Spin Pumping Research Articles

Topological invariants based on generalized position operators and application to the interacting Rice-Mele model

We discuss different properties and the potential of several topological invariants based on position operators to identify phase transitions, and compare with more accurate methods, such as crossing of excited energy levels and jumps in Berry phases. The invariants have the form $\text{Im}\phantom{\rule{4.pt}{0ex}}\text{ln}\ensuremath{\langle}exp[i(2\ensuremath{\pi}/L){\mathrm{\ensuremath{\Sigma}}}_{j}{x}_{j}({m}_{{}_{\ensuremath{\uparrow}}}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{n}}_{j\ensuremath{\uparrow}}+{m}_{\ensuremath{\downarrow}}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{n}}_{j\ensuremath{\downarrow}})]\ensuremath{\rangle}$, where $L$ is the length of the system, ${x}_{j}$ the position of site $j$, and ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{n}}_{j\ensuremath{\sigma}}$ the operator of the number of particles at site $j$ with spin $\ensuremath{\sigma}$. We show that ${m}_{\ensuremath{\sigma}}$ should be integers, and in some cases of magnitude larger than 1, to lead to well-defined expectation values. For the interacting Rice-Mele model (which contains the interacting Su-Schrieffer-Heeger and the ionic Hubbard model as specific cases), we show that three different invariants give complementary information and are necessary and sufficient to construct the phase diagrams in the regions where the invariants are protected by inversion symmetry. We also discuss the consequences for pumping of charge and spin, and the effect of an Ising spin-spin interaction or a staggered magnetic field.

Spin to Charge Conversion in Semiconducting Antiferromagnetic Co3O4

Power efficient and faster spintronic devices require high spin current propagation efficiency from a ferromagnet (FM) to high spin orbit coupling material. Antiferromagnets (AFM) exhibiting high spin orbit coupling and faster spin dynamics have drawn great attention in such studies. Efficient spin current propagation in metallic AFMs via conduction electron showed promising results in spin to charge conversion physics. Recently, the insulating AFMs are also found to exhibit efficient spin current propagation due to the presence of magnonic modes. The semiconducting AFM Co3O4 is expected to show promising results in spin to charge conversion studies due to the simultaneous presence of magnon modes and conduction electrons. In this context, spin pumping and inverse spin Hall effect studies are carried out on a series of Co3O4(t nm)/Co(22 nm) samples where the t value varies from 0 to 26 nm. Damping analysis reveals that the spin diffusion length of Co3O4 is 2.5 ± 0.5 nm which is large in comparison to metallic AFMs. The maximum Hall angle of Co3O4 is found to be 0.26. A high spin transparency of 65.8% indicates efficient spin current propagation in the samples.

Hysteresis and training effect in the electric control of spin current in Pt/Y3Fe5O12 heterostructures

We have reported on the hysteresis and training effect of spin current in $\mathrm{Pt}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ heterostructures during subsequent cycles of ionic liquid gate voltage ${V}_{g}$. The inverse spin Hall effect voltage in spin pumping and spin Hall magnetoresistance exhibit diode-like behaviors in the first half cycle of ${V}_{g}$ and also show hysteresis in the first cycle of ${V}_{g}$. Both the diode-like behavior and the hysteresis become weak and even vanish in the second cycle of ${V}_{g}$ due to the training effect. The above experimental results can be well explained by the screening charge doping model, in which the charge and the local magnetic moment are asymmetrically distributed in the Pt layer. The applicability of this model is further confirmed by measurements of anisotropic magnetoresistance and ferromagnetic resonance. The diode-like behavior is attributed to interplay between the asymmetrically distributed local magnetic moment and the spin current relaxation in the Pt layer. The hysteresis and the training effect arise from the incompletely reversible process between oxidation and reduction of Pt atoms and the evolution of the surface morphology at the ionic liquid/Pt interface under electric gating. This work provides new insights to improve the functional performance of electrically controlled spin current devices.

Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations

Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.

