Spin Backflow Research Articles

Reduction of spin backflow with underlying bulk WSe2 in Pt/Py/Al trilayers

Reduction of spin backflow with underlying bulk WSe2 in Pt/Py/Al trilayers

Modulating spin current induced effective damping in β–W/Py heterostructures by a systematic variation in resistivity of the sputtered deposited β–W films

Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal (HM) under the ferromagnetic resonance (FMR) condition, we report an enhancement in effective damping in β- W/Py bilayers by systematically varying resistivity (ρ W ) of β-W films. Different resistivity ranging from 100 μΩ-cm to 1400 μΩ-cm with a thickness of 8 nm can be achieved by varying the argon pressure (P Ar ) during the growth by the method of sputtering. The coefficient of effective damping α eff is observed to increase from 0.010 to 0.025 with ρ W , which can be modulated by P Ar . We observe a modest dependence of α eff on the sputtering power (p S ) while keeping the P Ar constant. α eff dependence on both P Ar and p S suggests that there exists a strong correlation between α eff and ρ W . It is thus possible to utilize ρ W as a tuning parameter to regulate the α eff , which can be advantageous for faster magnetization dynamics switching. The thickness dependence study of Py in the aforementioned bilayers manifests a higher spin mixing conductance ( geff↑↓ ) which suggests a strong spin pumping from Py into the β-W layer. The effective spin current (J S(eff)) is also evaluated by considering the spin-back flow in this process. Intrinsic spin mixing conductance ( gW↑↓ ) and spin diffusion length (λ SD ) of β-W are additionally investigated using thickness variations in β-W. Furthermore, the low-temperature study in β-W/Py reveals an intriguing temperature dependence in α eff which is quite different from α b of single Py layer and the enhancement in α eff at low temperature can be attributed to the spin-induced pumping from Py layer into β-W.

Large anomalous Hall magnetoresistance in partially oxidized single GdOx layer

This study investigated angular-dependent magnetoresistance in a partially oxidized single GdOx wire, wherein nonmetallic conduction was dominant. The resistivity with magnetic field being parallel (ρx), transverse (ρy), and perpendicular (ρz) to current exhibited a relation of ρz>ρx≈ρy, which is in contrast to that in case of conventional magnetoresistances. This unusual angular dependence can be attributed to the anomalous Hall magnetoresistance (AHMR), which is a novel magnetoresistance originating from anomalous Hall effect-induced spin accumulation, subsequent spin backflow, and spin-to-charge conversion, and was recently observed in metallic ferromagnets. The observed AHMR was as large as 1.5% at T = 5 K, which is orders of magnitude larger than that in case of metallic systems. Thus, the findings of this study are expected to motivate the evolution of further theoretical approaches towards a complete understanding of spin-dependent magnetotransport in granular systems.

The thickness dependence of the field-like spin–orbit torque in heavy metal/CoFeB/MgO heterostructures

We investigated the ferromagnet (FM) and heavy metal (HM) thickness dependence of the electric current-induced spin orbit torque (SOT), especially the field-like (FL) torque component in HM/CoFeB/MgO heterostructures. For Pt/CoFeB/MgO and Ta/CoFeB/MgO structures, after subtracting the dead-layer thickness of CoFeB, the damping-like (DL) effective field follows 1/tFM dependence, while the FL effective field deviates from 1/tFM dependence at the ultra-thin FM thickness range, indicating that an extra origination of FL torque, i.e., spin backflow at the FM/MgO interface, is responsible for the large FL torque in HM/CoFeB/MgO structures with a ultra-thin CoFeB layer. For Ta/Pt(tPt)/CoFeB(1)/MgO structures, the FL-SOT exhibits a gradual change similar to the DL-SOT, suggesting that the spin Hall effect is the dominant origination of spin current, which enhances the FL-SOT in the HM/CoFeB/MgO structures by the spin backflow effect when tCoFeB is less than the spin dephasing length. We also demonstrated that the obvious dead-layer thickness at the Ta/CoFeB interface reduces the effective CoFeB thickness and enhances the spin backflow effect further.

Fully Spin-Transparent Magnetic Interfaces Enabled by the Insertion of a Thin Paramagnetic NiO Layer

Spin backflow and spin-memory loss have been well established to considerably lower the interfacial spin transmissivity of metallic magnetic interfaces and thus the energy efficiency of spin-orbit torque technologies. Here, we report that spin backflow and spin-memory loss at Pt-based heavy metal-ferromagnet interfaces can be effectively eliminated by inserting an insulating paramagnetic NiO layer of optimum thickness. The latter enables the thermal magnon-mediated essentially unity spin-current transmission at room temperature due to considerably enhanced effective spin-mixing conductance of the interface. As a result, we obtain dampinglike spin-orbit torque efficiency per unit current density of up to 0.8 as detected by the standard technology ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient, integration-friendly, and high-endurance spin-current generators that provide >100 times greater energy efficiency than sputter-deposited topological insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and advance spin-torque technologies.

