Interfacial Spin Mixing Conductance Research Articles

Strong variation of spin-orbit torques with relative spin relaxation rates in ferrimagnets

Spin-orbit torques (SOTs) have been widely understood as an interfacial transfer of spin that is independent of the bulk properties of the magnetic layer. Here, we report that SOTs acting on ferrimagnetic FexTb1-x layers decrease and vanish upon approaching the magnetic compensation point because the rate of spin transfer to the magnetization becomes much slower than the rate of spin relaxation into the crystal lattice due to spin-orbit scattering. These results indicate that the relative rates of competing spin relaxation processes within magnetic layers play a critical role in determining the strength of SOTs, which provides a unified understanding for the diverse and even seemingly puzzling SOT phenomena in ferromagnetic and compensated systems. Our work indicates that spin-orbit scattering within the magnet should be minimized for efficient SOT devices. We also find that the interfacial spin-mixing conductance of interfaces of ferrimagnetic alloys (such as FexTb1-x) is as large as that of 3d ferromagnets and insensitive to the degree of magnetic compensation.

Effective spin dynamic control of CoFeB/Nd heterostructure by matched resistivity

The spin dynamics modulation has attracted extensive attention in the past decades. Rare-earth (RE) metals are essential participants in this context due to the large spin–orbit coupling. Here, with neodymium (Nd) capping, we achieve the enhancement on spin dynamic damping of Co40Fe40B20 (CFB) films by three times larger than that of CFB single layer. Based on the spin pumping theory, the interfacial spin mixing conductance Geff↑↓ is calculated as 7.3 × 1015 cm−2, which is one order larger than that of CFB/Pt. It leads to the large spin current transparency at CFB/Nd interface. By comparing of the resistivity of each layer, we found that the matched resistivity at two sides of the CFB/Nd interface plays an important role in the enhancement of Geff↑↓. As a consequence, a high spin transparency of the CFB/Nd interface is obtained as 82%. In addition, damping enhancement of CFB is not changed promptly by inserting 1–2 nm Cu layer, but it is suppressed when the Cu layer is thicker than 3 nm, which is related to the thickness dependence of the Cu resistivity. Our study broadens the horizon for the application of rare-earth (RE) in spintronics.

Spin pumping in nanolayers of WS2/Co2FeAl heterostructures: Large spin mixing conductance and spin transparency

Materials with high spin–orbit coupling (SOC) are a prerequisite for the realization of spin–orbit torque-based magnetic memories. Transition metal dichalcogenides (TMDs) are an apt choice for such applications due to their high SOC strength. In this work, we have investigated the spin pumping phenomenon at the interface between thin tungsten disulphide (WS2) films and Co2FeAl (CFA) Heusler alloy films by performing ferromagnetic resonance (FMR) measurements on WS2/CFA heterostructures capped with the 4 nm thin Al film. While Raman spectroscopy conclusively proves the number of monolayers in the WS2 films, atomic force microscopy and x-ray reflectivity measurements were used to quantify the smoothness of the grown interfaces (<0.4 nm) as well as the individual layer thicknesses in the heterostructure stacks. Here, we vary the WS2 layer numbers and CFA thicknesses to quantify the spin pumping parameters such as spin mixing conductance, and spin transparency. FMR measurements revealed that damping enhancement reached ∼41% with a monolayer of WS2. Interfacial effective spin mixing conductance and spin transparency of the WS2/CFA interface are found to be 7.47 ± 0.97 nm−2 and 73.35 ± 9.52%, respectively. Thus, high-quality TMDs can be used as efficient materials for magnetic memory device applications.

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: a first-principles Floquet-nonequilibrium Green function study

Motivated by the recent experiment (Vaidya et al 2020 Science 368 160) on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF2/Cu and Pt/MnF2/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use a more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green’s functions in order to take into account: SO-proximitized AFI as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; strong SO coupling at the interface; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting rather than insulating as in the isolated AFI. The DC component of pumped spin current vs. precession cone angle of the Néel vector l of AFI does not follow putative , except for very small angles for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. In addition, the angular dependence differs for opposite directions of precession of the Néel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.