Room-temperature spin injection from a ferromagnetic semiconductor

Spin injection using ferromagnetic semiconductors at room temperature is a building block for the realization of spin-functional semiconductor devices. Nevertheless, this has been very challenging due to the lack of reliable room-temperature ferromagnetism in well-known group IV and III-V based semiconductors. Here, we demonstrate room-temperature spin injection by using spin pumping in a BiSb/(Ga,Fe)Sb heterostructure, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC) and BiSb is a topological insulator (TI). Despite the very small magnetization of (Ga,Fe)Sb at room temperature (45 emu/cc), we detected spin injection from (Ga,Fe)Sb by utilizing the large inverse spin Hall effect (ISHE) in BiSb. Our study provides the first demonstration of spin injection at room temperature from a FMS.

Magnetic Damping and Dzyaloshinskii-Moriya Interactions in Pt/Co2FeAl/MgO Systems Grown on Si and MgO Substrates.

Spin-pumping-induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI) have been studied in Pt/Co2FeAl/MgO systems grown on Si or MgO substrates as a function of Pt and Co2FeAl (CFA) thicknesses. For this, we combined vibrating sample magnetometry (VSM), microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). VSM measurements of the magnetic moment at saturation per unit area revealed the absence of a magnetic dead layer in both systems, with a higher magnetization at saturation obtained for CFA grown on MgO. The key parameters governing the spin-dependent transport through the Pt/CFA interface, including the spin mixing conductance and the spin diffusion length, have been determined from the CFA and the Pt thickness dependence of the damping. BLS has been used to measure the spin wave non-reciprocity via the frequency mismatch between the Stokes and anti-Stokes lines. iDMI has been separated from the contribution of the interface perpendicular anisotropy difference between Pt/CFA and CFA/MgO. Our investigation revealed that both iDMI strength and spin pumping efficiency are higher for CFA-based systems grown on MgO due to its epitaxial growth confirmed by MS-FMR measurements of the in-plane magnetic anisotropy. This suggests that CFA grown on MgO could be a promising material candidate as a spin injection source via spin pumping and for other spintronic applications.

Underlying mechanism of the driving force for generating spin currents thermally in a ferrimagnetic insulator due to a temperature gradient

We previously proposed the idea that the effective thermal-energy transfer from the phonon system to the spin system in liquid phase epitaxial (LPE) films of Bi-substituted yttrium iron garnet (Bi:YIG), a ferrimagnetic insulator, takes place through enhanced spin-orbit coupling (SOC) with local 3d-electron spins in Fe3+ ions, and that the spin-thermoelectric (STE) generation in a paramagnetic platinum (Pt) layer is raised by increased spin pumping to the Pt layer from the Bi:YIG film. However, we could not specifically relate the underlying mechanism to the driving force for the generation of spin currents due to nonequilibrium dynamics of magnons driven by the temperature gradient ∇T in a magnetic insulator. In this paper, we discuss the origin of the driving force for generating spin currents, which are increased through enhanced SOC in LPE yttrium iron garnet films.

Spin Pumping of Magnons Coherently Coupled to a Cavity Dark Mode

Cavity magnonics provides a versatile platform to tune the photon-magnon interaction bringing about promising applications for quantum information processing. Here, we implement a metamaterial planar cavity to suppress the radiative damping by engineering the symmetry of the cavity configuration. Then, the mode with suppressed radiative damping, i.e., dark mode, is coupled to the magnon mode in a thin yttrium iron garnet film ($5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$), which is detected by performing the reflection measurements and spin-pumping experiment. Pronounced anticrossing of the ferromagnetic resonance and the cavity dark mode is observed, where the largest coupling strength reaches 2.5% of the resonant cavity frequency. In addition, the spin wave resonances along the thickness direction are identified, which enhances the spin-pumping signal when they coalesce with the hybridized cavity-magnon-polariton mode. The dark mode excited by the planar metamaterial cavity leads to an improvement in the coherent manipulation of the spin current.