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

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

Spin wave modulation by topographical perturbation in Y3Fe5O12 thin films

We present a comparison of the spin wave propagation in Au/Y3Fe5O12 and Pt/Y3Fe5O12 bilayers. Microwave technique with a co-planner waveguide arrangement was used to excite and detect the spin wave. We observed a suppression in the propagating spin wave intensity when a metal stripe is placed on the surface of Y3Fe5O12 in the spin wave propagation path due to the spin pumping from Y3Fe5O12 to nonmagnetic metal stripe. However, a significant difference in the suppression property was observed with the Au and Pt electrode layers, which cannot be explained by the enhancement of the damping constant induced by spin pumping alone. The significant suppression of the spin wave propagation in the Au/Y3Fe5O12 bilayer system is attributed to the spin backflow and two magnon scattering.

Magnetic Damping Constant of CoFeB/Pt Thin Films With Varying the Thicknesses of Pt and Insertion Layer of Al

The effect of the thicknesses of Pt and the insertion of Al layer on the effective magnetic damping constant of CoFeB/Pt thin films caused by the spin-pumping effect has been studied by an angular-dependent ferromagnetic resonance method. In CoFeB/Pt bilayers, a roughly linear dependence of the effective damping constant on Pt thickness ( $t{\mathbf {Pt}}$ ) was observed before saturated at $t_{\mathbf {Pt}} = 2$ nm. While in CoFeB/Al/Pt trilayers, a sharp drop of damping constant was observed while Al thickness is around 0.5 nm, then the damping constant nearly remains when Al thickness further increases to 5 nm. It indicates a decreased spin pumping efficiency due to Al insertion between CoFeB and Pt layers. The decreased spin pumping efficiency was ascribed to the spin memory loss at CoFeB/Al and/or Al/Pt interfaces rather than the spin backflow effect. The results also suggest that the linearly increased damping constant in CoFeB/Pt layers mainly comes from the magnetic proximity effect (MPE) of Pt, which could be eliminated by insertion a thin layer of Al. However, in order to obtain a large-spin current injection efficiency and less MPE to improve the spin-pumping effect in CoFeB/Pt, a proper insertion layer rather than Al with small-spin memory loss need to be investigated.

Spin backflow: A non-Markovian effect on spin pumping

The miniaturization of spintronic devices, specifically, nanoscale devices employing spintronics, has attracted intensive attention from a scientific as well as engineering perspective. In this paper, we study a non-Markovian effect on spin pumping to describe spin current generation driven by the magnetization of arbitrary precession frequency in a quantum dot attached to an electron lead. Although the Markovian approximation can be used when driving is sufficiently slow compared with relaxation times in electron tunneling, recent developments in nanospintronic devices show that we need to include non-Markovian effects. In contrast to the one-way-only nature of the spin current generation under Markovian dynamics, we find that non-Markovian dynamics exhibit a temporal backflow of spin, called spin backflow for brevity. We capture the phenomenon by introducing its quantifier, and show that the backflow reduces the amount of spin current significantly when the frequency exceeds the relaxation rate. This prevents an unphysical divergence of the spin current in the high-frequency limit that occurs under the Markovian approximation. We believe our analysis provides an understanding of the spin pumping particularly in regard to producing a more efficient spin current generation over shorter time scales by going beyond the conventional Markovian approximation.

Irrelevance of magnetic proximity effect to spin-orbit torques in heavy-metal/ferromagnet bilayers

The magnetic proximity effect (MPE) is a well-established magnetic phenomenon that occurs at certain heavy metal (HM)/ferromagnet (FM) interfaces. However, there is still an active debate as to whether the presence of a MPE affects spin transport through such a HM/FM interface. Here we demonstrate that the MPE at Pt/Co and Au0.25Pt0.75/Co interfaces can be enhanced substantially by thermal annealing protocols. From this ability, we show that the MPE has no discernable influence on either the damping-like or the field-like spin-orbit torques exerted on the FM layer due to the spin Hall effect of the HM layer, indicating a minimal role of the MPE compared to other interfacial effects, e.g. spin memory loss and spin backflow.

