Spin Torque Ferromagnetic Resonance Measurements Research Articles

Charge-spin interconversion in nitrogen sputtered Pt via extrinsic spin Hall effect.

We experimentally investigate the charge-spin interconversion by introducing a lighter impurity, namely nitrogen (N) into Pt by varying the nitrogen gas flow rate,Qfrom 0 to 20%, and studying the Onsager reciprocity of spin Hall effect (SHE) via complementary methods of spin-torque ferromagnetic resonance and spin-pumping inverse SHE measurements, respectively. We notice a reduction in the crystalline nature of Pt upon nitrogen incorporation. We observe the influence of extrinsic side-jump scattering induced SHE by measuring the spin Hall efficiency,θSHin a wide temperature range from 10 to 296 K. This work establishes the incorporation of N in Pt by using sputtering, to enhance the SHE via extrinsic scattering as well as to observe the reciprocal effects of charge-spin interconversion.

Enhanced effective spin Hall efficiency contributed by the extrinsic spin Hall effect in Pt1- x Ta x /CoFeB structures

High effective spin Hall efficiency and low magnetic damping constant are essential to achieve efficient spin-charge conversion for energy-efficient spintronic devices. We report the measurements of effective spin Hall efficiency and magnetic damping constant of Pt1- x Ta x /CoFeB structures using spin-torque ferromagnetic resonance (ST-FMR) techniques. With the increase of Ta content, the spin Hall efficiency increases and peaks x = 0.15 with the value of θSHeff=0.35 , before decreases for further Ta content. This non-monotonic variation is attributed to the interaction between the contribution of the extrinsic skew scattering mechanism and the spin-orbit coupling (SOC). The weakening of the SOC in the Pt1-x Ta x layer leads to a decrease in the effective magnetic damping constant with increasing Ta concentration. High-temperature ST-FMR measurements confirm the influence of the Ta concentration on the spin Hall effect and magnetic damping mechanism. Additionally, the decrease in spin mixing conductance and the increase in spin diffusion length leads to a decrease in the interfacial spin transparency.

Spin Torque Ferromagnetic Resonance Measurements in a Bulk Rashba System

While many researchers have focused on the interfacial Rashba effect, bulk Rashba materials have received considerable interest due to their potential to enhance spin–orbit torque (SOT). By utilizing GeTe as a bulk Rashba material in the role of a spin–orbit channel, GeTe/Ni81Fe19 and GeTe/Co40Fe40B20 bilayers are fabricated, and SOTs are investigated using the spin torque ferromagnetic (FM) resonance technique. In this method, damping‐like and field‐like SOTs are extracted individually, excluding thermal effects. Upon analyzing the data, a remarkable field‐like SOT efficiency of 0.40 is obtained from the GeTe/Ni81Fe19 system. This high efficiency is attributed to the enhancement of interfacial spin–orbit coupling through the bulk Rashba effect of the GeTe channel. Moreover, noticeable distinctions in SOTs are observed between the Co40Fe40B20 and Ni81Fe19 interfaces, underscoring the importance of selecting the appropriate FM layer for optimizing SOT efficiency. This study highlights the promising potential of bulk Rashba materials like GeTe in advancing SOT‐based devices.

The influence of Ti ultrathin insertion layer on the effective magnetic damping and effective spin Hall angle

We report the influence of ultrathin Ti insertion layer on the effective magnetic damping and effective spin Hall angle in Co/[Pt/Ti]n/Pt structures via spin-torque ferromagnetic resonance measurements. The effective magnetic damping shows a non-monotonic variation as a function of insertion layers number n, reaching a minimum at n = 5. Our analysis shows that when n is less than 5, the damping is mainly related to the thickness of the bottom Pt layer, and when it is greater than 5, the attenuation of the spin currents leads to increased damping. The effective magnetic damping first decreases as the number of layers n increases, reaching a minimum at n=5, and then increases with further increases in n. The observation can be ascribed to a competition between the increased longitudinal resistivity, which is due to the strong interfacial scattering, and the reduced effective spin Hall conductivity that originates from the shortening of the carrier lifetime. Additionally, the extracted interfacial spin transparency is found to be improved with the effect of the insertion layer.