Interfacial Tuning of Anisotropic Gilbert Damping

Tuning of the anisotropic Gilbert damping Δα has been realized in ultrathin single-crystalline Fe films grown on GaAs (001). A nonmonotonic dependence of Δα on film thickness t is observed upon varying t about 10ML (∼1.4 nm). Δα increases for 16 ML>t>8.5 ML, and then decreases for 8.5 ML>t>6.5 ML accompanied by a sign reversal of Δα for t=6.5 ML. The sign reversal of Δα is captured by first-principle calculations, which show that the anisotropic density of states changes sign upon decreasing t. Moreover, t^{-1} dependence of the anisotropic damping indicates the emergence of an anisotropic effective spin mixing conductance according to the theory of spin pumping. The results establish new opportunities for controlling the Gilbert damping and for fundamental studies of magnetization dynamics in reduced dimension.

Sizable spin-to-charge conversion in PLD-grown amorphous (Mo, W)Te2−x films

We report on the spin-to-charge conversion (SCC) in Mo0.25W0.75Te2−x (MWT)/Y3Fe5O12 (YIG) heterostructures at room temperature. The centimeter-scale amorphous MWT films are deposited on liquid-phase-epitaxial YIG by pulsed laser deposition technique. The significant SCC voltage is measured in the MWT layer with a sizable spin Hall angle of ∼0.021 by spin pumping experiments. The control experiments by inserting MgO or Ag layer between MWT and YIG show that the SCC is mainly attributed to the inverse spin Hall effect rather than the thermal or interfacial Rashba effect. Our work provides a novel spin-source material for energy-efficient topological spintronic devices.

Spin and spin current—From fundamentals to recent progress

Along with the progress of spin science and spintronics research, the flow of electron spins, i.e., spin current, has attracted interest. New phenomena and electronic states were explained in succession using the concept of spin current. Moreover, as many of the conventionally known spintronics phenomena became well organized based on spin current, it has rapidly been recognized as an essential concept in a wide range of condensed matter physics. In this article, we focus on recent developments in the physics of spin, spin current, and their related phenomena, where the conversion between spin angular momentum and different forms of angular momentum plays an essential role. Starting with an introduction to spin current, we first discuss the recent progress in spintronic phenomena driven by spin-exchange coupling: spin pumping, topological Hall torque, and emergent inductor. We, then, extend our discussion to the interaction/interconversion of spins with heat, lattice vibrations, and charge current and address recent progress and perspectives on the spin Seebeck and Peltier effects. Next, we review the interaction between mechanical motion and electron/nuclear spins and argue the difference between the Barnett field and rotational Doppler effect. We show that the Barnett effect reveals the angular momentum compensation temperature, at which the net angular momentum is quenched in ferrimagnets.

Spin blocking effect at Ni/Pt heterojunction

Spin current, the flow of spin angular momentum, can carry and transport energy and/or information without generating Joule heating, which makes spin-based devices become one of the potential aspects for the next-generation information processing devices. It is important to investigate the generation, transport, and detection of spins for developing spin-based devices, in which the spin transport and its related phenomena attract ongoing interest due to the complex interactions between spins and condensed matter system. Here, spin transport phenomenon is studied at a heterojunction consisting of ferromagnetic metal nickel and nonmagnetic heavy metal platinum, where transport spins are found to be totally blocked. Two series of spin-pumping devices, i.e. the yttrium iron garnet (YIG)/Ni/Pt trilayer devices and the contrastive YIG/Ni bilayer devices, are made in this work. The YIG serves as a substrate and spin-pump layer, on which nickel film and platinum film are deposited by a dc magnetron sputtering system. Spin currents are generated from YIG and injected into nickel layers by spin pumping technology. The voltage signals corresponding to the inverse spin Hall effect are detected and analyzed comparatively for both YIG/Ni/Pt trilayer device and YIG/Ni bilayer device. It is found that the platinum layers in YIG/Ni/Pt trilayer devices act only as charge current shunting but do not contribute to the spin-charge conversion. This implies that the spin current cannot transport through the Ni/Pt interface even when the nickel layer is as thin as 1 nm, in other words, the spin current is blocked at the Ni/Pt interface. Our result proposes a heterojunction that can block transport spins totally, which has never been discussed before, and the present study may expand the views and promote the development of spin-based devices.

