Spin-torque Ferromagnetic Resonance Research Articles

Simultaneous measurement of the exchange parameter and saturation magnetization using propagating spin waves

The exchange interaction in ferromagnetic ultra thin films is a critical parameter in magnetization-based storage and logic devices, yet the accurate measurement of it remains a challenge. While a variety of approaches are currently used to determine the exchange parameter, each has its limitations, and good agreement among them has not been achieved. To date, neutron scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic resonance spectroscopy, and Kerr microscopy have all been used to determine the exchange parameter. Here, we present a method that exploits the wavevector selectivity of Brillouin light scattering to measure the spin wave dispersion in both the backward volume and Damon–Eshbach orientations. The exchange, saturation magnetization, and magnetic thickness are then determined by a simultaneous fit of both dispersion branches with general spin wave theory without any prior knowledge of the thickness of a magnetic “dead layer.” In this work, we demonstrate the strength of this technique for ultrathin metallic films, typical of those commonly used in industrial applications for magnetic random-access memory.

Spin–orbit torque in a single ferrimagnetic GdFeCo layer near the compensation temperature

We report spin–orbit torque (SOT) based on spin-torque ferromagnetic resonance (ST FMR) in a single ferrimagnetic layer. Temperature-dependent anomalous Hall resistance (Rxy) shows a magnetic compensation temperature (Tm) of about 205 K. Temperature-dependent ST FMR is performed to quantify SOT; the torque is exerted to the total moment, and the SOT sign diverges as the temperautre approaches Tm. X-ray photoelectron spectroscopy revealed that SOT arises due to the broken symmetry of the bulk spatial inversion along the normal direction of the film. Our finding provides useful information about the controlled temperature of bulk spin–orbit coupling in single layer GdFeCo.

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.

Conversion between spin and charge currents in topological-insulator/nonmagnetic-metal systems

The charge current in a topological insulator (TI) will induce a spin accumulation (Edelstein effect or EE), from which the spin current will be generated. Inversely, the spin current injection into the TI will induce a charge current called the inverse Edelstein effect (IEE). Some experimental and theoretical works have been done for the understanding of either EE or IEE. However, little experimental work incorporating both processes in the same TI sample has been done. In this work, we propose a phenomenological model to understand EE and IEE in the TI-based system. Based on this model, efficiencies of EE and IEE can be directly derived, which is consistent with previous theoretical work based on Boltzmann transport theory and obeying the energy conservation law. We also measure EE and IEE efficiencies experimentally in a TI/Ru/CoFeB system by spin-torque ferromagnetic resonance and spin pumping, respectively. The experimental results are consistent with our model, which proves that the spin-charge conversion in TI can be understood in the framework of (I)EE instead of (inverse) spin Hall effect. By combining theories and experiments, we find that enhancing interfacial transparency is crucial for enhancing EE efficiency, and avoiding metallic contact is crucial for enhancing IEE efficiency.

Separation of Artifacts from Spin‐Torque Ferromagnetic Resonance Measurements of Spin‐Orbit Torque for the Low‐Symmetry Van der Waals Semi‐Metal ZrTe3

AbstractSpin‐orbit torques generated by exfoliated layers of the low‐symmetry semi‐metal ZrTe3 are measured using the spin‐torque ferromagnetic resonance (ST‐FMR) technique. When the ZrTe3 has a thickness greater than about 10 nm, artifacts due to spin pumping and/or resonant heating can cause the standard ST‐FMR analysis to overestimate the true magnitude of the torque efficiency by as much as a factor of 30, and to indicate incorrectly that the spin‐orbit torque depends strongly on the ZrTe3 layer thickness. Artifact‐free measurements can still be achieved over a substantial thickness range by the method developed recently to detect ST‐FMR signals in the Hall geometry as well as the longitudinal geometry. ZrTe3/Permalloy samples generate a conventional in‐plane anti‐damping spin torque efficiency = 0.014 ± 0.004, and an unconventional in‐plane field‐like torque efficiency = 0.003 ± 0.001. The out‐of‐plane anti‐damping torque is negligible. It is suggested that artifacts similarly interfere with the standard ST‐FMR analysis for other van der Waals samples thicker than about 10 nm.

