Current-induced Magnetization Switching Research Articles

Fabrication of highly oriented Mn4N/Pt epitaxial bilayer structure on MgO(001) for spintronics applications

Magnetization control by electric current or spin current is an attractive technology with potential applications in memory and logic devices, and Mn4N has attracted much attention in recent years as a candidate material for current-induced magnetization switching and magnetic domain wall motion, as well as a skyrmion material. For these applications, a bilayer structure of Mn4N and heavy metals must be formed with high crystalline quality. In this study, we have achieved the epitaxial growth of Mn4N thin films on MgO(001) with a Pt underlayer. The reflection high-energy electron diffraction images and x-ray diffraction ω-rocking curves show that the Mn4N films fabricated on the Pt underlayers exhibit higher crystalline quality than the Mn4N films directly formed on the MgO. Furthermore, anomalous Hall effect loops with bump-like structures were observed. This is considered to be due to the topological spin texture. The Mn4N/Pt epitaxial bilayer structure has potential for use in spin–orbit torque applications and is a candidate configuration for topological spin textures.

Nonuniform Current-Driven Formation and Displacement of the Magnetic Compensation Point in Variable-Width Nanoscale Ferrimagnets.

Nano- and microstructures based on ferrimagnets can demonstrate high efficiency and dynamics of current-induced magnetization switching combined with high stability of spin textures such as bubble domains and skyrmions, which are of practical importance for the development of spintronics and spin-orbitronics. This set of features is usually associated with magnetic momentum or angular momentum compensation states. Here, we experimentally show that the compensation state can be realized locally using nonuniform Joule heating. This effect is observed in the variable-width current guide made of the ferrimagnetic W/Co76Tb24/Ru thin films, where the position of a region heated to the compensation temperature depends linearly on the current pulse amplitude. This approach makes it possible to observe the simultaneous coexistence of Co-dominant and Tb-dominant regions, where current pulses induce spin-orbit torques in opposite directions, leading to local magnetization switching. It is found that the position of a Néel domain wall constraining the switched region lies in the vicinity of the coordinate corresponding to the compensation point but does not coincide with it due to high mobility under the action of spin current. Our findings open an alternative approach for engineering of ferrimagnetic nanodevices with advanced properties for future applications in spintronics, spin-orbitronics, and nanoelectronics.

Ultra-thin lithium aluminate spinel ferrite films with perpendicular magnetic anisotropy and low damping

Ultra-thin films of low damping ferromagnetic insulators with perpendicular magnetic anisotropy have been identified as critical to advancing spin-based electronics by significantly reducing the threshold for current-induced magnetization switching while enabling new types of hybrid structures or devices. Here, we have developed a new class of ultra-thin spinel structure Li0.5Al1.0Fe1.5O4 (LAFO) films on MgGa2O4 (MGO) substrates with: 1) perpendicular magnetic anisotropy; 2) low magnetic damping and 3) the absence of degraded or magnetic dead layers. These films have been integrated with epitaxial Pt spin source layers to demonstrate record low magnetization switching currents and high spin-orbit torque efficiencies. These LAFO films on MGO thus combine all of the desirable properties of ferromagnetic insulators with perpendicular magnetic anisotropy, opening new possibilities for spin based electronics.

Spin–orbit torque in perpendicularly magnetized [Pt/Ni] multilayers

The performance of spin–orbit torque (SOT) in heavy metal/ferromagnetic metal periodic multilayers has attracted widespread attention. In this paper, we have successfully fabricated a series of perpendicular magnetized [Pt(2–t)/Ni(t)]4 multilayers, and studied the SOT in the multilayers by varying the thickness of Ni layer t. The current induced magnetization switching was achieved with a critical current density of 1 × 107 A/cm2. The damping-like SOT efficiency ξ DL was extracted from an extended harmonic Hall measurement. We demonstrated that the ξ DL can be effectively modulated by t Pt/t Ni ratio of Pt and Ni in the multilayers. The SOT investigation about the [Pt/Ni] N multilayers might provide new material candidates for practical perpendicular SOT-MRAM devices.

Giant Orbital-to-Spin Conversion for Efficient Current-Induced Magnetization Switching of Ferrimagnetic Insulator.

