Current-induced Effective Field Research Articles

Perpendicular magnetization switching of RuO2(1 0 0)/[Pt/Co/Pt] multilayers

Current-induced perpendicular magnetization switching holds great potential for high-density magnetic storage technology. We report the growth of perpendicularly magnetized [Pt/Co/Pt] multilayers on a collinear altermagnet RuO2(100) thin films. The current-induced effective magnetic field was determined through anomalous Hall effect loop measurements with current applied along the [010] or [001] direction of RuO2(100). Magnetization switching was successfully achieved with the assistance of an in-plane magnetic field. Furthermore, by adjusting the nominal top Pt thickness of the [Pt/Co/Pt] multilayers, we could change the polarity of the current-induced magnetization switching, demonstrating an effective manipulation of spin polarization direction in RuO2/[Pt/Co/Pt]. These results provide a practical approach to the development of perpendicular magnetic devices based on RuO2 and have advanced the study of spin-splitting altermagnets.

The central role of tilted anisotropy for field-free spin–orbit torque switching of perpendicular magnetization

The discovery of efficient magnetization switching upon device activation by spin Hall effect (SHE)-induced spin–orbit torque (SOT) changed the course of magnetic random-access memory (MRAM) research and development. However, for electronic systems with perpendicular magnetic anisotropy (PMA), the use of SOT is still hampered by the necessity of a longitudinal magnetic field to break magnetic symmetry and achieve deterministic switching. In this work, we demonstrate that robust and tunable field-free current-driven SOT switching of perpendicular magnetization can be controlled by the growth protocol in Pt-based magnetic heterostructures. We further elucidate that such growth-dependent symmetry breaking originates from the laterally tilted magnetic anisotropy of the ferromagnetic layer with PMA, a phenomenon that has been largely neglected in previous studies. We show experimentally and in simulation that in a PMA system with tilted anisotropy, the deterministic field-free switching exhibits a conventional SHE-induced damping-like torque feature, and the resulting current-induced effective field shows a nonlinear dependence on the applied current density. This relationship could be potentially misattributed to an unconventional SOT origin.

Voltage-gated field-free spin–orbit torque switching in Pt/Co/Ir/MgO wedged structures

The ability to efficiently manipulate magnetization is of great significance for practical applications of spin–orbit torque (SOT) devices. In this study, we report the voltage-controlled, field-free SOT switching in perpendicular magnetized Pt/Co/Ir/MgO structures with wedge iridium interlayers. The insertion of a thin iridium interlayer at ferromagnet/oxide can significantly reduce the perpendicular magnetic anisotropy depending on the Ir thickness. The wedging of the iridium layer breaks lateral structural symmetry, resulting in deterministic switching without the assistance of in-plane magnetic fields. In such a structure, the SOT critical switching currents are remarkably decreased by 29% when a positive 6 V gate voltage is applied. Further quantitative analysis shows that multiple factors contribute to the decrease in switching currents, including a 23% reduction in magnetic anisotropy energy, a reduction in nucleation field, and a minor enhancement in damping-like torque under gate voltage. Moreover, the probabilistic hindrance that gate voltage poses to field-free switching is revealed by the decrease in current-induced perpendicular effective fields from symmetry-breaking. Our research shows that energy-efficient SOT switching can be controlled by gating and offers insight into the mechanism behind voltage-gated SOT switching.

Electric Field Control of Spin-Orbit Torque Magnetization Switching in a Spin-Orbit Ferromagnet Single Layer.

To achieve a desirable magnitude of spin-orbit torque (SOT) for magnetization switching and realize multifunctional spin logic and memory devices utilizing SOT, controlling the SOT manipulation is vitally important. In conventional SOT bilayer systems, researchers have tried to control the magnetization switching behavior via interfacial oxidization, modulation of spin-orbit effective field, and effective spin Hall angle; however, the switching efficiency is limited by the interface quality. A current-induced effective magnetic field in a single layer of a ferromagnet with strong spin-orbit interactions, the so-called spin-orbit ferromagnet, can be utilized to induce SOT. In spin-orbit ferromagnet systems, electric field application has the potential for manipulating the spin-orbit interactions via carrier concentration modulation. In this work, it is demonstrated that SOT magnetization switching can be successfully controlled via an external electric field using a (Ga, Mn)As single layer. By applying a gate voltage, the switching current density can be solidly and reversibly manipulated with a large ratio of 14.5%, which is ascribed to the successful modulation of the interfacial electric field. The findings of this work help further the understanding of the magnetization switching mechanism and advance the development of gate-controlled SOT devices.

