Spin-orbit Torque Induced Magnetization Switching Research Articles

Noncollinear spin texture-driven torque in deterministic spin–orbit torque-induced magnetization switching

To reveal the role of chirality on field-free spin–orbit torque (SOT) induced magnetization switching, we propose an existence of z-torque through the formation of noncollinear spin texture during SOT-induced magnetization switching in a laterally two-level perpendicular magnetic anisotropy (PMA) system. For the investigation of torque, we simulate magnetization dynamics in the two-level PMA system with SOT, which generates the noncollinear spin texture. From the spatial distribution of magnetic energy, we reveal the additional z-directional torque contribution in the noncollinear spin texture, which is unexpected in the conventional SOT-induced magnetization switching in collinear spin texture. The z-directional torque originates from the interaction between the chirality of the noncollinear spin texture and the interfacial Dzyaloshinskii-Moriya interaction of the system. Furthermore, the experimental observation of the asymmetric magnetization switching to the direction of the current flow in the two-level PMA system supports our theoretical expectation.

Ultrafast spin–orbit torque-induced magnetization switching in a 75°-canted magnetic tunnel junction

We investigate the switching dynamics of a 75°-canted Spin–orbit torque (SOT) device with an in-plane easy axis using the micro-magnetic simulation. The switching time (τ) is evaluated from the time evolution of the magnetization. The device with a strong out-of-plane magnetic anisotropy (μ0Hkeff = −0.08 T) shows τ = 0.19 ns while a device with a strong in-plane magnetic anisotropy (μ0Hkeff = −0.9 T) shows τ = 0.32 ns. The increase of the damping constant (α) results in the increase of τ for both devices and the sub-nanosecond switching could be retained as α < 0.14 in the device with μ0Hkeff = −0.08 T, while this was achieved as α < 0.04 in the device with μ0Hkeff = −0.9 T. Furthermore when the field-like coefficient (β) is increased, it leads to a decrease in τ, which can be reduced to 0.03 ns by increasing β to 1 in the device with μ0Hkeff = −0.08 T. In order to achieve the same result in the device with μ0Hkeff = −0.9 T, β must be increased to 6. These results indicate a way to achieve ultrafast field-free SOT switching of a few tens of picoseconds in nanometer-sized magnetic tunnel junction (MTJ) devices.

All-Spin Artificial Neural Network Based on Spin–Orbit Torque-Induced Magnetization Switching

All-Spin Artificial Neural Network Based on Spin–Orbit Torque-Induced Magnetization Switching

Spin–orbit torque induced magnetization switching in the W/CoFeB/Zr/MgO multilayers with high thermal stability

We demonstrate the spin–orbit torque (SOT) induced perpendicular magnetization switching in an annealed W/CoFeB/Zr/MgO multilayer with high thermal stability. It is found that the thermal stability factor can reach 79 after annealing at 540 °C. With an increase in the annealing temperature, the absolute damping-like efficiency almost keeps a high constant value (about 0.3). The tungsten in the W/CoFeB/Zr/MgO multilayer could convert from the high resistive β-W to a mediate resistive amorphous-like structure. Therefore, the absolute spin Hall conductance increases from 765 of β-W to 1420 (ℏ/e)(Ω cm)−1 of the amorphous-like tungsten. These results pave a realistic way for the practical application of tungsten in the SOT-based spintronics devices with high thermal stability and SOT efficiency.

Field-Free Spin-Orbit Torque-Induced Magnetization Switching in the IrMn/CoTb Bilayers with a Spontaneous In-Plane Exchange Bias.

Spin-orbit torque (SOT)-induced magnetization switching in ferrimagnetic materials is promising for application in a new generation of information storage devices. Here, we demonstrate SOT-induced field-free magnetization switching of the perpendicularly magnetized CoTb ferrimagnet layer in the IrMn/CoTb bilayer, in which the in-plane magnetic inversion symmetry is broken by a spontaneous in-plane exchange bias (IEB) established by isothermal crystallization of the IrMn layer. We obtain a significant SOT effective field acting on the CoTb layer and a large effective spin Hall angle in this system, derived by the second harmonic voltage measurement method. Moreover, the IrMn/CoTb bilayer demonstrates multistate magnetic switching behavior with different amplitudes of current pulses at zero field, which can mimic the synaptic weight updates in the neuromorphic network. These findings make the IrMn/CoTb bilayer with spontaneous IEB a promising candidate for potential applications in multilevel storage and neuromorphic computing.