Effect of interfacial spin mixing conductance on gyromagnetic ratio of Gd substituted Y3Fe5O12

Due to its low intrinsic damping, Y3Fe5O12 and its substituted variations are often used for ferromagnetic layer at spin pumping experiment. Spin pumping is an interfacial spin current generation in the interface of ferromagnet and non-magnetic metal, governed by spin mixing conductance parameter G↑↓. G↑↓ has been shown to enhance the damping of the ferromagnetic layer. The theory suggested that the effect of G↑↓ on gyromagnetic ratio only comes from its negligible imaginary part. In this article, we show that the different damping of ferrimagnetic lattices induced by G↑↓ can affect the gyromagnetic ratio of Gd-substituted Y3Fe5O12.

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.

Interface-driven spin pumping and inverse Rashba-Edelstein effect in FeGaB/Ag/BiSb multilayers

Spin-orbit-coupling (SOC) plays a critical role in spin-to-charge conversion (SCC) mechanism. The SCC process is usually investigated by injecting a spin current from ferromagnet (FM) into a large SOC normal metal (SOC-NM) and then measuring the voltage generated by inverse spin Hall effect (ISHE). Recently, a new mechanism of SCC at inversion symmetry breaking interfaces between topological insulators (TIs) and magnetically ordered materials has been introduced, which is driven by the inverse Rashba-Edelstein effect (IREE). Motivated by this result, we have investigated the IREE effect of a simple binary three-dimensional TI Bi85Sb15 (BiSb). The model system used here is a trilayer of FeGaB/Ag/BiSb with different Ag thickness made insitu with a combination of dc magnetron sputtering and pulsed laser ablation. The SCC efficiency has been evaluated by measuring ferromagnetic resonance (FMR) driven ISHE voltage and extraction of the Gilbert damping (α) and interfacial spin mixing conductance (g↑↓) from the FMR line shape analysis. It is interesting to note that the charge current (Ic) increases with Ag thickness. This observation indicated that the IREE process at the Ag/BiSb interface dominates over ISHE in the bulk of BiSb film. The conversion efficiency of IREE is determined by calculating the IREE length (λIREE), which increases with the thickness of the Ag layer. These findings offer a method to generate and detect the IREE in a room temperature deposited heterostructure using simple coplanar waveguide setup.

Probing the Spin Hall Characteristics of W/CoFeB/MgO Based Heterostructures for Spin‐Orbit Torque Based Magnetic Random Access Memory Application

AbstractIn this work, the spin Hall characteristics of W/CoFeB/MgO have been systematically studied to provide an in‐depth insight for the optimization of the SOT‐MRAM channel. The bcc and β phases of tungsten (W) are identified clearly from XRD and resistivity studies, which shows marked impact on the electrical characteristics. Moreover, the same is found have tremendous affect on W's spin Hall characteristics with a twelvefold increase of the spin Hall angle ΘSH. The evaluation of W/CoFeB interfacial spin mixing conductance using ab initio studies shows good coherency with the same evaluated experimentally, which is used to estimate interfacial spin transparency T</> to accurately evaluate intrinsic spin Hall conductivity σSH of W. The findings equivocate for a strong claim on β‐W's σSH to be around −0.23 ∓ 0.03, addressing the ongoing discrepancy. Increasing boron concentration in (Co1Fe3)100−xBx and thermal annealing temperature are found to reduce ΘSH effectively which is inferred to boron diffusion toward W. Alloying of W with Ta does not necessarily enhance ΘSH, for which very optimized relative concentration and the alloy growth phase is critical. The findings can significantly contribute to the ongoing SOT‐MRAM development for achieving various means of control for improvement in the device characteristics.

Absence of spin transport in amorphous YIG evidenced by nonlocal spin transport experiments

It remains highly controversial whether long-distance spin transport or only Joule heating without spin transport exists in the disordered magnetic amorphous ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (a-YIG). Here, we carefully check and analyze the origin of the observed electrical signals in the Py/a-YIG/Pt-based nonlocal spin-transport experiments with both horizontal and vertical geometries, based on the ferromagnetic resonance (FMR)-induced spin pumping and inverse spin Hall effect. In all lateral nonlocal devices with a distance from 2.0 \ensuremath{\mu}m to 200 nm and vertical structures with a $\ensuremath{\ge}\text{40-nm-thick}$ a-YIG spacer layer, the spin-current-induced direct voltage peak signal is absent under FMR conditions, indicating that a-YIG does not possess long-distance spin transport. Furthermore, we demonstrate that a weak spin-torque FMR voltage signal, which gradually reduces to zero with increasing a-YIG thickness in the vertical geometry, is attributed to the leaked spin rectification voltage generated in the Py layer due to the electric leakage between the top Pt layer and the bottom Py layer through the pinhole defects in the thin middle a-YIG layer. Additionally, we find an improvement of the interfacial spin-mixing conductance due to spin accumulation in the interface of a-YIG/Py, which can significantly enhance spin-orbit torque efficiency in the ferromagnet/heavy metal systems with a disordered magnetic a-YIG buffer layer.