Room‐Temperature Charge‐to‐Spin Conversion from Quasi‐2D Electron Gas at SrTiO3‐Based Interfaces

Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency, {\theta}_cs. The two types of amorphous insulators were selected to explicitly probe the overlooked contribution of oxygen vacancy to the {\theta}_cs. We proposed a mechanism to explain results of spin-torque ferromagnetic resonance (ST-FMR) measurements and developed an analysis protocol to reliably estimate the {\theta}_cs of the oxide based 2DEG. The resultant {\theta}_cs/t, where t is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1 for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly comparable to their crystalline counterparts. Furthermore, we also observe a large direct current modulation of resonance linewidth in SrTiO3/AlN samples, confirming its high {\theta}_cs and attesting an oxygen-vacancy-enabled charge-spin conversion. Our findings emphasize the defects' contribution to the charge-spin interconversion, especially in the oxide-based low dimensional systems, and provide a way to create and enhance charge-spin interconversion via defect engineering.

Room-Temperature Switching of Perpendicular Magnetization by Magnon Torques

Electron-mediated spin torque provides a fast and efficient method to manipulate magnetization; however, electron motion inevitably brings about the generation of Joule heat and corresponding power consumption. Magnon-mediated spin torque, without involving moving electrons, could circumvent the energy dissipation issue. In this work, we fabricate a sandwich structure of topological insulator/antiferromagnetic insulator/ferromagnet with perpendicular magnetic anisotropy. We find that the magnon current with spin angular momentum can traverse a 25-nm-thick antiferromagnetic $\mathrm{Ni}\mathrm{O}$ layer and effectively switch the perpendicular magnetization of $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ at room temperature with a critical switching current density of 4.1 \ifmmode\times\else\texttimes\fi{} ${10}^{6}\phantom{\rule{0.2em}{0ex}}\mathrm{A}/{\mathrm{cm}}^{2}$. The magnon torque efficiency is characterized using spin-torque ferromagnetic resonance measurements to be 0.33 with a magnon diffusion length of 26.6 nm. Our work paves the way for manipulating perpendicular magnetization via magnon torques, facilitating the exploration of magnon-based spintronics with low power consumption.

Room‐Temperature Charge‐to‐Spin Conversion from Quasi‐2D Electron Gas at SrTiO3‐Based Interfaces

Interfacial two‐dimensional electron gases (2DEG), especially the SrTiO3‐based ones at the unexpected interface of insulators, have emerged to be promising candidates for efficient charge–spin interconversion. Herein, to gain insight into the mechanism of the charge–spin interconversion, quasi‐2DEG between insulating SrTiO3 and two types of aluminum‐based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, are focused on and their charge‐to‐spin conversion efficiency is estimated. The two types of amorphous insulators are selected to probe the overlooked contribution of oxygen vacancy. A mechanism to explain the results of spin–torque ferromagnetic resonance measurements is proposed and an analysis protocol to reliably estimate in quasi‐2DEG is developed. The resultant, thickness of the 2DEG, is estimated to be 0.244 and 0.101 nm−1 for SrTiO3/AlN and SrTiO3/Al2O3, respectively, which are strikingly comparable to their crystalline counterparts. Furthermore, a large direct current modulation of resonance linewidth in SrTiO3/AlN samples is developed, confirming and attesting an oxygen vacancy‐enabled charge–spin conversion. The findings emphasize the defects' contribution‐, especially in oxide‐based low‐dimensional systems, and provide a way to create and enhance charge–spin interconversion via defect engineering.

