Field-like Torque Research Articles

Growth optimization of TaN for superconducting spintronics

We have optimized the growth of superconducting TaN thin films on SiO2 substrates via dc magnetron sputtering and extract a maximum superconducting transition temperature of T c = 5 K as well as a maximum critical field μ 0 H c2 = (13.8 ± 0.1) T. This material is of interest for both different fields of quantum technology and superconducting spintronics as it represents a magnetic field-robust superconductor with strong spin–orbit interaction (SOI). After presenting the results of the growth optimization, we investigate in the second part the impact of the strong SOI in TaN on superconductor/ferromagnet heterostructures. To this end, we analyze the magnetization dynamics of both normal state and superconducting TaN/Ni80Fe20 (permalloy, Py)-bilayers as a function of temperature using broadband ferromagnetic resonance spectroscopy. In particular, we quantify the inverse current-induced torques of the bilayers and compare these results to NbN/Py-bilayers. In the normal state of TaN, we detect a positive damping-like current-induced torque σ d from the inverse spin Hall effect and a small field-like torque σ f attributed to the inverse Rashba–Edelstein effect at the TaN/Py-interface. In the superconducting state of TaN, we detect a negative σ d attributed to the quasiparticle mediated inverse spin Hall effect (QMiSHE) and the unexpected manifestation of a large positive field-like σ f of unknown origin matching our previous results for NbN/Py-bilayers. The QMiSHE can be used to probe spin currents in emergent quantum materials.

Interfacial spin-orbit torques and magnetic anisotropy in WSe2/permalloy bilayers

Transition metal dichalcogenides (TMDs) are promising materials for efficient generation of current-induced spin-orbit torques (SOTs) on an adjacent ferromagnetic layer. Numerous effects, both interfacial and bulk, have been put forward to explain the different torques previously observed. Thus far, however, there is no clear consensus on the microscopic origin underlying the SOTs observed in these TMD/ferromagnet bilayers. To shine light on the microscopic mechanisms at play, here we perform thickness dependent SOT measurements on the semiconducting WSe2/permalloy bilayer with various WSe2 layer thickness, down to the monolayer limit. We observe a large out-of-plane field-like torque with spin-torque conductivities up to . For some devices, we also observe a smaller in-plane antidamping-like torque, with spin-torque conductivities up to , comparable to other TMD-based systems. Both torques show no clear dependence on the WSe2 thickness, as expected for a Rashba system. Unexpectedly, we observe a strong in-plane magnetic anisotropy—up to about erg cm−3—induced in permalloy by the underlying hexagonal WSe2 crystal. Using scanning transmission electron microscopy, we confirm that the easy axis of the magnetic anisotropy is aligned to the armchair direction of the WSe2. Our results indicate a strong interplay between the ferromagnet and TMD, and unveil the nature of the SOTs in TMD-based devices. These findings open new avenues for possible methods for optimizing the torques and the interaction with interfaced magnets, important for future non-volatile magnetic devices for data processing and storage.

Underlying mechanism for exchange bias in single-molecule magnetic junctions

Magnetic proximity has been observed in a variety of solid-state magnetic devices, but has been less discussed at the molecular scale. In this study, the magnetotransport calculation is carried out using the generalized Landau-Lifshitz-Gilbert (LLG) equation combined with density functional theory (DFT) and our self-developed junpy calculated spin-torque effect. Except for the current driven spin torque, which is a promising approach for magnetization switch in magnetic random access memory, the equilibrium fieldlike spin torque also plays a crucial role in the strain-controlled exchange bias with current-controlled magnetic coercivity in single-molecule magnetic junctions. The tight-binding model is further employed to clarify the critical role of the interfacial spin filter effect arising from the hybridization between the linker and Co apex. These multidisciplinary DFT+junpy+LLG results may provide important and practical implications in the dual control of magnetic proximity and magnetization switching in molecular spintronics at low temperature, either by tensile strain or via smaller applied current density of the order of $\mathrm{MA}/{\mathrm{cm}}^{2}$.