Identifying the origin of the nonmonotonic thickness dependence of spin-orbit torque and interfacial Dzyaloshinskii-Moriya interaction in a ferrimagnetic insulator heterostructure

Electrical manipulation of magnetism via spin-orbit torques (SOTs) promises efficient spintronic devices. In systems comprising magnetic insulators and heavy metals, SOTs have started to be investigated only recently, especially in systems with interfacial Dzyaloshinskii-Moriya interaction (iDMI). Here, we quantitatively study the SOT efficiency and iDMI in a series of gadolinium gallium garnet (GGG) / thulium iron garnet (TmIG) / platinum (Pt) heterostructures with varying TmIG and Pt thicknesses. We find that the non-monotonic SOT efficiency as a function of the magnetic layer thickness is not consistent with the 1/thickness dependence expected from a simple interfacial SOT mechanism. Moreover, considering the insulating nature of TmIG, our results cannot be explained by the SOT mechanism established for metallic magnets where the transverse charge spin current can inject and dephase in the magnetic layers. Rather we can explain this non-monotonic behavior by a model based on the interfacial spin mixing conductance that is affected by the thickness-dependent exchange splitting energy by determining the phase difference of the reflected spin-up and spin-down electrons at the TmIG / Pt interface. By studying the Pt thickness dependence, we find that the effective DMI for GGG / TmIG / Pt does not depend on the Pt thickness, which indicates that the GGG / TmIG interface is the source of the iDMI in this system. Our work demonstrates that SOT and DMI can originate from two different interfaces, which enables independent optimization of DMI and SOT for advanced chiral spintronics with low damping magnetic insulators.

Determining complex spin mixing conductance and spin diffusion length from spin pumping experiments in magnetic insulator/heavy metal bilayers

Magnetic insulators are promising materials for the development of energy-efficient spintronics. Unlike metallic counterparts, the magnetic insulators are characterized by the imaginary part of the interfacial spin mixing conductance as well in a bilayer with heavy metals, and it is responsible for the field-like toque in spin–orbit torque devices. Here, we study the underlying theoretical constructs and develop a general strategy to determine the complex spin mixing conductance from the experimental results of ferromagnetic resonance and spin pumping. The results show that the imaginary part of the spin mixing conductance can be one order more than the real part and it matches the critical trend of spin mixing conductance with thickness of the heavy metal. The interpretation of experimental results also indicates that at small thicknesses, the interface contribution becomes significant and a bulk diffusion model cannot explain the results. A thickness-dependent spin diffusion length is necessary too that is tantamount to the Elliott–Yafet spin relaxation mechanism in the heavy metals. Also, we effectively explain the experimental results while inserting a copper layer with varying thicknesses in between the magnetic insulator and the heavy metal using spin-circuit formalism.

Spin transmission in IrMn through measurements of spin Hall magnetoresistance and spin-orbit torque

Understanding the transport of spin current in antiferromagnetic materials is indispensable to develop antiferromagnetic spintronic devices. In this work, we study the spin current transmission through an antiferromagnetic IrMn insertion layer in a W/IrMn($t$)/CoFeB structure by measuring the spin Hall magnetoresistance (SMR) and spin-orbit torque (SOT). The temperature dependences of SMR and SOT effective fields indicate that the spin current transmission reaches its maximum at the N\'eel temperature of IrMn. The enhancement is ascribed to the increase in the interfacial spin mixing conductance, which is related to the maximum magnetic susceptibility of the IrMn layer at the N\'eel temperature. The spin transmission decreases monotonically as a function of the IrMn thickness, which is different from the case with an insulating antiferromagnetic NiO insertion layer in previous works. In addition, we found that the spin transmission through an antiferromagnetic IrMn layer is independent of the exchange bias orientation. Our results suggest that the spin current transmission through the IrMn layer (from W layer to CoFeB layer) is mainly mediated by spin-polarized electrons rather than magnons, which is likely due to the absence of the effective excitation of magnons in the IrMn layer by the spin-polarized current.