Inactivation of damping-like torque in Tb-Gd-Fe film on Ta layer

Recently, bulk spin–orbit torques (SOTs) in transition metal-rare earth (TM-RE) ferrimagnets have been reported. In this study, to investigate bulk SOTs, we observed the current-induced effective field mediated by a damping-like torque (DLT), which is one of the components of SOTs, using TM-RE ferrimagnetic Tb x (Gd32Fe68)100-x films with a Ta layer. The DLT efficiency was inactivated by increasing the Tb composition x to be greater than 8, suggesting that the DLT was induced by the bulk SOTs of Tb x (Gd32Fe68)100-x as well as the spin Hall effect in the Ta. However, from the DLT measurement using a Tb8(Gd32Fe68)92 layer sandwiched between Ru layers, where Ru has a negligibly small DLT, was almost zero. This indicates that the inactivation of DLT cannot simply be explained by the bulk SOTs of Tb x (Gd32Fe68)100-x . However, we expect that this finding will provide meaningful insights for controlling magnetization using current.

Nonreciprocal dynamics of ferrimagnetic bimerons

Magnetic bimerons are topologically nontrivial spin textures in in-plane easy-axis magnets, which can be used as particle-like information carriers. Here, we report a theoretical study on the nonreciprocal dynamics of asymmetrical ferrimagnetic (FiM) bimerons induced by spin currents. The FiM bimerons have the ability to move at a speed of kilometers per second and do not show the skyrmion Hall effect at the angular momentum compensation point. Our micromagnetic simulations and analytical results demonstrate that spin currents are able to induce the nonreciprocal transport and a drift motion of the FiM bimeron even if the system is at the angular momentum compensation point. By analyzing the current-induced effective fields, we find that the nonreciprocal transport is attributed to the asymmetry of the bimeron structure. Our results are useful for understanding the physics of bimerons in ferrimagnets and may provide guidelines for building bimeron-based spintronic devices.

Current-induced perpendicular magnetization switching without external magnetic field in gate-induced asymmetric structure

We demonstrate current-induced switching of perpendicular magnetization without any external magnetic field by introducing lateral structural asymmetry through gate-induced redox reactions. A gate electrode was fabricated to cover only half of a heavy metal/ferromagnet/oxide wire, allowing a gate voltage to form an in-plane oxidation gradient. The essential role of structural asymmetry is evidenced by the fact that external-field-free deterministic switching is absent when the gate electrode covers an entire wire. We confirm that the current-induced perpendicular effective field deterministically drives the magnetization direction at zero field. Magnetic domain imaging and transport measurements clarify that the origin of the perpendicular effective field is not the Rashba effect, but the Oersted field at the wire edges.

Anisotropic bilinear magnetoresistance in (110) SrTiO3 -based two-dimensional electron gas

Bilinear magnetoresistance (BMR), the magnetoresistance that is linear against either magnetic field or applied current, is a hot topic of recent investigations. While most of the previous works focused on isotropic BMR, here we report on a strongly anisotropic BMR for (110) $\mathrm{SrTi}{\mathrm{O}}_{3}$-based two-dimensional electron gas (2DEG). Remarkably, the BMR measured along the [001] axis can be fivefold as large as that obtained along the $[1\overline{1}0]$ axis. A close relation is found between BMR and current-induced effective Rashba field, and it is the anisotropy of the Rashba field that causes the anisotropic BMR. Based on the analysis of anisotropic magnetoresistance, effective Rashba fields up to 4.5 T are determined. The band structure of the 2DEG is further calculated, ellipse-shaped Fermi rings are obtained, and the respective effects of different Fermi rings on BMR are distinguished. This work demonstrates the great potential of anisotropic 2DEG for the exploration of unusual effects.

Spin–orbit torque-induced magnetization switching in Pt/Co–Tb/Ta structures

Although transition metal (TM)-rare earth (RE) alloy film has potential application as an information storage medium in spintronic devices, study of the physical mechanism and microscopic process for the current-induced magnetization switching by spin–orbit torque (SOT) in TM-RE is still inadequate. In this work, we investigated the SOT effect and its driven magnetization switching in Pt/Co–Tb/Ta structures with various Co–Tb compositions. The results show that the current-induced SOT effective fields follow 1/Ms law near the compensation composition in this structure. Because of the large SOT effective field and the low coercivity for the Co–Tb layer near the compensation composition, the current-induced magnetization switching with a threshold current density as low as 1010 A/m2 was achieved in the system. The direct Kerr imaging on the switching process verifies two different current-induced switching mechanisms in the Pt/Co–Tb/Ta system.

Current-induced out-of-plane effective magnetic field in antiferromagnet/heavy metal/ferromagnet/heavy metal multilayer

We report that the electrical current induced an out-of-plane effective field in an antiferromagnet/heavy metal/ferromagnet/heavy metal (CoOx/Pt/Co/Pt) multilayer, which could change the magnetic hysteresis loop shift. The bottom CoOx layer can not only generate a bias field but also affect the interfacial Dzyaloshinskii–Moriya interaction. The superposition of the bias field and current-induced effective field could influence nucleation and propagation of the domain wall. Our demonstration and understanding could pave the way for manipulating the magnetization electrically.

