Current-induced Magnetization Reversal Research Articles

Interfacial engineering to manipulate the effective spin Hall angle of perpendicularly magnetized systems

The spin current can be generated by passing an electric current through a heavy metal. The spin current generation depends on the spin Hall angle (θsh) of the material. To manipulate the effective θsh, a thin layer of Au has been introduced at the bottom Pt/Co and the top Co/Pt interfaces in Ta/Pt/Co/Pt based perpendicularly magnetic anisotropy systems, and current-induced magnetization reversal of Co has been studied to estimate Jc. The introduction of the Au layer at the top Co/Pt interface (Ta/Pt/Co/Au/Pt) did not produce any significant reduction in the Jc. However, a significant reduction of Jc (∼34%) has been observed, while the Au layer has been deposited at the bottom Pt/Co interface (Ta/Pt/Au/Co/Pt), indicating an enhancement in the value of θsh. We also performed a micromagnetic simulation to understand the qualitative change of the θsh. Micromagnetic simulation suggested that the θsh becomes 0.07 in Ta/Pt/Au/Co/Pt multilayer compared to θsh=0.05 of the Ta/Pt/Co/Pt system. Pt/Co/Au/Co/Au exhibits a reduction in Jc up to ∼30% and corresponds to θsh=0.09. A Ta capping layer has been introduced to inject more spin current into the Co layer since Pt and Ta have opposite spin Hall angles. The Jc lowers up to ∼58% in Ta/Pt/Au/Co/Pt/Ta multilayer, corresponding to θsh=0.23. We also achieved field-free switching at Jc=1.55×1011 by depositing an in-plane magnetized layer of Co in Ta/Pt/Au/Co/Pt/Ta/Co/Pt multilayer.

Field-Free Spin-Orbit Torque Switching of Co/Pd Memory Layer in CPP-GMR With Perpendicularly Magnetized SAF Pinned Layer

For the development of high-capacity spin-orbit torque magnetic memory, a giant magneto-resistive (GMR) device with perpendicularly magnetized Co/Pd memory layer was fabricated, and current-induced magnetization reversal of the memory layer was investigated. A synthetic antiferromagnet (SAF) reference layer consisting of [Co/Pt]/Ru/[Co/Pt] multilayer was used for the GMR device to reduce the stray field in the pillar-shaped Co/Pd memory layer. Spin-transfer torque (STT) magnetization reversal of the memory layer as well as the field-assisted spin-orbit torque (SOT) magnetization reversal of the memory layer was confirmed, by applying a current pulse through the GMR or through the bottom electrode, respectively. Furthermore, by injecting SOT and STT simultaneously, deterministic SOT switching of the perpendicularly magnetized Co/Pd memory layer was achieved in the absence of an in-plane assisting magnetic field to break the symmetry. We confirmed that STT assisting current significantly reduced the SOT switching current. Moreover, the assist efficiency defined by Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> / Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">STT</sub> depended on the magnitude of in-plane magnetic field. Higher assist efficiency was obtained at lower in-plane magnetic field.

Large damping-like torque and efficient current-induced magnetization reversal in Ti/Tb–Co/Cr structures

Seeking the magnetic heterostructures with large current-induced torque efficiency is currently one of the core hotspots in spintronics. In this work, we report the large and composition-dependent damping-like (DL) torque in the structure consisting of light metal Cr and Ti layers and a ferrimagnetic Tb–Co layer. The DL torque efficiency in the structures reaches a maximum (around −0.55) as the Tb content in the Tb–Co layer is in the range of 0.15–0.18. This composition-dependent behavior is different from that observed in the usual heavy-metal/ferrimagnetic structures. We also demonstrated the efficient current-induced magnetization reversal in these structures with a low threshold current density down to 8×1010 A/m2. In comparison, only very low efficiency values of −0.06 and −0.086 are obtained in the Ti/Co/Cr and Ti/Tb–Co/SiN control samples, respectively, suggesting that the large DL torque in the Ti/Tb–Co/Cr structures may originate from the orbital Hall effect in the Cr metal. By fitting the Cr layer thickness dependence of the torque efficiency with a simple orbital current diffusion model, we obtained an effective orbital Hall angle of −0.57±0.02 for the Ti/Tb0.85Co0.15/Cr samples. This work demonstrated the possibility to enhance the orbital torque effect by using the magnetic layer containing the element with strong spin–orbit coupling.