Temperature dependence of spin pumping and inverse spin Hall effect in permalloy/Pt bilayers

The temperature dependence of the spin pumping process in Fe80Ni20 (Py)/Pt bilayers is analyzed for different Pt capping layer thickness. Samples have been grown by means of dc magnetron sputtering, at room temperature, on top of Si substrates. In order to get an optimized interface between the Py and the Pt layers, the Pt capping layers were deposited In situ. The values of the damping constant, α, obtained for the Py alone samples, around 0.007 at RT, make evident the good quality of the samples. The increase of α due to spin pumping into the Pt layer is clearly appreciated; α values in Py/Pt bilayers are twice the value in Py alone. Spin pumping is confirmed by direct measurements of the inverse spin Hall effect (ISHE) voltage signal (VISHE) measured across the samples. To separate ISHE and spin rectification contributions to VISHE the line shape analysis method has been used and the conditions for its applicability thoroughly discussed. VISHE increases almost linearly on lowering temperature for all the frequencies investigated and smoothly increases as frequency decreases showing roughly a linear dependence with the inverse of the frequency. The Pt layer thickness dependence of α and VISHE suggest a low interfacial spin mixing conductance and high spin memory loss.

Dysprosium Iron Garnet Thin Films with Perpendicular Magnetic Anisotropy on Silicon

AbstractMagnetic insulators, such as the rare‐earth iron garnets, are promising materials for energy‐efficient spintronic memory and logic devices, and their anisotropy, magnetization, and other properties can be tuned over a wide range through selection of the rare‐earth ion. Films are typically grown as epitaxial single crystals on garnet substrates, but integration of these materials with conventional electronic devices requires growth on Si. The growth, magnetic, and spin transport properties of polycrystalline films of dysprosium iron garnet (DyIG) with perpendicular magnetic anisotropy (PMA) on Si substrates and as single crystal films on garnet substrates are reported. PMA originates from magnetoelastic anisotropy and is obtained by controlling the strain state of the film through lattice mismatch or thermal expansion mismatch with the substrates. DyIG/Si exhibits large grain sizes and bulk‐like magnetization and compensation temperature. Polarized neutron reflectometry demonstrates a small interfacial nonmagnetic region near the substrate. Spin Hall magnetoresistance measurements conducted on a Pt/DyIG/Si heterostructure demonstrate a large interfacial spin mixing conductance between the Pt and DyIG comparable to other garnet/Pt heterostructures.

Influence on the Gilbert damping of yttrium-iron-garnet films by the spin-pumping effect

The effect of a platinum (Pt) capping layer on the magnetization dynamics in polycrystalline yttrium iron garnet (YIG) films of various thicknesses has been studied in order to understand the influence on the Gilbert damping of YIG films by the spin-pumping at the YIG/Pt interface. The Gilbert damping in each YIG film with the Pt capping layer is found to be much higher than that of the YIG film without the Pt capping layer, as expected due to the spin-pumping effect. In comparison with the samples without capping layer, an enhanced value of interfacial spin mixing conductance can also be observed for films with the capping layer. These results demonstrate the existence of spin-pumping effect at the YIG/Pt interface which plays an important role for the enhancement of Gilbert damping.

Ferromagnetic resonance of perpendicularly magnetized Tm3Fe5O12/Pt heterostructures

