Spin Pumping Research Articles

Electric readout of Bloch sphere spanned by twisted magnon modes

We present a magnonic type of Bloch sphere based on twisted spin-wave (magnon) eigenmodes with opposite intrinsic orbital angular momentum, which is topology-protected and damping-resistant. Taking advantage of the release of the chiral degeneracy of magnons by dynamic dipolar interactions and/or interfacial Dzyaloshinskii–Moriya interactions in ferromagnetic nanodisks, we show how these magnonic “qubit” states can be precisely launched and electrically detected through combined spin pumping and inverse spin Hall effect. The experimental feasibility is verified using full-edged numerical micromagnetic simulations for FeB nanodisks. Our investigations demonstrate the potential of twisted spin waves for magnonic information encoding in a flexible and realizable approach.

Enhancement of thermal spin pumping by orbital angular momentum of rare earth iron garnet

In a bilayer of ferromagnetic and non-magnetic metal, spin pumping can be generated by a thermal gradient. The spin current generation depends on the spin mixing conductance of the interface and the magnetic properties of the ferromagnetic layer. Due to its low intrinsic damping, rare earth iron garnet is often used for the ferromagnetic layer in the spin Seebeck experiment. However, it is actually a ferrimagnetic with antiferromagnetically coupled magnetic lattices and the contribution of rare earth magnetic lattice of rare earth iron garnet on thermal spin pumping is not well understand. Here we focus on the effect of magnetic properties of lanthanide and show that the orbital angular momentum of rare earth iron garnet enhances thermal spin current generation of lanthanide substituted yttrium iron garnet.

Significant and Nonmonotonic Dynamic Magnetic Damping in Asymmetric Co - Fe/Ru/Co - Fe Trilayers

Static and dynamic magnetic properties of $\mathrm{Co}$-$\mathrm{Fe}$(10 nm)/$\mathrm{Ru}$$({t}_{\mathrm{Ru}}=0\ensuremath{-}3\phantom{\rule{0.25em}{0ex}}\mathrm{nm})$/$\mathrm{Co}$-$\mathrm{Fe}$(5 nm) asymmetric trilayers are systematically investigated. The interlayer exchange coupling (IEC) strengths of bilinear $({J}_{1})$, biquadratic $({J}_{2})$, and their equivalent $({J}_{\mathrm{eff}})$ terms are determined; they show a weak nonmonotonic behavior with ${t}_{\mathrm{Ru}}$. Interestingly, the magnetic remanence ratio \ensuremath{\eta} of hard axis to easy axis has two distinct peaks; this can reasonably be interpreted by taking the strong ${J}_{2}$ term into account. Various pump-laser fluences are utilized to modulate the static IEC during the time-resolved magneto-optical Kerr effect measurements, by which individual magnetization precessions of the two $\mathrm{Co}$-$\mathrm{Fe}$ layers are achieved and the effects of dynamic IEC through mutual spin currents are highlighted. With the increase in ${t}_{\mathrm{Ru}}$, the magnetic damping factors of both layers display the same nonmonotonic behavior, which has been mainly ascribed to the spin pumping damping ${\ensuremath{\alpha}}_{\mathrm{SP}}$ associated with the dynamic IEC. Moreover, it is found that the variation trend of damping difference $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ between the two $\mathrm{Co}$-$\mathrm{Fe}$ layers is similar to that of ${J}_{\mathrm{eff}}$, revealing that the dynamic IEC effect is actually dominated by the static IEC and thereupon a theoretical formula is proposed to describe the correlation between $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ and ${J}_{\mathrm{eff}}$. These results suggest the feasibility of efficient control of spin pumping damping through controlled IEC, which has great significance for microwave spintronic devices based on asymmetric trilayer structures.

Terahertz Spin-Current Pulses from an Off-Resonant Antiferromagnet

The generation of terahertz (THz) spin current is highly desired for enhancing the speed and efficiency of information manipulation by ultrafast spintronics. We develop an analytical model to investigate the coherent and pure spin-current pulses generated by spin pumping in a canted insulating antiferromagnet (AFM) system with Dzyaloshinskii-Moriya interaction (DMI). As expected, the spin pumping signal is significant at resonance. However, we also predict efficient THz spin-current transient with a magnitude comparable with that of its resonant counterparts in the much higher off-resonance THz frequency regime. Its ultrafast temporal change compensates for the suppression of the dynamic magnetization amplitudes at off-resonance. In addition, a stronger DMI field enhances the amplitude of the spin-current transient. Our results represent an efficient approach to generate coherent THz spin-current pulses based on canted AFM heterostructures without requiring a static magnetic field.