Influence of parametric instability on spin pumping by dipole-exchange magnetostatic surface waves in YIG–Pt structures

The purpose of this work is to study the influence of four-magnon (4M) parametric instability on spin pumping by dipole-exchange magnetostatic surface waves (MSSW) with the help of the inverse spin Hall effect (ISHE) in structures based on yttrium-iron garnet (YIG) and platinum (Pt). Methods. The experiments were carried out using the delay line structures based on YIG(900 nm)/Pt(9 nm) where electromotive force (EMF) induced by ISHE demonstrates a growth at the frequencies of the resonant interaction between MSSW and volume exchange modes. The frequency dependencies of the amplitude and phase for the delay line structure and EMF (𝑈(𝑓)) from the platinum layer were studied as a function of the MSSW power. Results. It was shown that the resonant EMF growth at the frequencies of dipole-exchange resonances is caused by the presence of Van Hove singularities in the density of states for spin waves at such frequencies that leads to an increase in the efficiency of electron-magnon scattering at the YIG–Pt interface. A growth in MSSW power beyond the threshold of 4M instability development results in a “smoothing” of resonant particularities in the EMF frequency dependence 𝑈(𝑓) that can be explained by decreasing efficiency of spin pumping due to destruction of dipole-exchange resonances and related singularities in the density of states of spin waves. Conclusion. Obtained results may be of interest for the development of highly sensitive spin current detectors, as well as for the implementation of spintronic devices.

Частотная зависимость смешанной спиновой проводимости структур YIG|Pt при спиновой накачке ПМСВ

Spin pumping by traveling magnetostatic surface waves (MSSW) has been experimentally investigated in YIG|Pt structures made on the basis of 0.9, 4, 8, 14 and 18 µm thick YIG epitaxial films. It is found that at frequencies corresponding to the van Hove singularities in the density of states of the MSSW spectrum of the structure, the EMF value generated due to the inverse spin Hall effect increases. This increase is associated with an increase in the spin mixing conductance of the YIG|Pt interface due to an increase in the efficiency of electron-magnon scattering at the frequencies of van Hove singularities in the spin wave spectrum.

Frequency dependence of the spin mixing conductance of YIG|Pt structures upon MSSW spin pumping

Spin pumping by traveling magnetostatic surface waves (MSSW) has been experimentally investigated in YIG|Pt structures made on the basis of 0.9, 4, 8, 14 and 18 μm thick YIG epitaxial films. It is found that at frequencies corresponding to the van Hove singularities in the density of states of the MSSW spectrum of the structure, the EMF value generated due to the inverse spin Hall effect increases. This increase is associated with an increase in the spin mixing conductance of the YIG|Pt interface due to an increase in the efficiency of electron-magnon scattering at the frequencies of van Hove singularities in the spin wave spectrum. Keywords: spintronics, spin transport, spin waves, magnon density of states.

Automated mechanical exfoliation technique: a spin pumping study in YIG/TMD heterostructures.

Spintronics devices rely on the generation and manipulation of spin currents. Two-dimensional transition-metal dichalcogenides (TMDs) are among the most promising materials for a spin current generation due to a lack of inversion symmetry at the interface with the magnetic material. Here, we report on the fabrication of Yttrium Iron Garnet(YIG)/TMD heterostructures by means of a crude and fast method. While the magnetic insulator single-crystalline YIG thin films were grown by magnetron sputtering, the TMDs, namely MoS2 and MoSe2, were directly deposited onto YIG films using an automated mechanical abrasion method. Despite the brute force aspect of the method, it produces high-quality interfaces, which are suitable for spintronic device applications. The spin current density and the effective spin mixing conductance were measured by ferromagnetic resonance, whose values found are among the highest reported in the literature. Our method can be scaled to produce ferromagnetic materials/TMD heterostructures on a large scale, further advancing their potential for practical applications.