Spin and Charge Interconversion in Dirac-Semimetal Thin Films

We report spin-to-charge and charge-to-spin conversion at room temperature in heterostructure devices that interface an archetypal Dirac semimetal, Cd3As2, with a metallic ferromagnet, Ni0.80Fe0.20 (permalloy). The spin-charge interconversion is detected by both spin torque ferromagnetic resonance and ferromagnetic resonance driven spin pumping. Analysis of the symmetric and anti-symmetric components of the mixing voltage in spin torque ferromagnetic resonance and the frequency and power dependence of the spin pumping signal show that the behavior of these processes is consistent with previously reported spin-charge interconversion mechanisms in heavy metals, topological insulators, and Weyl semimetals. We find that the efficiency of spin-charge interconversion in Cd3As2/permalloy bilayers can be comparable to that in heavy metals. We discuss the underlying mechanisms by comparing our results with first principles calculations.

Inhomogeneous magnetic properties characterized by simultaneous electrical and optical detection of spin-torque ferromagnetic resonance

Magnetic properties of the Pt/Py microstrip were investigated using the simultaneous electrical and optical detection of spin-torque ferromagnetic resonance. From the measured optical signal using the heterodyne-magneto-optical Kerr effect (MOKE), we found that the inhomogeneous broadening and Gilbert damping constant were modified along the transverse direction of the microstrip. In addition, a difference in the precession phase was also observed owing to the non-uniform out-of-plane microwave field. Our study shows that the ferromagnetic resonance measurement based on the heterodyne-MOKE technique is a powerful tool for characterizing the static and dynamic magnetic properties of magnetic thin films with sub-micrometer spatial resolution.

Modulation of spin-torque ferromagnetic resonance with a nanometer-thick platinum by ionic gating

The spin Hall effect (SHE) and inverse spin Hall effect (ISHE) have played central roles in modern condensed matter physics especially in spintronics and spin-orbitronics, and much effort has been paid to fundamental and application-oriented research towards the discovery of novel spin–orbit physics and the creation of novel spintronic devices. However, studies on gate-tunability of such spintronics devices have been limited, because most of them are made of metallic materials, where the high bulk carrier densities hinder the tuning of physical properties by gating. Here, we show an experimental demonstration of the gate-tunable spin–orbit torque in Pt/Ni80Fe20 (Py) devices by controlling the SHE using nanometer-thick Pt with low carrier densities and ionic gating. The Gilbert damping parameter of Py and the spin-memory loss at the Pt/Py interface were modulated by ionic gating to Pt, which are compelling results for the successful tuning of spin–orbit interaction in Pt.

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.

Spin-orbit torque in a Ni-Fe single layer

We evaluated the self-induced spin-orbit torque (SOT) in a single ferromagnetic layer. Spin-torque ferromagnetic resonance (ST-FMR) was measured for the very thin Ni-Fe (permalloy, Py) layers with and without structural inversion symmetry: asymmetric and symmetric Py layers. For both structures, the fieldlike component coming from the Oersted field clearly appeared. On the other hand, the dampinglike component was observed only for asymmetric Py with thickness $\ensuremath{\le}3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. This dampinglike torque is attributable to the significant spatial change in the properties of Py. We propose a toy model to analyze the self-induced SOT.

Coexistence of low-frequency spin-torque ferromagnetic resonance and unidirectional spin Hall magnetoresistance

We investigate the DC voltage under an applied microwave current for platinum(Pt)/cobalt(Co), tantalum(Ta)/Co, and tungsten(W)/Co bilayers, where the microwave-induced alternating magnetic field (${B}_{\mathrm{rf}}$) is parallel to an external static magnetic field (${B}_{\mathrm{ext}}$). Whereas spin torque ferromagnetic resonance (ST FMR) signals do not appear because of the parallel configuration of ${B}_{\mathrm{rf}}$ and the magnetization of the Co layer, we recently found a clear hysteresis signal around ${B}_{\mathrm{ext}}=0\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ only when the microwave frequency (${f}_{\mathrm{MW}}$) is low, typically less than 10 GHz. This low-frequency ST FMR (LFST FMR) signal enables us to detect magnetization switching induced by spin-orbit torque (SOT) with high sensitivity. In this study, we found an additional background (BG) signal superimposed on the LSFT FMR signal. The additional BG signal also has hysteresis behavior and appears even at high ${f}_{\mathrm{MW}}$, where the LSFT FMR signal completely disappears. The sign of the BG signal is changed by changing the nonmagnetic material from Pt to Ta or W. By measuring ${f}_{\mathrm{MW}}$, the microwave current, the temperature, and the magnetic field angle dependences, we conclude that the BG signal is induced by spin-dependent unidirectional spin Hall magnetoresistance (SD USMR), which is generated by a spin current induced by the spin Hall effect of nonmagnetic metals and spin-dependent electron mobility in ferromagnetic metals. The SD USMR signal appears in wide ranges of ${f}_{\mathrm{MW}}$ and ${B}_{\mathrm{ext}}$ because the SD USMR originates from the nonlinearity of current-dependent resistance and is not related to magnetization dynamics. From the magnitude of the BG signal, the spin Hall angles of Pt and Ta are calculated to be 0.026 \ifmmode\pm\else\textpm\fi{} 0.006 and \ensuremath{-}0.042 \ifmmode\pm\else\textpm\fi{} 0.006, respectively. In addition, we demonstrate SOT magnetization switching using the BG signal.