Deterministic and stochastic aspects of current-induced magnetization reversal in perpendicular nanomagnets

Deterministic and stochastic aspects of current-induced magnetization reversal in perpendicular nanomagnets

Current-induced magnetization reversal in (Ga,Mn)(Bi,As) epitaxial layer with perpendicular magnetic anisotropy

Pulsed current-induced magnetization reversal is investigated in the layer of (Ga,Mn)(Bi,As) dilute ferromagnetic semiconductor (DFS) epitaxially grown under tensile misfit strain causing perpendicular magnetic anisotropy in the layer. The magnetization reversal, recorded through measurements of the anomalous Hall effect, appearing under assistance of a static magnetic field parallel to the current, is interpreted in terms of the spin–orbit torque mechanism. Our results demonstrate that an addition of a small fraction of heavy Bi atoms, substituting As atoms in the prototype DFS (Ga,Mn)As and increasing the strength of spin–orbit coupling in the DFS valence band, significantly enhances the efficiency of current-induced magnetization reversal, thus reducing considerably the threshold current density necessary for the reversal. Our findings are of technological importance for applications to spin–orbit torque-driven nonvolatile memory and logic elements.

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density (> 106 A/cm2) is one of major obstacles to realize practical applications of the current-driven magnetization reversal devices. In this work, we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack. Under the assistance of an out-of-plane magnetic field, the magnetization can be fully reversed with a current density of about 104 A/cm2. The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque. It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque, and the current-driven magnetization reversal will be more efficient in low-temperature range, while the magnetic field is helpful for the magnetization reversal in high-temperature range.

Spin-orbit torque assisted magnetization reversal of 100 nm-long vertical pillar

Long vertical pillars, with a width of the order of nanometers and with perpendicular shape anisotropy (PSA), have high thermal stability. The advantage of using longer pillars is that they can increase the memory areal density while maintaining robust thermal stability. The current-induced magnetization reversal of long pillars is a significant challenge in spintronic applications such as high-density magnetic memories. However, the magnetization of pillars that are more than 100 nm long has never been reversed by spin-orbit torque (SOT) or spin injection from another ferromagnet (FM). Against this background, we propose a novel magnetization reversal method for pillars based on both SOT and spin transfer torque without using a FM for spin injection. Furthermore, this SOT-assisted method significantly reduces the reversal time, as was demonstrated by micromagnetic simulation. Using a spin-polarized current and SOT, the magnetization was reversed in pillars with length ⩾100 nm. The magnetization of pillars with PSA and those with both high perpendicular magnetic anisotropy and PSA was successfully reversed. The findings of this study are physically novel and significant for practical applications. Consequently, the proposed new writing scheme paves the way for next-generation spintronic devices.

Enhancing the spin–orbit torque efficiency in Pt/CoFeB/Pt based perpendicularly magnetized system

In the recent past, current-induced magnetization reversal (CIMR) has been extensively studied on the heterostructures prepared with heavy metal (HM) and ferromagnet (FM) having spin–orbit torque (SOT). The critical current density (Jc) for magnetization switching depends on the SOT efficiency. In this study, we have chosen Ta/Pt/CoFeB/Pt/Ta/CoFeB/Pt heterostructure having perpendicular magnetic anisotropy (PMA) with broken lateral symmetry and an in-plane magnetized (IMA) ferromagnetic layer on top of the Ta layer. In this heterostructure, we have utilized the SOT originated by the spin-Hall effect that arises from two materials having opposite spin Hall angles i.e. the bottom Pt and the top Ta layer. We have also introduced lateral space inversion asymmetry (SIA) that combines both damping-like and field-like field. The enhanced spin Hall angle, the stray field by IMA ferromagnetic layer, and the lateral SIA effect jointly enhances the SOT efficiency of the system to reduce the Jc for CIMR. We have found that the SOT efficiency and the effective spin Hall angle of our newly designed heterostructure to be 106 Oe/1011A/m2 and 0.23, respectively. Thus we have achieved CIMR with a lower critical current density of 8.65×1010 A/m2 and 3.35×1010 A/m2 at 0 Oe and 600 Oe field applied in-plane (IP), respectively. Due to the lower Jc for CIMR our newly designed heterostructure has potential in low-power magnetic data storage applications.