Implementation of a full Wheatstone-bridge GMR sensor by utilizing spin–orbit torque induced magnetization switching in synthetic antiferromagnetic layer

A giant magnetoresistance (GMR) sensor with a Wheatstone bridge structure and an out-of-plane linear response was developed. The spin-valve structure consists of a synthetic antiferromagnetic [(Co/Pt)n/Ru/(Pt/Co)n] reference layer with perpendicular magnetic anisotropy, a Cu spacer layer, and a Co-free layer with in-plane easy magnetization. By utilizing the spin–orbit torque induced magnetization switching in the synthetic antiferromagnetic layer, the magnetization of the reference layers in the adjacent bridge arms is set to the opposite direction, achieving a GMR sensor with a full Wheatstone bridge structure. The sensor exhibits linear response to the out-of-plane magnetic field with adjustable dynamic ranges from hundreds to thousands of Oe, depending on the thickness of the Co-free layer. A similar Wheatstone bridge sensor consisting of magnetic tunnel junctions was also proposed. The sensor with out-of-plane linear response may have promising applications in three-dimensional magnetic field detection and current sensing field.

Inversion symmetry breaking in spin–orbit torque-induced magnetization switching to improve the recording density of multi-level magnetoresistive random-access memory

In this study, we prepared a multi-layer Tb–Fe/Pt/Tb–Fe wire to develop a multi-level magnetic memory. By applying current, magnetizations of the Tb–Fe layers were inversion symmetrically switched by spin– orbit torque (SOT) generated from the middle Pt layer. Measurements of SOT showed that its efficiency had opposite polarities in the top and bottom Tb–Fe layers. The switching current density of the top and bottom Tb–Fe layers shifted in opposite directions under a uniform perpendicular magnetic field. Because the perpendicular magnetic field broke the inversion symmetry of SOT generated from the middle Pt layer, it could be used to control the switching current. Our results prove that the additional uniform and perpendicular magnetic field can enhance the controllability of the magnetization state in case of multi-level SOT-induced magnetization switching.

Role of the chiral spin configuration in field-free spin–orbit torque-induced magnetization switching by a locally injected spin current

For deterministic magnetization switching by spin–orbit torque (SOT) in a perpendicular magnetic anisotropy system, an additional in-plane direction magnetic field is essential to break the lateral symmetry. Realizing chirality in a magnetic ordering system can be one approach for achieving asymmetry in the lateral direction for field-free magnetization switching. However, systematic analysis of the influence of the chiral spin system on deterministic switching is still scarce. We investigate the field-free SOT-induced magnetization switching by using a chiral spin configuration experimentally and theoretically with micromagnetic simulations. We designed a system in which only part of the ferromagnetic layer overlaps with the heavy metal layer in the Pt/Co/MgO structure. Therefore, a spin current exerts only on a local area of the ferromagnetic layer, which results in a Néel-type chiral spin configuration. The induced chiral spin configuration can be stabilized (or destabilized) depending on the sign of the interfacial Dzyaloshinskii–Moriya interaction and the direction of the current. The stabilized spin configuration plays a crucial role in the deterministic switching in the zero field. We expect our findings to widen the perspective on chirality-based all-electrical SOT device applications.

Antiferromagnet-mediated spin–orbit torque induced magnetization switching in perpendicularly magnetized L1-MnGa

Current-induced magnetization switching plays an essential role in spintronic devices exhibiting nonvolatility, high-speed processing, and low-power consumption. Here, we report on the spin–orbit torque-induced magnetization switching in perpendicularly magnetized L10-MnGa/FeMn/Pt trilayers grown by molecular-beam epitaxy. An antiferromagnetic FeMn layer is inserted between the spin current generating Pt layer and spin absorbing MnGa layer. Due to the exchange bias effect, the trilayers show field-free spin–orbit torque switching. Overall, the spin transmission efficiency decreases monotonically as the FeMn thickness increases. It is found that the spin current can be transmitted through an 8 nm-thick FeMn layer as evidenced by partial switching of the L10-MnGa. The damping-like spin–orbit torque efficiency shows a peak value at tFeMn = 1.5 nm due to the enhanced interfacial spin transparency and crystalline quality of the FeMn. These results help demonstrate the efficacy of emerging spintronic devices containing antiferromagnetic elements.