Enhanced subterahertz spin-current transients via modulation of cross-sublattice damping in uniaxial antiferromagnets

We present an analytical model to compute the subterahertz (sub-THz) spin current transients injected from the insulating uniaxial antiferromagnet (AFM) into the adjacent nonmagnetic layer excited by spin pumping under the antiferromagnetic resonance condition, where both intra- and cross-sublattice damping parameters are treated on an equal footing. As expected, the sub-THz spin-pumping signal decreases with larger intra-sublattice damping dissipation. Interestingly, it is found that the amplitude of the spin current transient is enhanced with increasing cross-sublattice damping. On the other hand, the spin pumping is reduced by increasing the cross-sublattice spin-mixing conductance. These trends indicate that the intrinsic origin of the cross-sublattice damping in the bulk AFM enhances the spin current transients while its extrinsic origin, directly related to the interfacial cross-sublattice spin-mixing conductance, has the opposite effect. Our results suggest the important role of the previously neglected cross-sublattice damping in modulating the sub-THz spin current pulses for ultrafast spintronic applications.

Spin rectification effect induced by planar Hall effect and its strong impact on spin-pumping measurements

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion in various systems. The reversing sign of the symmetric Lorentzian charge current with respect to the opposite magnetic field is generally accepted as the key criterion to identifying its pure spin current origin. However, we herein find that the rectified voltage due to the planar Hall effect can exhibit a similar spurious signal, complicating and even misleading the analysis. The distribution of microwave magnetic field and induction current has a strong influence on the magnetic field symmetry and line shape of the obtained signal. We further demonstrate a geometry where the spin-charge conversion and the rectified voltage can be readily distinguished with a straightforward symmetry analysis.

Global and local topological quantized responses from geometry, light, and time

To describe a spin-$\frac{1}{2}$ particle on the Bloch sphere with a radial magnetic field and topological states of matter from the reciprocal space, we introduce $C$ square (${C}^{2}$) as a local formulation of the global topological invariant. For the Haldane model on the honeycomb lattice, this ${C}^{2}$ can be measured from the Dirac points through circularly polarized light related to the high-symmetry $M$ point(s). For the quantum spin Hall effect and the Kane-Mele model, the ${\mathbb{Z}}_{2}$ topological number robust to interactions can be measured locally from a correspondence between the Pfaffian and light. We address a relation with a spin pump and the quantum spin Hall conductance. The analogy between light and magnetic nuclear resonance may be applied for imaging, among other applications.

Inverted fine structure of a 6H-SiC qubit enabling robust spin-photon interface

Controllable solid-state spin qubits are currently becoming useful building blocks for applied quantum technologies. Here, we demonstrate that in a specific type of silicon-vacancy in the 6H-SiC polytype the excited-state fine structure is inverted, compared to 4H-SiC. From the angular polarization dependencies of the emission, we reconstruct the spatial symmetry and determine the optical selection rules depending on the local deformation and spin–orbit interaction. We show that this system is well suited for the implementation of robust spin–photon entanglement schemes. Furthermore, the inverted fine structure leads to unexpected behavior of the spin readout contrast. It vanishes and recovers with lattice cooling due to two competing optical spin pumping mechanisms. Our experimental and theoretical approaches provide a deep insight into the optical and spin properties of atomic-scale qubits in SiC required for quantum communication and distributed quantum information processing.

Large exchange bias and spin pumping in ultrathin IrMn/Co system for spintronic device applications

Few nanometers thick antiferromagnetic/Ferromagnetic bilayer based spintronic devices have emerged as a potential nanostructured bilayer for achieving ultrahigh-speed magnetization switching, low power dissipation, terahertz magnetization dynamics, and are compatible with CMOS technology. The systematic investigation of the exchange bias (EB) in the Ir22Mn78/Co system is performed by varying Co thickness (tCo) in the range of 6–20 nm with 10 nm thin Ir22Mn78 layer on top, using longitudinal magneto-optic Kerr effect (L-MOKE) and ferromagnetic resonance (FMR) measurements at room temperature. Here, we report the occurrence of a record high EB field in this bilayer system, which is 13.11 mT (9.04 mT) statically (dynamically) for 6 nm thick of Co. The percentage change of 313.5 % (251.7%) in exchange bias field is found through FMR (MOKE) measurements with respect to tCo variation from 20 to 6 nm. Additionally, the spin pumping mechanism is also studied in the above stated material system by using the FMR technique. The observed linear dependence of effective Gilbert’s damping with respect to the 1/tCo, indicates the occurrence of spin pumping phenomena. The study suggests that tunability of both the exchange interaction and spin pumping behavior in this Ir22Mn78/Co system, makes this system suitable for future antiferromagnetic spintronic devices.