Spin\u2013orbit torque driven magnetization switching in W/CoFeB/MgO-based type-Y three terminal magnetic tunnel junctions

We have studied current induced magnetization switching in W/CoFeB/MgO based three terminal magnetic tunnel junctions. The switching driven by spin—orbit torque (SOT) is evaluated in the so-called type-Y structure, in which the magnetic easy-axis of the CoFeB layer lies in the film plane and is orthogonal to the current flow. The effective spin Hall angle estimated from the bias field dependence of critical current (Ic) is ~ 0.07. The field and current dependence of the switching probability are studied. The field and DC current induced switching can be described using a model based on thermally assisted magnetization switching. In contrast, the 50 ns long pulse current dependence of the switching probability shows significant deviation from the model, even if contribution from the field-like torque is included. The deviation is particularly evident when the threshold switching current is larger. These results show that conventional thermally assisted magnetization switching model cannot be used to describe SOT induced switching using short current pulses.

Anatomy of Type- x Spin-Orbit-Torque Switching

Using a type-x spin-orbit-torque (SOT) switching scheme, in which the easy axis (EA) of the ferromagnetic (FM) layer and the charge-current flow direction are collinear, it is possible to realize a lower-power-consumption, higher-density, and better-performance SOT magnetoresistive random-access memory (SOT MRAM) compared with the conventional type-y design. Here, we systematically investigate type-x SOT switching properties through both macrospin and micromagnetic simulations. The out-of-plane external field and anisotropic field dependence of the switching-current density (${J}_{\mathrm{sw}}$) is first examined in the ideal type-x configuration. Next, we study the FM-layer canting-angle ($\ensuremath{\varphi}$) dependence of ${J}_{\mathrm{sw}}$ through macrospin simulations and experiments, which show the transformation of switching dynamics from type x to type y with increasing $\ensuremath{\varphi}$. By further integrating fieldlike torque (FLT) into the simulated system, we find that a positive FLT can assist type-x SOT switching, while a negative one brings about complex dynamics. More crucially, with the existence of a sizable FLT, type-x switching mode results in a lower critical switching current than that of type y at a current pulse width less than about 10 ns, indicating the advantage of employing the type-x design for ultrafast switching using material systems with FLT. Our work provides a thorough examination of the type-x SOT scheme with various device and materials parameters, which can be informative for designing next-generation SOT MRAM.

Spin-transfer torque driven localized spin excitations in the presence of field-like torque

We study the existence of localized one-spin excitation in the Heisenberg one- dimensional ferromagnetic spin chain in the presence of perpendicular and parallel external magnetic fields and current with spin-transfer torque and field-like torque. The Landau–Lifshitz–Gilbert–Slonczewski (LLGS) equation is exactly solved for the one spin excitation in the absence of onsite anisotropy for the excitations of spin with fields perpendicular and parallel to the chain. We show the removal of damping in the spin excitations by appropriately introducing current and also the enhancement of angular frequency of the oscillations due to field-like torque in the case of both perpendicular and parallel field. The exactness of the analytical results is verified by matching with numerical counterparts. Further, we numerically confirm the existence of in-phase and anti-phase stable synchronized oscillations for two spin-excitations in the presence of current with perpendicular field and field-like torque. We also show that the one-spin excitation is stable against thermal noise and gets only slightly modified against thermal fluctuations.

Critical switching current of a perpendicular magnetic tunnel junction owing to the interplay of spin-transfer torque and spin-orbit torque

Using the linearized Landau–Lifshitz-Gilbert (LLG) equation in the rotation coordinate, we calculate the critical switching current density of a perpendicular magnetic tunnel junction (MTJ), where magnetization switching is achieved by the interplay of spin-transfer and spin-orbit torques. In terms of the critical current density, we find that as the current density inducing the spin-orbit torque increases, the current density inducing the spin-transfer torque decreases nonlinearly. In the presence of the spin-orbit field-like torque (β), the critical switching current density by the spin-transfer torque is proportional to the damping constant (α) as the conventional spin-transfer torque switching, while the critical switching current density by the spin-orbit torque increases with when . We also investigated the β dependence of the critical switching current density. For a given , the critical switching current density for the spin-transfer torque, decreases linearly with increasing β and nonlinearly decreases with increasing for a given β. Further, we discuss the β dependence of the critical switching current density on energy.