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.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Spin-orbit torque induced magnetization switching in ferrimagnetic Heusler alloy D22-Mn3Ga with large perpendicular magnetic anisotropy

Magnetization switching induced by spin–orbit torque is of fundamental interest for developing spintronic devices with low-power consumption and nonvolatility. Here, we report on the spin–orbit torque induced magnetization switching behavior of (001) oriented tetragonal Heusler alloy D022-Mn3Ga films with an intrinsic ferrimagnetic spin structure grown on the GaAs(001) substrate by molecular-beam epitaxy. The out-of-plane hysteresis loop and anomalous Hall effect demonstrated a large perpendicular magnetic anisotropy and low saturation magnetization of D022-Mn3Ga thin films. The spin–orbit torque induced magnetization switching has been realized in D022-Mn3Ga/Pt heterostructure based Hall devices under an in-plane external field. It is found that the critical switching current density Jc is much smaller than that of the L10-MnGa/heavy metal system. Besides, both a dampinglike effective field HDL and a fieldlike effective field HFL are quantified by performing harmonic Hall voltage measurements. All these results indicate that ferrimagnetic D022-Mn3Ga can be a promising candidate material for realizing high-density and energy-efficient spintronic devices.

Lowering critical current density for spin-orbit torque induced magnetization switching by ion irradiation

In ferromagnet/heavy metal heterostructures, critical current density (JC) refers to the minimum current density required to generate spin–orbit torque (SOT) for effective magnetization manipulation, including switching of magnetization and moving of domain walls and magnetic skyrmions. This critical current density is a key factor for next-generation SOT-based magnetic random access memory, racetrack memory, and logic devices. In this work, the critical current density for magnetization switching and the thermal stability of Pt/Co/Ta heterostructures in response to ion irradiation are studied. It is found that ion irradiation represents a promising approach for the wide tuning of the magnetic properties, such as the coercive force and the perpendicular magnetic anisotropy constant. It is also found that JC is significantly reduced after 500 eV Ar+ irradiation. Meanwhile, the ratio between the thermal stability factor E/kBT and JC increases with an increase in the irradiation dose, although E/kBT decreases slightly with the irradiation dose. This work demonstrates that JC can be significantly reduced by an appropriate ion irradiation process and thereby demonstrates a promising approach for effective reduction of the power consumption in SOT-based spintronic devices.

Modulation of spin-orbit torque induced magnetization switching in Pt/CoFe through oxide interlayers

We investigate the spin–orbit torque (SOT) induced magnetization switching in the Pt/CoFe structure via inserting NiO or MgO interlayers. The effective spin Hall angles are enhanced by inserting a NiO layer and decreased by a MgO interlayer, through the harmonic Hall measurement. Both the MgO and NiO interlayers decrease the critical switching current and the minimum in-plane field required for completely switching, which is decreased down to about 50 Oe for a sample with MgO and 25 Oe for a sample with NiO. This originates from the reduction of the Dzyaloshinskii-Moriya interaction by inserting the oxide interlayers, which is confirmed by the Kerr images for the switching process. We confirm that the oxide interlayer is an efficient way to modify the SOT-induced magnetization switching and reveal the possibility of the SOT-induced switching for double MgO-based perpendicular magnetic tunneling junctions.

Spin-orbit torque from a ferromagnetic metal

The switching of magnetization by current-induced spin-orbit torque (SOT) has potential applications for energy-efficient spintronic devices. In the past, most conventional works have been focused on SOT in heavy metals. Here the SOT from a ferromagnetic metal is investigated, and two mechanisms of the field-free SOT induced magnetization switching are demonstrated to be from the interlayer exchange coupling and the tilted perpendicular magnetic anisotropy. We exclude the spin torque contribution from the anomalous Hall effect and the interfacial Rashba effect combined with spin precession. A spin Hall angle ${\ensuremath{\theta}}_{\mathrm{SH}}=\ensuremath{-}0.022$ of CoFeB is obtained by the current-induced hysteresis loop shift method, and the obtained ${\ensuremath{\theta}}_{\mathrm{SH}}$ is comparable with heavy metals. This work demonstrates that a considerable SOT can come from a ferromagnetic metal, and indicates the unconventional origin of spin-orbit coupling.