Influence of heat flow control on dynamical spin injection in CoFeB/Pt/CoFeB trilayer

A dynamical spin injection based on the ferromagnetic resonance in a ferromagnetic/nonmagnetic bi-layered structure, is a powerful mean for generating and manipulating the spin current. Although the mechanism of the dynamical spin injection is mainly attributed to the spin pumping, the detailed mechanism and the quantitative understanding for related phenomena are still controversial. As an another important contribution to the dynamical spin injection, the heating effect due to the resonant precessional motion of the magnetization is pointed out recently. In order to quantify the contribution from the heating effect, we here investigate the dynamical spin injection in a CoFeB/Pt/CoFeB trilayer. Although the contribution from the spin pumping diminishes because of the symmetric spin injection from the upper and lower interfaces, a significant inverse spin Hall voltage has been clearly observed. We show that the observed voltage can be quantitatively understood by the thermal spin injection due to a heating effect during the ferromagnetic resonance. A proper combination between the spin pumping and the heat-flow control in the multi-layered system is a key for the efficient dynamical spin injection.

Assessing the relaxation mechanisms contributions on magnetoimpedance effect in YIG/W bilayers

Damping parameter plays a key role in the magnetization dynamics of ferri/ferromagnetic materials with potential technological applications. Experimentally the damping parameter can be estimated from the linewidth of the ferromagnetic resonance experiment, which is altered by distinct mechanisms associated with extrinsic contributions, such as spin pumping and two-magnon scattering mechanisms. Here we perform a systematic investigation of the magnetization dynamics through magnetoimpedance effect in Y5Fe3O12(t)/W(100 nm) bilayers having t YIG between 25 and 125 nm. By modifying the YIG thickness, we explore the contributions of such extrinsic mechanisms to α eff . From the structural features, quasi-static magnetic properties, and ferromagnetic resonance results, we interpret our findings for the magnetization dynamics. Our results disclose that the bilayer structure allows us to explore the magnetoimpedance effect in a wide range of frequency and magnetic fields, providing insights on the effective damping parameter.

Unified formulation of interfacial magnonic pumping from noncollinear magnets

We establish a general formulation for spin current pumped by magnons at the interface between a normal metal and a magnetic insulator, valid for any noncollinear magnetic configuration in the linear spin wave regime. This current is generated by driving the system in a nonequilibrium state, covering setups such as thermal spin injection (spin Seebeck effect) or spin voltage by irradiation of the insulator (spin pumping). We illustrate the formula in the special case of a honeycomb topological ferromagnet, for simplicity, and cover both the spin Seebeck and spin pumping setups. We show how the topological parameters influence the spin current and propose a way to obtain a contribution mainly from the topological edge magnons in the spin pumping case.

Distinguishing spin pumping from spin rectification in lateral spin pumping device architectures based on doped organic semiconductors

Over the last two decades organic spintronics has developed into a striving field with exciting reports of long spin diffusion lengths and spin relaxation times in organic semiconductors (OSCs). Easily processed and inexpensive, OSCs are considered a potential alternative to inorganic materials for use in spintronic applications. Spin currents have been detected in a wide range of materials; however, there is still uncertainty over the origin of the signals. Recently, we explored spin transport through an organic semiconductor with lateral spin injection and detection architectures, where the injected spin current is detected nonlocally via spin-to-charge conversion in an inorganic detector. In this work we show that the widely used control experiments like linear power dependence and inversion of the signal with the magnetic field are not sufficient evidence of spin transport and can lead to an incorrect interpretation of the signal. Here, we use in-plane angular dependent measurements to separate pure spin signal from parasitic effects arising from spin rectification. Apart from well established anisotropic magnetoresistance and the anomalous Hall effect, we observe another spurious effect originating in Py and having the same angular symmetry as the inverse spin Hall effect (ISHE), which suggests it might be a self-induced ISHE.

The effect of hydrogen gas on Pd/[Co/Pd]30/Pd multilayer thin films

It is known that large perpendicular magnetic anisotropy is found at interfaces of cobalt (Co) with palladium (Pd). We investigated Pd/[Co/Pd]30/Pd multilayer thin films with very thin Co and Pd layers with ferromagnetic resonance (FMR) spectroscopy in air and in hydrogen gas. A number of samples characterised by different Pd thicknesses were studied. A ferromagnetic resonance (FMR) peak shift similar to Co/Pd bilayer thin films was observed in the presence of hydrogen gas in the sample environment for all samples. The FMR peak shift is larger for samples with thicker Pd layers. Additionally, we found a dependence of the linewidth on the Pd layer thickness and explain this with the presence of spin pumping.