Spin-orbit torques and magnetotransport properties of α−Sn and β−Sn heterostructures

Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\alpha$-Sn. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\alpha$-Sn and $\beta$-Sn-based ferromagnetic heterostructures. We show that $\alpha$-Sn grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\alpha$-Sn grown without Bi surfactant and the non-topological phase, $\beta$-Sn, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\alpha$-Sn with Bi are 0.12 and 0.18, respectively. Further, we show that $\alpha$-Sn grown with and without Bi presents a spin Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\alpha$-Sn ferromagnetic heterostructures, showing that $\alpha$-Sn is a promising material for current-induced magnetization control in spintronic devices.

Current-induced torques in magnetic Weyl semimetal tunnel junctions.

We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the non-equilibrium torques acting on the ferromagnet and discuss the existence of large field-like torques acting on the magnetic Weyl semimetal which exceeds the theoretical maximum of conventional magnetic tunnel junctions. The latter are derived from the Fermi arc spin texture and display a counter-intuitive dependence on the Weyl nodes separation. Our results shed light on the new physics of multilayered spintronic devices comprising of magnetic Weyl semimetals, which might open doors for new energy efficient spintronic devices.

Interfacial and bulk spin Hall contributions to fieldlike spin-orbit torque generated by iridium

We present measurements of spin orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a damping-like component of spin orbit torque with a maximum spin torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of 0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large field like spin orbit torque. Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism so that in the ultrathin limit there is a non-zero FLT with a maximum torque conductivity -5.0E4 in the SI unit. When the Ir film thickness becomes comparable to or greater than its spin diffusion length, 1.6 nm, there is also a smaller bulk contribution to the fieldlike torque.

Current-induced dynamics of skyrmion tubes in synthetic antiferromagnetic multilayers

Topological spin textures can be found in both two-dimensional and three-dimensional nanostructures, which are of great importance to advanced spintronic applications. Here we report the current-induced skyrmion tube dynamics in three-dimensional synthetic antiferromagnetic (SyAF) bilayer and multilayer nanostructures. It is found that the SyAF skyrmion tube made of thinner sublayer skyrmions is more stable during its motion, which ensures that a higher speed of the skyrmion tube can be reached effectively at larger driving current. In the SyAF multilayer with a given total thickness, the current-induced deformation of the SyAF skyrmion tube decreases with an increasing number of interfaces; namely, the rigidity of the SyAF skyrmion tube with a given thickness increases with the number of ferromagnetic (FM) layers. For the SyAF multilayer with an even number of FM layers, the skyrmion Hall effect can be eliminated when the thicknesses of all FM layers are identical. Larger damping parameter leads to smaller deformation and slower speed of the SyAF skyrmion tube. Larger fieldlike torque leads to larger deformation and a higher speed of the SyAF skyrmion tube. Our results are useful for understanding the dynamic behaviors of three-dimensional topological spin textures and may provide guidelines for building SyAF spintronic devices.

Field-Free Deterministic Magnetization Switching Induced by Interlaced Spin-Orbit Torques.

Spin-orbit torque (SOT) magnetic random access memory is envisioned as an emerging nonvolatile memory due to its ultrahigh speed and low power consumption. The field-free switching scheme in SOT devices is of great interest to both academia and industry. Here, we propose a novel field-free deterministic magnetization switching scheme in a regular magnetic tunnel junction by using two currents sequentially passing interlaced paths, with less requirements of the manufacturing process or additional physical effects. The switching is bipolar since the final magnetization state depends on the combination of current paths. The functionality and robustness of the proposed scheme are validated through both macrospin and micromagnetic simulation. The influences of field-like torque and the Dzyaloshinskii-Moriya interaction effect are further researched. Our proposed scheme shows good scalability and is expected to realize novel digital logic and even computing-in-memory platforms.