Direct detection of spin-orbit effective fields through magneto-optical Kerr effect

Current-induced effective magnetic fields provide efficient methods to electrically control the magnetization switching in ultrathin magnetic films. It is getting clearer that the two terms of spin-orbit torque (SOT), i.e., the dampinglike SOT and the fieldlike SOT, play different roles during the magnetization switching process. Fast and direct estimation of both effective fields are crucial for highly efficient magnetization switching. Methods to extract the amplitudes of the effective fields have been intensively investigated, but they are either complex or require multiparameter fitting, especially for the fieldlike field. We introduce the direct detection of SOT effective fields using hysteresis-loop shift through the magneto-optical Kerr effect in the out-of-plane Pt/Co/W and in-plane Pt/Fe/MgO trilayer systems. The results show strong agreement with transport measurements. Benefiting from noncontact optical detection, our approach in the detection of the spin-orbit effective field has great advantages in the simplified device fabrication processing, and it is suitable not only for metallic systems but also for insulating systems, which paves the way to direct detection of SOT effective fields.

Magnetization switching induced by magnetic field and electric current in perpendicular TbIG/Pt bilayers

Magnetic insulators (MIs) have attracted great attention because of their low Gilbert damping, long spin transmission length, and no Ohmic loss. In this study, the high quality TbIG films with perpendicular magnetic anisotropy were epitaxially grown on GGG (111) substrates. In TbIG/Pt bilayers, the angular dependence of coercivity is found to obey the Kondorsky model, suggesting the magnetization reversal mechanism of magnetic domain nucleation and expansion. The transverse component of spin Hall magnetoresistance (SMR), which is analogous to the planar Hall resistance in a ferromagnetic metal, is found to be about seven times larger than the SMR-induced anomalous Hall resistance (analogous to the anomalous Hall resistance in a ferromagnetic metal). Moreover, the phase diagrams of the current-induced magnetization switching with different angles and magnitudes of the assisting magnetic field were drawn for the TbIG/Pt bilayers. The current-induced damping-like effective field (HDL) characterized by the harmonic measurements was evaluated to be about 164 Oe/108 A cm−2. By providing a comprehensive investigation of magnetization switching behaviors in MIs, our results will promote the application of ultralow-dissipation MI based spintronic devices.

Current-induced effective magnetic field in La0.67Sr0.33MnO3/LaAlO3/SrTiO3 structures

We investigate the current-induced effective magnetic field Heff in La0.67Sr0.33MnO3 (18)/LaAlO3(0,2,6)/SrTiO3 (LSMO/LAO/STO) structures by using the planar Hall effect (PHE), where numbers in parentheses give the nominal thickness of the given layer in unit cells. In all the structures, applying an in-plane current creates an in-plane Heff orthogonal to the current direction, and the direction of Heff for the LSMO/LAO(6)/STO structure is opposite to that for the LSMO/LAO(0,2)/STO structures. At low temperature, the sign of the PHE coefficient originating from the spin-orbit interaction (SOI) for the LSMO/LAO(6)/STO structure is also opposite to that for the LSMO/LAO(0,2)/STO structures, which suggests that the SOI in the LSMO layer is modified by the LAO(6)/STO structure. The direction of Heff is consistent with that induced by the SOI at the interface of the LSMO layer, and thus the opposite-polarity Heff detected in the LSMO/LAO(6)/STO structure can be related to the SOI variation induced by the LAO(6)/STO structure.

Field-Free Spin-Orbit Torque Switching of Perpendicular Magnetization by the Rashba Interface.

Current-induced spin-orbit torques (SOTs) enable efficient electrical manipulation of the magnetization in heterostructures with a perpendicular magnetic anisotropy through the Rashba effect or spin-Hall effect. However, in conventional SOT-based heterostructures, an in-plane bias magnetic field along the current direction is required for the deterministic switching. Here, we report that the field-free SOT switching can be achieved by introducing a wedged oxide interface between a heavy metal and a ferromagnet. The results demonstrate that the field-free SOT switching is determined by a current-induced perpendicular effective field (Hzeff) originating from the interfacial Rashba effect due to the lateral structural symmetry-breaking introduced by the wedged oxide layer. Furthermore, we show that the sign and magnitude of Hzeff exhibit a significant dependence on the interfacial oxygen content, which can be controlled by the inserted oxide thickness. Our findings provide a deeper insight into the field-free SOT switching by the interfacial Rashba effect.