Giant Dampinglike-Torque Efficiency in Naturally Oxidized Polycrystalline TaAs Thin Films

We report the measurement of efficient charge-to-spin conversion at room temperature in Weyl semimetal/ferromagnet heterostructures with both oxidized and pristine interfaces. Polycrystalline films of the Weyl semimetal, TaAs, are grown by molecular beam epitaxy on (001) GaAs and interfaced with a metallic ferromagnet (Ni$_{0.8}$Fe$_{0.2}$). Spin torque ferromagnetic resonance (ST-FMR) measurements in samples with an oxidized interface yield a spin torque efficiency as large as $\xi_{\mathrm{FMR}}=0.45\pm 0.25$ for a 8 nm Ni$_{0.8}$Fe$_{0.2}$ layer thickness. By studying ST-FMR in these samples with varying Ni$_{0.8}$Fe$_{0.2}$ layer thickness, we extract a damping-like torque efficiency as high as $\xi_{\mathrm{DL}}=1.36\pm 0.66$. In samples with a pristine (unoxidized) interface, the spin torque efficiency has opposite sign to that observed in oxidized samples ($\xi_{\mathrm{FMR}}=-0.27\pm 0.14$ for a 5 nm Ni$_{0.8}$Fe$_{0.2}$ layer thickness). We also find a lower bound on the spin Hall conductivity ($424 \pm 110 \frac{\hbar}{e}$ S/cm) which is surprisingly consistent with theoretical predictions for the single crystal Weyl semimetal state of TaAs.

Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

Spin torque is a promising tool for driving magnetization dynamics for computing technologies. These torques can be easily produced by spin-orbit effects, but for most conventional spin source materials, a high degree of crystal symmetry limits the geometry of the spin torques produced. Magnetic ordering is one way to reduce the symmetry of a material and allow exotic torques, and antiferromagnets are particularly promising because they are robust against external fields. We present spin torque ferromagnetic resonance (ST-FMR) measurements and second harmonic Hall measurements characterizing the spin torques in antiferromagnetic iron rhodium alloy. We report extremely large, strongly temperature-dependent exotic spin torques with a geometry apparently defined by the magnetic ordering direction. We find the spin torque efficiency of iron rhodium to be (207 \ifmmode\pm\else\textpm\fi{} 94)% at 170 K and (88 \ifmmode\pm\else\textpm\fi{} 32)% at room temperature. We support our conclusions with theoretical calculations showing how the antiferromagnetic ordering in iron rhodium gives rise to such exotic torques.

Energy-Efficient W100−xTax/ Co-Fe-B/MgO Spin Hall Nano-Oscillators

We investigate a W-Ta alloying route to reduce the auto-oscillation current densities and the power consumption of nano-constriction based spin Hall nano oscillators. Using spin-torque ferromagnetic resonance (ST-FMR) measurements on microbars of W$_{\text{100-x}}$Ta$_{\text{x}}$(5 nm)/CoFeB(t)/MgO stacks with t = 1.4, 1.8, and 2.0 nm, we measure a substantial improvement in both the spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a 34\% reduction in threshold auto-oscillation current density, which translates into a 64\% reduction in power consumption as compared to pure W-based SHNOs. Our work demonstrates the promising aspects of W-Ta alloying for the energy-efficient operation of emerging spintronic devices.

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

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

Crystalline Orientation–Dependent Spin Hall Effect in Epitaxial Platinum

We report on the spin Hall effect in epitaxial Pt films with well-defined crystalline (200), (220), and (111) orientations and smooth surfaces. The magnitude of the spin Hall effect has been determined by spin–torque ferromagnetic resonance measurements on epitaxial Pt/Py heterostructures. We observed a 54% enhancement of the charge-to-spin conversion efficiency of the epitaxial Pt when currents are applied along the in-plane <002> direction. Temperature-dependent harmonic measurements on epitaxial Pt/Co/Ni heterostructures compared to a polycrystalline Pt/Co/Ni suggest the extrinsic mechanism underlying spin Hall effect in epitaxial Pt. Our work contributes to the development of energy-efficient spintronic devices by engineering the crystalline anisotropy of non-magnetic metals.