Large amplitude spin-Hall oscillations due to field-like torque

Large amplitude spin-Hall oscillations are identified in a ferromagnetic layer with two perpendicular in-plane easy axis in the presence of field-like torque without any polarizer and external field. The analytical study confirms the possibility of oscillations in the presence of field-like torque. The investigation shows that the oscillation frequency can be tuned from ∼2 GHz to ∼80 GHz by current and enhanced by field-like torque. Further, the enhancement of frequency along with the Q-factor by current and field-like torque is also observed.

Large fieldlike torque in amorphous Ru2Sn3 originated from the intrinsic spin Hall effect

We investigated temperature dependent current driven spin-orbit torques in magnetron sputtered Ru2Sn3 (4 and 10 nm) /Co20Fe60B20 (5 nm) layered structures with in-plane magnetic anisotropy. The room temperature damping-like and field-like spin torque efficiencies of the amorphous Ru2Sn3 films were measured to be 0.14 +- 0.008 (0.07 +- 0.012) and -0.03 +- 0.006 (-0.20 +- 0.009), for the 4 (10 nm) films respectively, by utilizing the second harmonic Hall technique. The large field-like torque in the relatively thicker Ru2Sn3 (10 nm) thin film is unique compared to the traditional spin Hall materials interfaced with thick magnetic layers with in-plane magnetic anisotropy which typically have dominant damping-like and negligible field-like torques. Additionally, the observed room temperature field-like torque efficiency in Ru2Sn3 (10 nm)/CoFeB (5 nm) is up to three times larger than the damping-like torque (-0.20 +- 0.009 and 0.07 +- 0.012, respectively) and thirty times larger at 50 K (-0.29 +- 0.014 and 0.009 +- 0.017, respectively). The temperature dependence of the field-like torques show dominant contributions from the intrinsic spin Hall effect while the damping-like torques show dominate contributions from the extrinsic spin Hall effects, skew scattering and side jump. Through macro-spin calculations, we found that including field-like torques on the order or larger than the damping-like torque can reduce the switching critical current and decrease magnetization procession for a perpendicular ferromagnetic layer.

Out-of-plane magnetization oscillation in spin Hall device assisted by field-like torque

An excitation of a large-amplitude out-of-plane magnetization oscillation in a ferromagnet by the spin Hall effect is of great interest for practical applications such as microwave generators and neuromorphic computing. However, both experimental and theoretical works have revealed that only small-amplitude oscillation around an in-plane easy axis can be excited via the spin Hall effect. Here, we propose that an out-of-plane oscillation can be excited due to an assistance of field-like torque. We focus on an in-plane magnetized ferromagnet with an easy axis parallel to the current direction. We notice that the field-like torque with an appropriate sign provides an additional field, modifying the dynamic trajectory of the magnetization, and drives the auto-oscillation. The condition on the sign of the field-like torque is satisfied for a typical nonmagnet used in spin Hall devices such as tungsten.

Effects of Interfacial Oxidization on Magnetic Damping and Spin-Orbit Torques.

We investigate the effects of interfacial oxidation on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques in heavy-metal (Pt)/ferromagnet (Co or NiFe)/capping (MgO/Ta, HfOx, or TaN) structures. At room temperature, the capping materials influence the effective surface magnetic anisotropy energy density, which is associated with the formation of interfacial magnetic oxides. The magnetic damping parameter of Co is considerably influenced by the capping material (especially MgO) while that of NiFe is not. This is possibly due to extra magnetic damping via spin-pumping process across the Co/CoO interface and incoherent magnon generation (spin fluctuation) developed in the antiferromagnetic CoO. It is also observed that both antidamping and field-like spin-orbit torque efficiencies vary with the capping material in the thickness ranges we examined. Our results reveal the crucial role of interfacial oxides on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques.