Determination of spin Hall angle and spin diffusion length in β -phase-dominated tantalum

The dc spin-to-charge conversions of tantalum (Ta) in $\mathrm{Ta}/\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness $({t}_{\mathrm{Ta}})$-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are $\ensuremath{\beta}$-phase dominated. The damping enhancement shows a fast increase with ${t}_{\mathrm{Ta}}$ when ${t}_{\mathrm{Ta}}l1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and reaches a saturation value at $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The ISHE induced charge voltages have opposite signs for Ta and Pt. From ${t}_{\mathrm{Ta}}$-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with ${t}_{\mathrm{Ta}}$ and saturate at $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. Our findings can be understood with a recently developed theory [Phys. Rev. Lett. 114, 126602 (2015)], which includes spin backflow and a spin loss at the interface. With a fitted spin loss factor of $0.02\ifmmode\pm\else\textpm\fi{}0.02$, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be ${\ensuremath{\theta}}_{\mathrm{SH}}=\ensuremath{-}0.0062\ifmmode\pm\else\textpm\fi{}0.001$ and ${\ensuremath{\lambda}}_{\mathrm{sd}}=5.1\ifmmode\pm\else\textpm\fi{}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively.

Anomalous Hall magnetoresistance in a ferromagnet

The anomalous Hall effect, observed in conducting ferromagnets with broken time-reversal symmetry, offers the possibility to couple spin and orbital degrees of freedom of electrons in ferromagnets. In addition to charge, the anomalous Hall effect also leads to spin accumulation at the surfaces perpendicular to both the current and magnetization direction. Here, we experimentally demonstrate that the spin accumulation, subsequent spin backflow, and spin–charge conversion can give rise to a different type of spin current-related spin current related magnetoresistance, dubbed here as the anomalous Hall magnetoresistance, which has the same angular dependence as the recently discovered spin Hall magnetoresistance. The anomalous Hall magnetoresistance is observed in four types of samples: co-sputtered (Fe1−xMnx)0.6Pt0.4, Fe1−xMnx/Pt multilayer, Fe1−xMnx with x = 0.17–0.65 and Fe, and analyzed using the drift-diffusion model. Our results provide an alternative route to study charge–spin conversion in ferromagnets and to exploit it for potential spintronic applications.

Tunable magnetotransport in Fe/hBN/graphene/hBN/Pt(Fe) epitaxial multilayers

Theoretical and computational analysis of the magnetotransport properties and spin-transfer torque field-induced switching of magnetization density in vertically-stacked multilayers is presented. Using epitaxially-capped free layers of Pt and Fe, atom-resolved magnetic moments and spin-transfer torques are computed at finite bias. The calculations are performed within linear response approximation to the spin-density reformulation of the van der Waals density functional theory. Dynamical spin excitations are computed as a function of a spin-transfer torque induced magnetic field along the magnetic easy axis, and the corresponding spin polarization perpendicular to the easy axis is obtained. Bias-dependent giant anisotropic magnetoresistance of up to 3200% is obtained in the nonmagnetic-metal-capped Fe/hBN/graphene/hBN/Pt multilayer architecture. Since this specific heterostructure is not yet fabricated and characterized, the predicted high performance has not been demonstrated experimentally. Nevertheless, similar calculations performed on the Fe/hBN/Co stack show that the tunneling magnetoresistance obtained at the Fermi-level is in excellent agreement with results of recent magnetotransport measurements on magnetic tunnel junctions that contain the monolayer hBN tunnel region. The magnitude of the spin-transfer torque is found to increase as the tunneling spin current increases, and this activates the magnetization switching process due to increased charge accumulation. This mechanism causes substantial spin backflow, which manifests as rapid undulations in the bias-dependent tunneling spin currents. The implication of these findings on the design of nanoscale spintronic devices with spin-transfer torque tunable magnetization density is discussed. Insights derived from this study are expected to enhance the prospects for developing and integrating artificially assembled van der Waals multilayer heterostructures as the preferred material platform for efficient engineering of spin switches for spintronic applications.