Broadband ferromagnetic resonance is used to investigate magnetization dynamics, damping, interfacial spin transport, and perpendicular magnetic anisotropy (PMA) of (111)-oriented epitaxial thin films of the ferrimagnetic insulator Tm3Fe5O12 (TmIG) on substrates of (111)-oriented Gd3Ga5O12. A PMA field of ∼162 mT is found at 350 K, in the temperature range where spin–orbit torque switching was previously reported [Avci et al., Nat. Mater. 16, 309–314 (2017)]. A Landé g-factor of 1.56 strongly supports large intrinsic spin–orbit coupling due to the presence of the heavy rare earth Tm. Gilbert damping coefficients α are compared for three samples: a 28 nm thin TmIG film (α ∼ 0.014), a TmIG (28 nm)/Pt (6 nm) bilayer (α ∼ 0.022), and a TmIG (28 nm)/Cu (3 nm)/Pt (6 nm) trilayer (α ∼ 0.024). Applying the spin pumping formalism, we find that the real part of the effective interfacial spin mixing conductance Geff↑↓ = 5.7 × 1014 Ω−1 m−2 is comparable to that of well-studied garnet/Pt interfaces. Our work strengthens the candidacy of TmIG for spintronics applications requiring PMA in insulating thin films.

Optical determination of spin diffusion length and interfacial spin mixing conductance in epitaxial Pd/Fe bilayers

We present a detailed study of the photoinduced magnetization precession and magnetic relaxation in epitaxial Pd/Fe films on MgO(0 0 1) substrates via all-optical pump-probe techniques. We explicitly formulate the correlation between the Gilbert damping and the effective damping in the optical approach. Furthermore, a non-local Gilbert damping induced by spin pumping is demonstrated self-consistently by the Gilbert damping dependence on Pd thickness and the relationship of damping with Fe thickness . The non-local Gilbert damping enables the determination of spin diffusion length in Pd and interfacial spin mixing conductance . Our work paves the way toward the optical determination of spin transport parameters.

Gate voltage tuning of spin current in Pt/yttrium iron garnet heterostructure

Electric gating provides a versatile tool to create new spintronic materials and functionalities. Here, we have shown significant tuning effects of the gate voltage via ionic liquid on spin pumping and spin Hall magnetoresistance in Pt/yttrium iron garnet heterostructure. It is found that the interfacial spin mixing conductance is greatly enhanced upon application of the gate voltage. In addition, and interestingly, the spin relaxation time also increases with the gate voltage. Moreover, suppressed spin Hall magnetoresistance and enhanced spin polarization of Pt atoms both suggest that the spin Hall angle is reduced by the gate voltage. All these results can be consistently explained by the enhanced charge carrier density and the subsequent elevated Fermi level induce by the gate voltage. By tackling the key parameters in controlling the generation and propagation of pure spin current via voltage gating, the present work provides novel strategies to improve the performance of the spin current devices.

Inverse spin Hall effect in ITO/YIG exited by spin pumping and spin Seebeck experiments**Project supported by the National Key Basic Research Project of China (Grant No. 2016YFA0300600), Chinese Academy of Sciences (Grant No. KJCX2-YW-W24), the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11604375), and the Laboratory of Microfabrication of Institute of Physics, Chinese Academy

Spin currents, which are excited in indium tin oxide (ITO)/yttrium iron garnet (YIG) by the methods of spin pumping and spin Seebeck effect, are investigated through the inverse spin Hall effect (ISHE). It is demonstrated that the ISHE voltage can be generated in ITO by spin pumping under both in-plane and out-of-plane magnetization configurations. Moreover, it is observed that the enhancement of spin Hall angle and interfacial spin mixing conductance can be achieved by an appropriate annealing process. However, the ISHE voltage is hardly seen in the presence of a longitudinal temperature gradient. The absence of the longitudinal spin Seebeck effect can be ascribed to the almost equal thermal conductivity of ITO and YIG and specific interface structure, or to the large negative temperature dependent spin mixing conductance.

Correlation between the Dzyaloshinskii-Moriya interaction and spin-mixing conductance at an antiferromagnet/ferromagnet interface

The rich interaction phenomena at antiferromagnet (AFM)/ ferromagnet (FM) interfaces are key ingredients in AFM spintronics, where many underlying mechanisms remain unclear. Here we report a correlation observed between interfacial Dzyaloshinskii-Moriya interaction (DMI) Ds and effective spin mixing conductance g at IrMn/CoFeB interface. Both Ds and g are quantitatively determined with Brillouin light scattering measurements, and increase with IrMn thickness in the range of 2.5~7.5 nm. Such correlation likely originates from the AFM-states-mediated spin-flip transitions in FM, which promote both interfacial DMI and spin pumping effect. Our findings provide deeper insight into the AFM-FM interfacial coupling for future spintronic design.