Experimental demonstration of voltage-gated spin-orbit torque switching in an antiferromagnet/ferromagnet structure

Efficient manipulation of magnetization is crucial for magnetic random-access memory (MRAM). Here we experimentally demonstrate spin-orbit-torque-driven (SOT-driven) magnetization switching of perpendicularly magnetized IrMn/CoFeB/MgO structures with the assist of a gate voltage. It is found that with the gate voltage changing from \ensuremath{-}0.6 to 0.6 V, the SOT critical switching current density ${J}_{c}$ is reduced by 59%. Furthermore, the mechanism of voltage-induced ${J}_{c}$ reduction is explored. We find that the dampinglike torque decreases with positive voltage, while the fieldlike torque shows a weak voltage dependence, which demonstrates that the voltage control of spin-orbit torque (VCSOT) effect has no positive effect on the reduction of ${J}_{c}$. In addition, a significant decrease of anisotropy energy is observed when a positive voltage is applied. These results indicate that both VCSOT effect and voltage control of magnetic anisotropy (VCMA) effect exist in voltage-gated SOT switching, while the VCMA effect plays a main role in reduction of ${J}_{c}$. This work shows low-power magnetization switching and helps to understand the mechanism of voltage-gated SOT switching.

Temperature-Dependent Spin Transport and Current-Induced Torques in Superconductor-Ferromagnet Heterostructures.

We investigate the injection of quasiparticle spin currents into a superconductor via spin pumping from an adjacent ferromagnetic metal layer. To this end, we use NbN-Ni_{80}Fe_{20}(Py) heterostructures with a Pt spin sink layer and excite ferromagnetic resonance in the Permalloy layer by placing the samples onto a coplanar waveguide. A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between the sample and the coplanar waveguide, interpreted in terms of inverse current-induced torques, in our heterostructures as a function of temperature. Below the superconducting transition temperature T_{c}, we observe a suppression of the dampinglike torque generated in the Pt layer by the inverse spin Hall effect, which can be understood by the changes in spin current transport in the superconducting NbN layer. Moreover, below T_{c} we find a large fieldlike current-induced torque.

W layer thickness dependence of the spin–orbit effective fields in NiFe/W bilayers

Spin–orbit torques (SOTs) generated by in-plane current injection in a ferromagnetic metal (FM)/heavy metal (HM) bilayer offers a new route to electrically manipulate magnetization. Here, we report on two sizable spin–orbit field contributions from the spin Hall effect and Rashba effect in NiFe/W bilayers by using the planar Hall effect. Both spin–orbit fields decrease with increasing W layer thickness. Importantly, the spin–orbit field contributing from the spin Hall effect decreases faster than the one from the Rashba effect as the thickness of W layer increases, leading to the sign change of the field-like torque at thicker W. Our results illustrate the co-contributions of the Rashba effect and the spin Hall effect to the field-like SOT in NiFe/W bilayer giving more insight into the effect of the field-like SOT in a FM/HM bilayer.

Origin of spin–orbit torque in single-layer CoFeB investigated via in-plane harmonic Hall measurements

The current-induced spin–orbit torque in single-layer ferromagnetic CoFeB thin films is quantitatively investigated by using in-plane harmonic Hall measurements. After the subtraction of thermal contributions such as the anomalous and ordinary Nernst effects, the obtained overall spin–orbit torque is successfully decomposed into damping-like (DL) and field-like (FL) terms. The DL and FL torques exhibit opposite trends of ferromagnetic layer thickness dependence before saturation, giving rise to distinctively different spin torque efficiencies: the DL torque efficiency shows a strong thickness dependence, while the FL torque efficiency is almost independent of the thickness. Such a result shows strong evidence that the DL torque originates from a spin-Hall-like charge-spin conversion in the ferromagnet, while the FL torque stems from interfacial effects such as the Rashba–Edelstein effect. With both DL and FL torques quantified in the single-layer CoFeB, our results exhibit an important step toward the understanding of nontrivial spin–orbit torques in single-layer ferromagnetic thin films.