin current tunnelling in Ni80Fe20/Si3N4/Fe40Co37Zr11N12 magnetically collinear trilayer films with defined in-plane uniaxial anisotropy

Ferromagnetic trilayer film systems with nonferromagnetic Si3N4 interface layers were fabricated by using magnetron sputtering. A post-annealing process was carried out at 400 °C for 2 h in a static magnetic field, in order to induce an in-plane uniaxial anisotropy. The present study introduces the exchange interaction between ferromagnetic layers which influences the static magnetisation properties as well as the natural resonance frequency behaviour and its damping features, i.e., damping and anti-damping effects. This is expressed by the total damping parameter αitot = αGi ± αspi, which is enhanced or reduced by a spin transfer torque damping parameter ± αspi whereupon Gilbert bulk damping αGi is considered to be independent. The static polarisation loops are slightly but noticeably different, caused by an interface with thickness up to 100 nm, which reflects a decreasing exchange interaction. With varying the interface thickness, the films also show a distinct variation in their frequency spectra. This leads to the conclusion that spin currents caused by spin pumping arise, and tunnelling within a Si3N4 insulating barrier generates spin transfer torque. This can be assumed because the observed ferromagnetic resonance dual lines show remarkable changes in their Full Width at Half Maxima (FWHM) at frequencies of approximately 600 MHz caused by the Ni80Fe20 layers and 2.32 GHz caused by the Fe40Co37Zr11N12 layers. Regarding the FWHM of the resonance frequency spectra in association with the damping constant αeff on the dependence of the interface layer thickness, one can observe an exponential curve shape for both ferromagnetic layers, which is obviously a sign for the interaction between the layers through a spin current tunnelling mechanism.

Reproducible low Gilbert damping yttrium iron garnet by magnetron sputtering

Reproducible growth of single-crystalline (111) yttrium iron garnet (YIG) with very low Gilbert damping (α) has been reported by the use of several techniques, and continues to attract the attention of many groups in sprintronics and magnonics. We report on the growth of YIG thin films onto GGG (111) substrates by magnetron sputtering, followed by an ex-situ oxygen flow heat treatment. The magnetic properties were studied by ferromagnetic resonance, and the structural and morphological properties by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. The annealing parameters and thickness dependency on the magnetic properties was systematically investigated. A decrease in α for almost ten times was found preventing the YIG growth at the edges of the substrate by the use of lithography. For YIG 60 nm thick α were measured as (1.72 ± 0.08) × 10−4, Ms = 179 ± 2 emu cm−3, and surface roughness as 0.23 nm. To quantify the growth as being of good quality, we measured the spin current injection via spin pumping to an adjacent bismuth-doped copper layer with spin mixing conductance g↑↓ = 2.16 × 1018 m−2, for a platinum layer g↑↓ = 2.17 × 1018 m−2, with the detection of a 7 μV in the inverse spin Hall effect voltage, quantities which are in good agreement with the best values reported so far.