Highly efficient spin-orbit torque in a perpendicular synthetic ferrimagnet

Perpendicular synthetic ferrimagnet (pSFi) due to the low stray field and net magnetization is expected to replace single ferromagnet as a free layer to optimize the memory devices for high storage density and low-energy consumption. Here, we investigate the dependence of exchange-coupling strength on the thickness of the spacer Ru and current-induced magnetization reversal due to spin-orbit torque in $\mathrm{Ta}/\mathrm{Pt}/{[\mathrm{Pt}/\mathrm{Co}]}_{2}/\mathrm{Ru}/{[\mathrm{Co}/\mathrm{Pt}]}_{4}/\mathrm{Pt}$ structure with a perpendicular magnetic anisotropy. An oscillating interlayer exchange coupling as a function of Ru spacer layer thickness with a period of 1.16 nm is determined by combining the anomalous Hall effect and the polar magnetic-optical Kerr effect. Furthermore, current-controllable magnetization reversal experiments reveal that the magnetizations of top and bottom layers with antiferromagnetic coupling switch simultaneously due to the combination of spin-orbit torque generated from the two adjacent heavy-metal layers and the interlayer exchange torque. The SOT efficiency $\ensuremath{\chi}\ensuremath{\sim}57.52\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}/({10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2})$, corresponding to an effective spin Hall angle $\phantom{\rule{4pt}{0ex}}{\ensuremath{\xi}}_{DL}\ensuremath{\sim}0.68$, is estimated by analyzing current-dependent anomalous Hall resistance with our proposed quasistatic balance equation of magnetic moments. In addition to the advantage of minimizing stray field acting on the storage layer, the observed low switching current density and high spin-orbit torque efficiency suggest that the pSFi structure with a high thermal stability factor has great potential to realize the high-density and low-power consumption of nonvolatile magnetic memory devices.

Asymmetric magnetic domain wall motion under lateral and normal space inversion asymmetry with interfacial Dzyaloshinskii–Moriya interaction

The magnetic anisotropy of a typical magnetic sample with an easy axis perpendicular to the surface has many advantages over an in-plane easy axis due to advantages like better thermal stability, higher data storage density, lower energy consumption, etc. The perpendicular magnetic anisotropy arises due to the spatial inversion asymmetry (SIA) at the interfaces between a ferromagnet (FM) and a heavy metal (HM). In such systems, one can achieve a current-induced magnetization reversal (CIMR) via spin–orbit torque with the assistance of an in-plane (IP) magnetic field. This IP field requirement for the CIMR process is an obstacle for the practical application. Recent studies on CIMR at magnetic fields free condition are achieved by breaking the lateral structural symmetry by achieving thickness-gradient in a particular layer. In previous reports, the SIA normal to the surface in a heterostructure leads to an asymmetric contribution as chiral damping. In this work, a system Ta/Pt/CoFeB/Pt with normal and lateral SIA has been prepared by introducing a thickness-gradient in the FM CoFeB layer, which leads to create a tilted perpendicular magnetic anisotropy. The presence of the lateral SIA gives rise to another asymmetric component besides the chiral damping component. In this study, the total asymmetric component of the Ta/Pt/CoFeB/Pt heterostructure has been calculated into two contributions Ht and Hch, to extract the distinct information of the tilt asymmetry due to the lateral SIA and the chiral damping due to the normal SIA, respectively. In this study, by varying the bottom Pt thickness of the Ta/Pt/CoFeB/Pt heterostructures we have observed that the two asymmetry contributions Ht and Hch both decreased with increasing bottom Pt thickness.

Experimental observation and micromagnetic understanding of the current-induced magnetization reversal process in Ta/Pt/[Co/Ni] n Co/Ta multilayers

We investigate a current-induced magnetization reversal process mediated by spin–orbit torque (SOT) in Ta/Pt/(Co/Ni) n Co/Ta multilayers experimentally and by micromagnetic simulation. Experimental results show that the current-induced magnetization reversal in these samples is completed by nucleation and subsequent domain wall (DW) propagation. For samples with a small number (n) of Co/Ni bi-layers, which show strong Dzyaloshinskii–Moriya interaction (DMI), when the in-plane magnetic field (Hx ) is weaker than a critical value, the tilted DWs form in the final stabilized state, corresponding to a partial magnetization reversal. Since Hx is close to the DMI effective field, the domains expand asymmetrically to semicircular areas, finally resulting in complete magnetization reversal. For the sample with large n and weak DMI, the reversed domains grow obliquely along a special orientation, which varies with the magnitude of Hx . These experimental phenomena are well reproduced by micromagnetic simulation after taking into account the SOT effect and the DMI-modulated magnetic moment distribution in the DW. The agreement betweenexperimental observation and micromagnetic simulation suggests that the current-induced magnetization reversal process in the Ta/Pt/[Co/Ni] n Co/Ta system can be well understood within the framework of SOT and the DMI scenario.