Terahertz Generation via Picosecond Spin-to-Charge Conversion in IrMn3/Ni−Fe Heterojunction

Experimental investigations of ultrafast electro-optical properties in magnetic materials manifest their great potential for emerging spintronic optoelectronic devices. Here, using time-resolved terahertz emission spectroscopy, we construct a spintronic terahertz emitter consisting of an ${\mathrm{Ir}\mathrm{Mn}}_{3}/\mathrm{Ni}\ensuremath{-}\mathrm{Fe}$ heterojunction. A femtosecond spin current pulse is generated in the thin film of the $\mathrm{Ni}\ensuremath{-}\mathrm{Fe}$ layer when it absorbs a femtosecond laser pulse, and then the spin current is converted into a transient charge current by the metallic ${\mathrm{Ir}\mathrm{Mn}}_{3}$ layer on picosecond timescales. We timely record the terahertz emission associated with this ultrafast conversion process by means of electro-optic sampling. Besides, the spin-to-charge conversion efficiency of the ${\mathrm{Ir}\mathrm{Mn}}_{3}/\mathrm{Ni}\ensuremath{-}\mathrm{Fe}$ heterojunction is determined via quantitative analysis of the spin torque ferromagnetic resonance results. We have both optically verified and electrically studied the spin-to-charge conversion of the ${\mathrm{Ir}\mathrm{Mn}}_{3}/\mathrm{Ni}\ensuremath{-}\mathrm{Fe}$ heterojunction. Our results enlarge the material choice range of spintronic terahertz emitters, which may promote further investigations of ultrafast spin-to-charge conversion in different heterojunction materials.

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.

Controllable charge-spin conversion by Rashba-Edelstein effect at Cu/Ta interface

The Rashba-Edelstein effect typically originates from inversion symmetry breaking at an interface that allows charge to spin conversion efficiency comparable to or larger than the SHE. Its potential value is in converting charge current into spin current. Here, we demonstrate experimentally an efficient charge to spin conversion at a Cu/Ta Rashba interface. We exploit spin-torque ferromagnetic resonance to detect a spin current from Cu/Ta interface into adjacent FeNi magnetic layer. The Ta thickness dependence of charge to spin conversion efficiency unambiguously ascribed that interface effect caused FeNi magnetization to precess by generating nonzero spin current that exerts torques rather than bulk effect. The possibility of controllable charge-spin conversion efficiency at room temperature opens another route for modulating the charge to spin conversion by interface engineering.

Resonant Measurement of Nonreorientable Spin-Orbit Torque from a Ferromagnetic Source Layer Accounting for Dynamic Spin Pumping

Using a multilayer containing a cobalt detector layer, a copper spacer, and a Permalloy source layer, we show experimentally how the nonreorientable spin-orbit torque generated by the Permalloy source layer---the component of spin-orbit torque that does not change when the Permalloy magnetization is rotated---can be measured using spin-torque ferromagnetic resonance (ST FMR) with line-shape analysis. We find that dynamic spin pumping between the magnetic layers exerts torques on the magnetic layers as large as, or larger than, the spin-orbit torques, so that if dynamic spin pumping is neglected, the result will be a large overestimate of the spin-orbit torque. Nevertheless, the two effects can be separated by performing ST FMR as a function of the frequency. We measure a nonreorientable spin torque ratio ${\ensuremath{\xi}}_{\mathrm{DL}}=0.04\ifmmode\pm\else\textpm\fi{}0.01$ for the spin-current flow from Permalloy through an 8-nm $\mathrm{Cu}$ spacer to the $\mathrm{Co}$ and a strength of dynamic spin pumping that is consistent with previous measurements by conventional ferromagnetic resonance.