Spin-charge conversion in Bi([formula omitted][formula omitted]) [formula omitted]/Ag(111) structure

Working with spin pumping and spin-torque ferromagnetic resonance measurements, we investigate the spin-charge conversion in epitaxial Bi(3×3) R30°/Ag(111) structure grown by molecular beam epitaxy. The results show that the sign of the spin-charge conversion in our Bi/Ag structure is opposite to that in Sánchez et al. (2013), indicating the spin-charge conversion in our Bi/Ag structure is dominated by the (inverse) spin Hall effect rather than the (inverse) Rashba–Edelstein effect.

Determination of the spin Hall angle by the inverse spin Hall effect, device level ferromagnetic resonance, and spin torque ferromagnetic resonance: A comparison of methods

The spin torque ferromagnetic resonance (STFMR) is one of the popular methods for measurement of the spin Hall angle, θSH. However, in order to accurately determine θSH from STFMR measurements, the acquired data must be carefully analyzed. The resonance linewidth should be determined to an accuracy of a fraction of an Oe, while the dynamical interaction leading to the measured response consists of the conventional field-induced ferromagnetic resonance (FMR), the spin-torque induced FMR, and the inverse spin Hall effect (ISHE). Additionally, the signal often deteriorates when DC is passed through the device. In this work, we compare the STFMR method with two other FMR-based methods that are used to extract θSH. The first is a device-level FMR, and the second is based on the ISHE. We identify artifacts that are caused by the noise floor of the instrumentation that make the measurement of θSH illusive even when the signal to noise ratio seems to be reasonable. Additionally, we estimate a 10% error in θSH that results from neglecting the magnetic anisotropies as in conventional measurements. Overall, we find the STFMR to be the most robust of the three methods despite the complexity of the interaction taking place therein. The conclusions of our work lead to a more accurate determination of θSH and will assist in the search of novel materials for energy efficient spin-based applications.

Self-induced spin-orbit torques in metallic ferromagnets

We present a phenomenological theory of spin-orbit torques in a metallic ferromagnet with spin-relaxing boundaries. The model is rooted in the coupled diffusion of charge and spin in the bulk of the ferromagnet, where we account for the anomalous Hall effects as well as the anisotropic magnetoresistance in the corresponding constitutive relations for both charge and spin sectors. The diffusion equations are supplemented with suitable boundary conditions reflecting the spin-sink capacity of the environment. In inversion-asymmetric heterostructures, the uncompensated spin accumulation exerts a dissipative torque on the order parameter, giving rise to a current-dependent linewidth in the ferromagnetic resonance with a characteristic angular dependence. We compare our model to recent spin-torque ferromagnetic resonance measurements, illustrating how rich self-induced spin-torque phenomenology can arise even in simple magnetic structures.

Large spin-to-charge conversion in ultrathin gold-silicon multilayers

Investigation of the spin Hall effect in gold has triggered increasing interest over the past decade, since gold combines the properties of a large bulk spin diffusion length and strong interfacial spin-orbit coupling. However, discrepancies between the values of the spin Hall angle of gold reported in the literature have brought into question the microscopic origin of the spin Hall effect in Au. Here, we investigate the thickness dependence of the spin-charge conversion efficiency in single Au films and ultrathin Au/Si multilayers by non-local transport and spin-torque ferromagnetic resonance measurements. We show that the spin-charge conversion efficiency is strongly enhanced in ultrathin Au/Si multilayers, reaching exceedingly large values of 0.99 +/- 0.34 when the thickness of the individual Au layers is scaled down to 2 nm. These findings reveal the coexistence of a strong interfacial spin-orbit coupling effect which becomes dominant in ultrathin Au, and bulk spin Hall effect with a relatively low bulk spin Hall angle of 0.012 +/- 0.005. Our experimental results suggest the key role of the Rashba-Edelstein effect in the spin-to-charge conversion in ultrathin Au.