Quantitative comparison of electrically induced spin and orbital polarizations in heavy-metal/ 3d -metal bilayers

Electrical control of magnetization is of crucial importance for integrated spintronics devices. Spin-orbit torques (SOT) in heavy-metal/ferromagnetic heterostructures have emerged as a promising tool to achieve efficiently current-induced magnetization reversal. However, the microscopic origin of the SOT is being debated, with the spin Hall effect (SHE) due to nonlocal spin currents and the spin Rashba-Edelstein effect (SREE) due to local spin polarization at the interface being the primary candidates. We investigate the electrically induced out-of-equilibrium spin and orbital polarizations in pure Pt films and in Pt/$3d$-metal (Co, Ni, Cu) bilayer films using ab initio electronic structure methods and linear-response theory. We compute atom-resolved response quantities that allow us to identify the induced spin-polarization contributions that lead to fieldlike (FL) SOTs, mostly associated with the SREE, and dampinglike (DL) SOTs, mostly associated with the SHE, and compare their relative magnitude, dependence on the magnetization direction, as well as their Pt-layer thickness dependence. We find that both the FL and DL components contribute to the resulting SOT at the Pt/Co and Pt/Ni interfaces, with the former contributions being larger at the Pt interface layer and the latter larger in the Co or Ni layers. Our calculations show that the electrically induced transverse orbital polarization is exceedingly larger than the induced spin polarization and present even without spin-orbit coupling, in contrast to the spin polarization.

Field-free spin–orbit torque driven multi-state reversal in wedged Ta/MgO/CoFeB/MgO heterostructures

We report a current-induced four-state magnetization reversal under zero magnetic field in a wedged Ta/MgO/CoFeB/MgO heterostructure with a perpendicular magnetic anisotropy. Anomalous Hall effect and magneto-optical Kerr effect microscopy measurements were performed to demonstrate that the field-free multi-level reversal is jointly determined by the spin–orbit torque effective field that originates from the lack of the lateral inversion symmetry in the wedged stacking structure and the current-induced Oersted field. Moreover, the creation of robust intermediate Hall resistance states in the multi-state switching strongly depends on the current-induced Joule heating. Our results provide a route for the field-free multi-level state reversal, which is significant for fabricating the non-volatile and energy-efficient multi-level memories or artificial neuron devices.

Sub-ns Field-Free Switching in Perpendicular Magnetic Tunnel Junctions by the Interplay of Spin Transfer and Orbit Torques

The interplay of spin-transfer torque (STT) and spin-orbit torque (SOT) highlights the potential of current-induced magnetization reversal for the ultrahigh-speed and ultralow-power memory applications. However, the ultrafast field-free switching and its dynamic mechanism have not been clarified yet. Here, we experimentally demonstrate the sub-nanosecond field-free switching by the interplay of STT and SOT in perpendicular magnetic tunnel junctions (MTJs). The dynamic investigations of the STT and SOT switching reveal that the applied SOT current can highly decrease the incubation time and current density of STT, which leads to the achievement of ultra-fast switching. Besides, the timing scheme investigations of SOT and STT pulses are performed. The firstly applied SOT current further decreases the transition time of the magnetization switching, which could result from the different nucleation and motions of domains. The interplay and the timing operations of STT and SOT provide more flexible solutions for high-speed, large-scalability and energy efficient memory applications.