Spin-orbit torque-induced magnetization switching in epitaxial Au/Fe4N bilayer films

Au/Fe4N bilayer films have been grown by the plasma-assisted molecular beam epitaxy system. After an extraordinarily small charge current is applied to the samples, magnetization of the Fe4N layer was reversed by spin-orbit torque. Analyses indicate that the magnetization reversal is realized via domain wall motion and it was confirmed by magnetic force microscopy measurements. By comparing the transport properties of the Au/Fe4N bilayer film with those of control samples before and after using a pulse current to stimulate the films, contributions of the thermal effect and spin transfer torque induced by current that flows in the Fe4N layer to the magnetization switching were analyzed and determined to be negligible. Kerr signals were observed simultaneously with applying a charge current to the samples at zero magnetic field, which could be explained by the spin Hall effect of the Au layer.

Observation of spin–orbit torque-induced magnetization switching in Gd-Fe perpendicular magnetized wire with in-plane exchange bias field

We fabricated a Ta/Gd-Fe/Ta and a Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire and investigated spin–orbit torque current-induced magnetization switching in these wires. Magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire can be observed by an electric current even if the external in-plane magnetic field is not applied at all. Moreover, we successfully observed the periodical magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire in zero magnetic field. This indicates that the present wire is a promising material to realize magnetic random access memory with low power consumption.

Modulated spin orbit torque via controlling the oxidation level of Ti in Pt/Co/TiOx heterostructures

We have investigated the oxidation effect on the spin orbit torque (SOT) in Pt/Co/TiOx heterostructures through controlling the oxidation time of Ti. The SOT induced magnetization switching in Pt/Co/Ti heterostructures is weak. However, it becomes evident as the oxidation of the Ti layer goes above a threshold level, and the magnetization can be completely switched when the oxidation time of the Ti layer is 11 min. With excessive oxidation, the SOT in the Pt/Co/TiOx films finally disappears. The average valence of the Ti was confirmed by X-ray photoelectron spectroscopy (XPS) spectra, which is about +3.619 for 11 min oxidation and the Ti2O3 will enhance the SOT efficiency. It is proposed that the oxidation will modulate the interfacial Rashba effect, the film qualities and the spin transparency at the interfaces, which will collectively dominate the SOT results.

Time and spatial evolution of spin–orbit torque-induced magnetization switching in W/CoFeB/MgO structures with various sizes

We study spin–orbit torque (SOT) switching in W/CoFeB/MgO structures with various dot sizes (120–3500 nm) using pulsed current of various widths τ (800 ps–100 ms) to examine the time and spatial evolution of magnetization switching. We show that the switching behavior and the resultant threshold switching current density Jth strongly depend on device size and pulse width. The switching mode in a 3500 nm dot device changes from probabilistic switching to reproducible partial switching as τ decreases. At τ = 800 ps, Jth becomes more than 3 times larger than that in the long-pulse regime. A decrease in dot size to 700 nm does not significantly change the switching characteristics, suggesting that domain-wall propagation among the nucleated multiple domains governs switching. In contrast, devices with further reduced size (120 nm) show normal full switching with increasing probability with current and insignificant dependence of Jth on τ, indicating that nucleation governs switching.

Stack Structure Dependence of Magnetic Properties of PtMn/[Co/Ni] Films for Spin-Orbit Torque Switching Device

We investigate the stack structure dependence of magnetic properties on thin films that consist of an antiferromagnetic PtMn and a ferromagnetic Co/Ni multilayer for field-free spin-orbit torque-induced magnetization switching devices. Magnetic parameters, such as the spontaneous magnetization, effective and interfacial magnetic anisotropies, and exchange bias field are quantified as a function of stack structure. Engineering of the stack allows the improvement of current-induced magnetization switching characteristics compared with a previous work, which is confirmed using patterned Hall cross devices.