Spin transfer torque ferromagnetic resonance induced spin pumping in the Fe/Pd bilayer system

Inconsistencies in estimates of the spin Hall angle (${\ensuremath{\theta}}_{SH}$) and spin diffusion length (${\ensuremath{\lambda}}_{SD}$) of nonmagnetic (NM) layers using the spin transfer torque ferromagnetic resonance (ST-FMR) in ferromagnetic FM/NM bilayer structures are attributed to the inverse spin Hall effect (ISHE) and interfacial parameter contributions, interface spin transparency, interfacial anisotropic magnetoresistance, and effective spin-mixing conductance. These contributions in Fe(10 nm)/Pd(2--8 nm) bilayer structures have been probed employing the simultaneous detection of ST-FMR and ISHE in conjunction with in-plane FMR measurements. The interfacial contributions are found to increase with an increase in Pd layer thickness (${t}_{NM}$), which can be linked to the spin pumping effect in conjunction with spin backflow. Correcting the ${t}_{NM}$ dependence of the ST-FMR spectra for the interfacial and ISHE contributions prior to estimating ${\ensuremath{\theta}}_{SH}$ and ${\ensuremath{\lambda}}_{SD}$ of the Pd layer, the estimated values are found to be $0.10\ifmmode\pm\else\textpm\fi{}0.03$ and $5.4\ifmmode\pm\else\textpm\fi{}1.2\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, respectively.

Effect of Ru thickness on spin pumping in Ru/Py bilayer

We report the effect of Ru thickness (tRu) on ferromagnetic resonance (FMR) line-width of Ru(tRu)/Py(23 nm) bilayer samples grown on Si(100)/SiO2 substrates at room temperature by magnetron sputtering. The FMR line-width is found to vary linearly with frequency for all thicknesses of Ru, indicating intrinsic origin of damping. For Ru thicknesses below 15 nm, Gilbert-damping parameter, α is almost constant. We ascribe this behavior to spin back flow that is operative for Ru thicknesses lower than the spin diffusion length in Ru, λsd. For thicknesses >15 nm (>λsd), the damping constant increases with Ru thickness, indicating spin pumping from Py into Ru.

Inverse spin Hall effect induced by spin pumping into semiconducting ZnO

The inverse spin Hall effect (ISHE) of n-type semiconductor ZnO thin films with weak spin-orbit coupling has been observed by utilizing the spin pumping method. In the ferromagnetic resonance condition, the spin pumping driven by the dynamical exchange interaction of a permalloy film injects a pure spin current into the adjacent ZnO layer. This spin current gives rise to a DC voltage through the ISHE in the ZnO layer, and the DC voltage is proportional to the microwave excitation power. The effect is sizeable even when the spin backflow is considered.

Spin Backflow and ac Voltage Generation by Spin Pumping and the Inverse Spin Hall Effect

The spin current pumped by a precessing ferromagnet into an adjacent normal metal has a constant polarization component parallel to the precession axis and a rotating one normal to the magnetization. The former is now routinely detected as a dc voltage induced by the inverse spin Hall effect (ISHE). Here we compute ac ISHE voltages much larger than the dc signals for various material combinations and discuss optimal conditions to observe the effect. The backflow of spin is shown to be essential to distill parameters from measured ISHE voltages for both dc and ac configurations.

Dynamics of spin proximity polarization in a hybrid ferromagnet/semiconductor structure

We study theoretically dynamics of spin proximity polarization of semiconductor electrons in a hybrid ferromagnet/semiconductor (F/S) structure. When the hybrid structure comes into equilibrium after a photoexcitation a spin current flowing through the interface produces a spin polarization of semiconductor electrons. In turn, a spin backflow arising in the thin semiconductor layer affects the interface spin current. We analyze the interplay of these processes by using the spin diffusion equation with appropriate boundary conditions at the F/S interface and show that the decay of the proximity polarization with time depends on the temporal behavior of the spin current emitted by the interface.

Spin-charge separation in the single-hole-doped Mott antiferromagnet

The motion of a single hole in a Mott antiferromagnet is investigated based on the t-J model. An exact expression of the energy spectrum is obtained, in which the irreparable phase string effect [Phys. Rev. Lett. 77, 5102 (1996)] is explicitly present. By identifying the phase string effect with spin backflow, we point out that spin-charge separation must exist in such a system: the doped hole has to decay into a neutral spinon and a spinless holon, together with the phase string. We show that while the spinon remains coherent, the holon motion is deterred by the phase string, resulting in its localization in space. We calculate the electron spectral function which explains the line shape of the spectral function as well as the ``quasiparticle'' spectrum observed in angle-resolved photoemission experiments. Other analytic and numerical approaches are discussed based on the present framework.

Spin backflow in the vicinity of spin polarons in the t − J model

We investigate the spin backflow current in a spin- 1 2 Heisenberg quantum antiferromagnet associated with the motion of a hole. The spin backflow is studied here with two different methods: (a) starting from the wave function of the hole-like quasiparticle obtained in the self-consistent Born approximation and (b) an approximation-free exact diagonalization technique. Spin currents calculated analytically are in a good agreement with small cluster diagonalization studies.