Current-induced switching of YIG/Pt bilayers with in-plane magnetization due to Oersted fields

We report on the switching of the in-plane magnetization of thin yttrium iron garnet (YIG)/Pt bilayers induced by an electrical current. The switching is either field-induced and assisted by a dc current, or current-induced and assisted by a static magnetic field. The reversal of magnetization occurs at a current density as low as 105 A/cm2 and magnetic fields of ∼40 μT, two orders of magnitude smaller than in ferromagnetic metals, and consistent with the weak uniaxial anisotropy of the YIG layers. We use the transverse component of the spin Hall magnetoresistance to sense the magnetic orientation of YIG while sweeping the current. Our measurements and simulations reveal that the current-induced effective field responsible for switching is due to the Oersted field generated by the current flowing in the Pt layer rather than by spin–orbit torques, and that the switching efficiency is influenced by pinning of the magnetic domains.

Current-induced spin-orbit effective field modulations in synthetic antiferromagnetic structures

Abstract We demonstrate the modulation of spin-orbit torque (SOT) fields in synthetic antiferromagnetic (SAF) structures sandwiched between two heavy metals of opposite spin Hall angles. The SOTs fields that are measured by using harmonic Hall voltage technique, increase with net areal magnetization of the Pt/SAF/Ta structures. The result suggests an antiparallel orientation of the SOT fields in the two ferromagnetic layers. The antiparallel alignment of the SOT fields switches the magnetization of SAF structures with a current density as low as 2.3 × 1011 A/m2.

Tuning of current-induced effective magnetic field through Rashba effect engineering in hybrid multiferroic structures

Current-induced effective magnetic fields offer a new pathway through spin orbit interaction (SOI) to switch magnetization and have recently attracted great interest. In the conventional heavy metal/ferromagnetic metal/oxide (HM/FM/Oxide) structure, significant efforts have been made to study the role of the HM in determining effective magnetic fields. However, very little attention has been paid to the oxide layer and its interface with FM, where the Rashba effect may affect the effective field. In this report, we present a pathway to tune the effective magnetic field by engineering the Rashba effect in a hybrid multiferroic multilayer structure. A ferroelectric oxide of BaTiO3, whose polarizations either up or down are controlled by interface engineering, was introduced into the conventional SOI multilayer with the structure of BaTiO3/CoFeB/Pt. The current-induced effective magnetic fields increase by more than 200% when the ferroelectric polarization of BaTiO3 changes from up to down. The changes in the effective magnetic field are mainly attributed to the different Rashba effective fields induced by the opposite ferroelectric polarizations. Our study offers a new path towards controlling the current-induced effective magnetic field and may pave the way for integrating other functional oxides into the spintronic devices.

Current polarity-dependent manipulation of antiferromagnetic domains.

Antiferromagnets have several favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields 1 . Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents 2 . In previous experiments, orthogonal in-plane current pulses were used to induce 90° rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry 3 . Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using X-ray magnetic linear dichroism microscopy, and can also be detected electrically. Switching by domain-wall motion can occur at much lower current densities than those needed for coherent domain switching.

In situ Kerr and harmonic measurement in determining current-induced effective fields in MgO/CoFeB/Ta

A combination of the harmonic measurement and in situ Kerr imaging was used to experimentally determine the spin–orbit (SO) effective fields in a MgO/CoFeB/Ta structure. Here, we evaluate the SO effective fields through an analytical energy approach by transforming the anomalous Hall effect and planar Hall effect (PHE) voltage into a field dependency while imaging the magnetisation behaviour by differential Kerr microscopy. The analytical fitting to the measurement data indicates the significant coexistence of both a transverse field, , and longitudinal field, , in the longitudinal (HL = –12 Oe, HT = 8 Oe per 106 A cm−2) and transverse (HL = –12 Oe, HT = –17 Oe per 106 A cm−2) measurement schemes, respectively, due to the PHE. Additionally, dendritic-like domains, indicating the influence of the interfacial Dzyaloshinskii–Moriya interaction (DMI) at the CoFeB/Ta interface, were observed by in situ Kerr imaging. Micromagnetic simulations confirm the dendritic domain formation and edge tilting of the magnetisation, as being due to the DMI.

Magnetic properties and evidence of current-induced perpendicular field in epitaxial ferrimagnetic Mn4N (002) film mixed with (111) phase

The magnetic properties and anomalous Hall effect of the Mn4N (002) film mixed with the (111) phase were investigated. The lessened magnetization of hysteresis loops at a low field and the peak at about 50 K of the temperature-dependent magnetization curves display that magnetocrystalline anisotropies play a significant role in the magnetic properties of the film. Moreover, the centres of the anomalous Hall loops are shifted to the left for positive currents and show the opposite shift for negative currents, which may indicate the presence of current-induced effective fields. It is proposed that the (111) phase in the film could not only supply the magnetization m derivation from the c aixs but also afford asymmetric interfaces to induce effective fields. The effective perpendicular field arises from the perpendicular components of the effective fields.