Resolving Discrepancies in Spin-Torque Ferromagnetic Resonance Measurements: Lineshape versus Linewidth Analyses

When spin-orbit torques are measured using spin-torque ferromagnetic resonance (ST-FMR), two alternative ways of analyzing the results to extract the torque efficiencies -- lineshape analysis and analysis of the change in linewidth versus DC current -- often give inconsistent results. We identify a source for these inconsistencies. We show that fits of ST-FMR data to the standard analysis framework leave significant residuals that we identify as due to (i) current-induced excitations of a small volume of magnetic material with magnetic damping much larger than the bulk of the magnetic layer, that we speculate is associated with the heavy-metal/magnet interface and (ii) oscillations of the sample magnetization at the modulation frequency due to heating. The dependence of the residual signals on DC current can interfere with an accurate extraction of spin-torque efficiencies by the linewidth method. We show that the discrepancies between the two types of analysis can be largely eliminated by extrapolating the window of magnetic fields used in the linewidth fits to small values so as to minimize the influence of the residual signals.

Geometric size dependence of spin-mixing conductance at Pt/YIG interface

The spin-mixing conductance (SMC) is an essential parameter for ferromagnetic-insulators-based spintronics devices. Here, we study the influence of device boundary on local spin transport properties in platinum (Pt)/yttrium iron garnet (YIG) films by reducing the lateral size of the devices. An obvious fluctuation of spin Hall magnetoresistance, Gilbert damping coefficient, and effective spin Hall angle with restricting the size of YIG films has been found at room temperature. By employing both harmonic and current-induced spin-torque ferromagnetic resonance measurements, we have found a negligible fluctuation of both the imaginary part of SMC and effective magnetization with varying the geometric size of devices. In contrast, the real part of SMC at the Pt/YIG interface has been rigidly altered. Our results show that the SMC dramatically depends on the boundary effect from Ar+-ion milling.

Composition dependence of spin–orbit torques in PtRh/ferromagnet heterostructures

We experimentally study the spin–orbit torque (SOT) in PtRh/heterostructures by varying the composition of PtRh alloy. By performing dc-biased spin-torque ferromagnetic resonance and second-harmonic measurements in PtxRh1−x/ferromagnet heterostructures, we find that the effective damping-like spin-torque efficiency and spin Hall conductivity are 0.18 and 3.8 × 105 ℏ/2e Ω−1 m−1 for Pt0.9Rh0.1, respectively, with a low resistivity of 46.9 µΩ cm. Furthermore, current induced SOT switching in PtRh/Co is investigated. The critical current density for SOT switching decreases with an increase in the Rh composition of the PtRh alloy, which can be understood by domain wall assisted switching. Due to a large spin Hall conductivity, a relatively low resistivity, and sustainability of the high temperature process, the PtRh alloy could be an attractive spin source for SOT applications.

Impact of trace amounts of interfacial oxidation on the spin–orbit torque in the Co/Pt heterostructures

We found that the exposure of a Co/Pt bilayer to air will result in a trace amount of oxidation at the Co/Pt interface, while the Pt layer is immune to oxidation. The appearance of CoOx results in a negative spin Hall magnetoresistance and unconventional spin–orbit torques (SOTs), which are observed through temperature-dependent transport and spin-torque ferromagnetic resonance measurements. These results can be understood by considering CoOx as an individual magnetic layer between Pt and Co, with two important characteristics: (1) its magnetization is aligned in the plane that is perpendicular to the magnetization of Co and (2) the spin transparency of CoOx increases with increasing temperature. These results help us understand the features of spin transport at the interface when oxidation occurs and further indicate that trace amounts of oxidation can be a highly effective method to control SOT in magnetic heterostructures.