A comparative first principles study of quantum well states in MgO barrier MTJs for STT-RAMs

This paper presents a comparative first principle study of tunneling magneto resistance (TMR), spin transfer torque (STT) and resistance area (RA) product of magnetic tunnel junctions (MTJ) having different quantum well states in middle of MgO barrier region. We report that such MTJs show enhanced STT and low RA product as compared to only MgO barrier MTJ with maintaining high TMR. Electronic structure of quantum well states adjudicates magnitude of TMR in MTJs. It is found that quantum well states having s-like character allow the transmission of Δ1 symmetry of Bloch wave whereas d-like character supports to the transmission of Δ2, Δ2′, and Δ5 symmetry. This increases transmission of spin polarized tunneling current with good spin filtering injection efficiency resulting in higher STT. It will produce less heat on applying critical switching current density (Jco), for current induced magnetization reversal by STT phenomenon without breakdown of the barrier.

Advanced Method for the Reliable Estimation of Spin-Orbit-Torque Efficiency in Low-Coercivity Ferromagnetic Multilayers

Proper estimation of the efficiency of spin-orbit torque (SOT) in ferromagnetic multilayers with small coercive force is not trivial, yet is in high demand for implementing current-induced magnetization reversal in memory cells. This study proposes a method for estimating the effective field of SOT based on analyzing the position of a domain wall displaced parallel to the current, combined with the spatial distribution of the Oersted field. This advanced method is more reliable than the extensively used second-harmonic and current-induced-magnetization-curve techniques, for a wide spectrum of materials, including Pt-based systems.

Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field

The field-and current-induced magnetization reversal of a Co-Fe-B layer in a perpendicular magnetic tunnel junction (pMTJ) is studied at room temperature. The magnetization switching probability from the parallel (P) state to the antiparallel (AP) state and from AP to P is found to be asymmetric. We observe that this asymmetry depends on the magnetic configuration of the synthetic antiferromagnetic (SAF). The state diagram and the energy landscape are compared for two SAF configurations. We conclude that the asymmetry is due to the inhomogeneity of the fringe field generated by the SAF. Our study highlights the role of the reference-layer fringe field on the free layer's energy barrier height, which has to be carefully controlled for memory applications.

Efficient current-induced magnetization reversal by spin-orbit torque in Pt/Co/Pt

Current-induced magnetization reversal due to spin-orbit torque is demonstrated in an anisotropy controlled Pt/Co/Pt trilayer. The samples were designed to have weak perpendicular magnetic anisotropy, with a measured anisotropy field of (1340±20) Oe. Reversal is shown to be dominated by a damping-like torque associated with the spin-Hall effect. A small in-plane magnetic field was required to break the symmetry and enable reversal. With a 273 Oe field, magnetization reversal occurred with a current density amplitude of only 5×1010 A m−2, which is shown to be consistent with a simple model. The field-like torque is negligible, so measurements indicate that the imaginary part of the spin-mixing conductance associated with Co/Pt interfaces must be negligible.

Effect of Tilted Magnetic Anisotropy on the Deterministic Current-Induced Magnetization Reversal in Quasi-Perpendicularly Magnetized Ta/Pt/CoFeB/Pt Multilayers

Deterministic current-induced magnetization reversal is observed in quasi-perpendicularly magnetized Ta/Pt/CoFeB/Pt multilayer thin films without the assistance of an in-plane field. In a quasi-perpendicularly magnetized system, the anisotropy is tilted slightly away from the normal to the film plane. This tilt in the anisotropy is obtained by growing the films in an oblique-angle sputter deposition technique, which results in a gradient in the thickness of the layers. To estimate the tilt, out-of-plane hysteresis loop measurements are performed in the presence of an in-plane bias field. The effect of the tilt is reflected in the shift in the hysteresis loop. A measurement of this shift at different relative azimuthal orientations of the in-plane projection of the tilt direction with respect to the in-plane bias field direction gives a tilt of 0.74° (±0.06°) in Ta(3 nm)Pt(3 nm)CoFeB(0.5 nm)Pt(1 nm) with all the layers having a thickness gradient. In addition, field-induced domain wall velocity measurements in the creep regime on Ta(3 nm)Pt(3 nm)CoFeB(0.5 nm)Pt(1 nm) thin film with a thickness gradient only in CoFeB layer give an elliptical profile for the domain wall. In the current-induced magnetization reversal studies, threshold current density of $1.06 \times 10^{11}$ Am−2 is required to deterministically switch the device fabricated from Ta(3 nm)Pt(3 nm)CoFeB(0.5 nm)Pt(1 nm) thin film with all the layers having a thickness gradient. On application of in-plane bias, there is a shift in the threshold current densities given by $2.6 \times 10^{8}$ Am−2 of in-plane bias field.

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.