Impact of gigahertz and terahertz transport regimes on spin propagation and conversion in the antiferromagnet IrMn

Control over spin transport in antiferromagnetic systems is essential for future spintronic applications with operational speeds extending to ultrafast time scales. Here, we study the transition from the gigahertz (GHz) to terahertz (THz) regime of spin transport and spin-to-charge current conversion (S2C) in the prototypical antiferromagnet IrMn by employing spin pumping and THz spectroscopy techniques. We reveal a factor of 4 shorter characteristic propagation lengths of the spin current at THz frequencies (∼0.5 nm) as compared to GHz experiments (∼2 nm). This observation may be attributed to different transport regimes. The conclusion is supported by extraction of sub-picosecond temporal dynamics of the THz spin current. We identify no relevant impact of the magnetic order parameter on S2C signals and no scalable magnonic transport in THz experiments. A significant role of the S2C originating from interfaces between IrMn and magnetic or non-magnetic metals is observed, which is much more pronounced in the THz regime and opens the door for optimization of the spin control at ultrafast time scales.

Quantitative Evaluation of Heating Effect on Dynamical Spin Injection Using CoFeB/Pt/CoFeB Trilayered Film

We have examined a dynamical spin injection in a CoFeB/Pt/CoFeB trilayered structure. Although the contribution of the spin pumping was eliminated by the symmetric spin injection from the upper and lower CoFeB layers, a clear voltage change due to the inverse spin Hall effect has been observed. We find that the observed signal can be quantitatively explained by considering the thermal spin injection due to the heating effect during ferromagnetic resonance (FMR). This innovative demonstration indicates the significant contribution of FMR heating in the dynamical spin injection.

Coherent Feedback Cooling of a Nanomechanical Membrane with Atomic Spins

Coherent feedback stabilises a system towards a target state without the need of a measurement, thus avoiding the quantum backaction inherent to measurements. Here, we employ optical coherent feedback to remotely cool a nanomechanical membrane using atomic spins as a controller. Direct manipulation of the atoms allows us to tune from strong-coupling to an overdamped regime. Making use of the full coherent control offered by our system, we perform spin-membrane state swaps combined with stroboscopic spin pumping to cool the membrane in a room-temperature environment to ${T}={216}\,\mathrm{mK}$ ($\bar{n}_{m} = 2.3\times 10^3$ phonons) in ${200}\,\mathrm{{\mu}s}$. We furthermore observe and study the effects of delayed feedback on the cooling performance. Starting from a cryogenically pre-cooled membrane, this method would enable cooling of the mechanical oscillator close to its quantum mechanical ground state and the preparation of nonclassical states.

Spin-orbit pumping

We study theoretically the effect of a rotating electric field on a diffusive nanowire and find an effect that is analogous to spin pumping, which refers to the generation of spin through a rotating magnetic field. The electron spin couples to the electric field because the particle motion induces an effective magnetic field in its rest frame. In a diffusive system the velocity of the particle, and therefore also its effective magnetic field, rapidly and randomly changes direction. Nevertheless, we demonstrate analytically and via a physical argument why the combination of the two effects described above produces a finite magnetization along the axis of rotation. This manifests as a measurable spin-voltage in the range of tens of microvolts.

The 50 nm-thick yttrium iron garnet films with perpendicular magnetic anisotropy

Yttrium iron garnet (YIG) films possessing both perpendicular magnetic anisotropy (PMA) and low damping would serve as ideal candidates for high-speed energy-efficient spintronic and magnonic devices. However, it is still challenging to achieve PMA in YIG films thicker than 20 nm, which is a major bottleneck for their development. In this work, we demonstrate that this problem can be solved by using substrates with moderate lattice mismatch with YIG so as to suppress the excessive strain-induced stress release as increasing the YIG thickness. After carefully optimizing the growth and annealing conditions, we have achieved out-of-plane spontaneous magnetization in YIG films grown on sGGG substrates, even when they are as thick as 50 nm. Furthermore, ferromagnetic resonance and spin pumping induced inverse spin Hall effect measurements further verify the good spin transparency at the surface of our YIG films.

Topological gaps in quasiperiodic spin chains: A numerical and K-theoretic analysis

Topological phases supported by quasi-periodic spin-chain models and their bulk-boundary principles are investigated by numerical and K-theoretic methods. We show that, for both the un-correlated and correlated phases, the operator algebras that generate the Hamiltonians are non-commutative tori, hence the quasi-periodic chains display physics akin to the quantum Hall effect in two and higher dimensions. The robust topological edge modes are found to be strongly shaped by the interaction and, generically, they have hybrid edge-localized and chain-delocalized structures. Our findings lay the foundations for topological spin pumping using the phason of a quasi-periodic pattern as an adiabatic parameter, where selectively chosen quantized bits of magnetization can be transferred from one edge of the chain to the other.