Interplay between Spin-Orbit Torques and Dzyaloshinskii-Moriya Interactions in Ferrimagnetic Amorphous Alloys.

Ferrimagnetic thin films are attractive for low‐power spintronic applications because of their low magnetization, small angular momentum, and fast spin dynamics. Spin orbit torques (SOT) can be applied with proximal heavy metals that also generate interfacial Dzyaloshinskii–Moriya interactions (DMI), which can stabilize ultrasmall skyrmions and enable fast domain wall motion. Here, the properties of a ferrimagnetic CoGd alloy between two heavy metals to increase the SOT efficiency, while maintaining a significant DMI is studied. SOT switching for various capping layers and alloy compositions shows that Pt/CoGd/(W or Ta) films enable more energy‐efficient SOT magnetization switching than Pt/CoGd/Ir. Spin‐torque ferromagnetic resonance confirms that Pt/CoGd/W has the highest spin‐Hall angle of 16.5%, hence SOT efficiency, larger than Pt/CoGd/(Ta or Ir). Density functional theory calculations indicate that CoGd films capped by W or Ta have the largest DMI energy, 0.38 and 0.32 mJ m−2, respectively. These results show that Pt/CoGd/W is a very promising ferrimagnetic structure to achieve small skyrmions and to move them efficiently with current.

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.

Reversible strain-induced spin–orbit torque on flexible substrate

We propose the use of mechanical strain and mild annealing to achieve reversible modulation of spin–orbit torque (SOT) and Gilbert damping parameter. X-ray diffraction results show that the residual spin–orbit torque enhancement and Gilbert damping reduction, due to the post-mechanical strain treatment, can be reset using mild annealing to alleviate the internal strain. The spin Hall efficiency of the heat- and strain-treated Pt/Co bilayer was characterized through spin-torque ferromagnetic resonance, and it was found that the device could switch between the strain enhanced SOT and the pristine state. The Gilbert damping parameter behaves inversely with the spin Hall efficiency, and therefore, strain can be used to easily tune the device switching current density by a factor of ∼2 from its pristine state. Furthermore, the resonance frequency of the Pt/Co bilayer could be tuned using purely mechanical strain, and from the endurance test, the Pt/Co device can be reversibly manipulated over 104 cycles demonstrating its robustness as a flexible device.

A quantum material spintronic resonator

In a spintronic resonator a radio-frequency signal excites spin dynamics that can be detected by the spin-diode effect. Such resonators are generally based on ferromagnetic metals and their responses to spin torques. New and richer functionalities can potentially be achieved with quantum materials, specifically with transition metal oxides that have phase transitions that can endow a spintronic resonator with hysteresis and memory. Here we present the spin torque ferromagnetic resonance characteristics of a hybrid metal-insulator-transition oxide/ ferromagnetic metal nanoconstriction. Our samples incorporate {mathrm {V}}_2{mathrm {O}}_3, with Ni, Permalloy ({hbox {Ni}}_{80}{hbox {Fe}}_{20}) and Pt layers patterned into a nanoconstriction geometry. The first order phase transition in {mathrm {V}}_2{mathrm {O}}_3 is shown to lead to systematic changes in the resonance response and hysteretic current control of the ferromagnetic resonance frequency. Further, the output signal can be systematically varied by locally changing the state of the {mathrm {V}}_2{mathrm {O}}_3 with a dc current. These results demonstrate new spintronic resonator functionalities of interest for neuromorphic computing.

Theory of spin-torque ferrimagnetic resonance

We report a theory of spin-torque-induced magnetic resonance in collinear ferrimagnets, i.e., spin-torque ferrimagnetic resonance (ST-FiMR). We find that the rectified DC voltage of ST-FiMR has several distinct features as compared to spin-torque ferromagnetic resonance (ST-FMR) signals. The most important feature of the ST-FiMR signal is that its magnitude due to dampinglike spin-orbit torque is almost linearly proportional to the net spin density, and thus its sign changes near the angular momentum compensation condition. Our result suggests that the conventional ST-FMR line-shape analysis is unable to correctly estimate the magnitude and sign of spin-orbit torque for compensated ferrimagnets. Therefore, the analysis based on the ST-FiMR theory, of which a complete analytic expression is provided in this work, must be applied.