Heavy metal deposition temperature tuned spin pumping efficiency control in permalloy/tantalum bilayers

Spin pumping is a key property for spintronic application that can be realized in heavy metal/ferromagnet bilayers. Here we demonstrate the possibility of improving spin pumping in permalloy (Py)/tantalum (Ta) bilayers through control of Ta heavy metal deposition temperature. Through a combination of structural and ferromagnetic resonance based magnetization dynamics study, we reveal the role of Ta deposition temperature in improving spin mixing conductance which is a key parameter for spin pumping across the Py/Ta interface. The results show that by depositing Ta above room temperature, a high spin mixing conductance of 7.7 ×1018 m−2 is obtained with α-Ta layer. The results present an understanding of the correlation between heavy metal deposition temperature and interface structure improvement and consequent control of spin pumping in Py/Ta bilayers.

Significant Role of Interfacial Spin–Orbit Coupling in the Spin-to-Charge Conversion in Pt/NiFe Heterostructure

For the spin-to-charge conversion (SCC) in heavy metal/ferromagnet (HM/FM) heterostructure, the contribution of interfacial spin-orbit coupling (SOC) remains controversial. Here, we investigate the SCC process of the Pt/NiFe heterostructure by the spin pumping in YIG/Pt/NiFe/IrMn multilayers. Due to the exchange bias of NiFe/IrMn structure, the NiFe magnetization can be switched between magnetically unsaturated and saturated states by opposite resonance fields of YIG layer. The spin-pumping signal is found to decrease significantly when the NiFe magnetization is changed from the saturated state to the unsaturated state. Theoretical analysis indicates that the interfacial spin absorption is enhanced for the above-mentioned NiFe magnetic state change, which results in the increased and decreased spin flow in the Pt layer and across the Pt/NiFe interface, respectively. These results demonstrate that in our case the interfacial SOC effect at the Pt/NiFe interface is dominant over the bulk inverse spin Hall effect in the Pt layer. Our work reveals a significant role of interfacial SOC in the SCC in HM/FM heterostructure, which can promote the development of high-efficiency spintronic devices through interfacial engineering.

Unraveling the spin current flow in Bi layers

Although heavy metal materials, due to their large spin-orbit interaction, would be expected to act as an efficient converter of spin current to charge current, Bismuth does not obey this rule. Heterostructures of yttrium iron garnet (YIG)/Bi are investigated by means of the spin pumping technique and the results show that Bi exhibits negligible spin-charge conversion. In addition to not converting spin to charge, Bi exhibits an exotic spin relaxation mechanism, which is driven by its large diamagnetic response. By considering a nonlinear stochastic model of diffusing spin particles, the results show that the Bi layer acts as a binary classifier device that categorizes the pumped spins into relaxed spins and those that flow upwards and eventually reach the top layer of Pt. This shows that the investigation reported here opens different perspectives involving fundamental aspects in the fields of machine learning and condensed matter physics.

Magnetic energy dissipative factors of spin-coated Y[formula omitted]Fe[formula omitted]O12 thin films

Y3Fe5O12 (YIG) is a ferrimagnetic insulator that shows significant potential in various applications such as spin pumping and optical devices. In the case of YIG, the uncompensated antiferromagnetic coupled Fe3＋ ions originate the ferrimagnetic ordering. A homogeneous YIG thin film with lower Gilbert damping constant is essential for its industrial applications. Therefore, the YIG’s intrinsic and extrinsic magnetic energy dissipative factors are crucial to explore. Here, we report the growth of uniform polycrystalline YIG thin films on thermally oxidized Si (100) substrates. X-ray diffraction confirms the single-phase formation of YIG. Surface morphology and thickness on thin films are studied using scanning electron microscopy. X-ray photoemission is used to probe the valence state of constituent elements of the ferrimagnetic thin films with saturation magnetization 3.11 μB/f.u. The lowest Gilbert damping constant (α) = 4.754 × 10−3 with an inhomogeneous contribution to the linewidth of 5.04 mT is observed due to extrinsic inhomogeneous growth of the deposited films.