It has long been believed that the attachment of two heavy metals such as Ta and Pt with opposite spin Hall angles results in a weakened net torque generation efficiency in magnetization switching devices. Here, we report a giant orbital-to-spin conversion in Ta/Pt/Tm3Fe5O12 (TmIG) heterostructures. We show that the torque generation efficiency is enhanced by an order of magnitude in the Ta/Pt/TmIG trilayer compared to that in the Pt/TmIG bilayer. This enhancement is further evidenced by the fact that the critical current density for the magnetization switching of the Ta/Pt/TmIG is an order of magnitude smaller than that of the Pt/TmIG. It is found that the orbital current generated from Ta through the orbital Hall effect (OHE) is converted to the spin current in the interior of Pt. Our discovery offers an extraordinary approach to enhance the torque generation for magnetization switching of insulators and provides an important piece of information for orbitronics.

Field-free spin-orbit torque switching in interlayer exchange coupled Co/Ta/CoTb

This study investigates a T-type field-free spin-orbit torque device with an in-plane magnetic layer coupled to a perpendicular magnetic layer via a non-magnetic spacer. The device utilizes a Co/Ta/CoTb structure, in which the in-plane Co layer and the perpendicular CoTb layer are ferromagnetically (FM) coupled through the Ta spacer. ‘T-type’ refers to the magnetization arrangement in the FM/spacer/FIM structure, where the magnetization in FM is in-plane, while in FIM, it is out-of-plane. This configuration forms a T-shaped arrangement for the magnetization of the two magnetic layers. Additionally, ‘interlayer exchange coupling (IEC)’ denotes the interaction between the two magnetic layers, which is achieved by adjusting the material and thickness of the spacer. Our results show that an in-plane effective field from the IEC enables deterministic current-induced magnetization switching of the CoTb layer. The field-driven and the current-driven asymmetric domain wall motion are observed and characterized by magneto-optic Kerr effect measurements. The functionality of multistate synaptic plasticity is demonstrated by understanding the relationship between the anomalous Hall resistance and the applied current pulses, indicating the potential for the device in spintronic memory and neuromorphic computing.

Symmetry breaking for current-induced magnetization switching

Electromagnetic phenomena, such as magnetization switching, are guided by parity and time-reversal symmetries. Magnetic field and magnetization are time-odd axial vectors. Therefore, the magnetic field can switch magnetization reversibly. In contrast, the electric field is a time-even polar vector that cannot directly switch magnetization. For magnetic recording, an electrical coil-generated local magnetic field is used to switch the magnetic bit. However, in order to integrate the magnetic functionality, e.g., nonvolatile magnetic memory with high speed and low energy consumption, into the chip, it is essential to implement the magnetization switching by an electrical current, where the current induces other axial vectors through spin-transfer torque or spin–orbit torque (SOT). As an energy-efficient tool of magnetization switching, current-induced SOT has been intensively studied for the past decade, which holds great promise in the next generation of magnetic memories and magnetic logic devices [A. Manchon et al., Rev. Mod. Phys. 91, 035004 (2019); X. Han et al., Appl. Phys. Lett. 118, 120502 (2021); C. Song et al., Prog. Mater. Sci. 118, 100761 (2021); Q. Shao et al., IEEE Trans. Magn. 57, 21076639 (2021); J. Ryu et al., Adv. Mater. 32, 1907148 (2020); Y. Cao et al., iScience 23, 101614 (2020)]. In this review, we will first give the basic principle of the symmetry considerations for current-induced magnetization switching. Then, different methods to break the mirror symmetry for deterministic SOT switching will be discussed, together with examples that contain recent progress. In the end, we will give a discussion on the challenges and perspectives of the symmetry designs for SOT, which aim to inspire future fundamental studies and device applications.

Field-Free Magnetization Switching in A1 CoPt Single-Layer Nanostructures for Neuromorphic Computing

Current-induced field-free magnetization switching in single-layer films has been widely investigated for the application of spin–orbit torque (SOT) drivers in low-power, high-density, and nano-sized memory. In this paper, we report field-free SOT switching with threefold angle dependence in A1 disordered single-layer CoPt using the MgO (111) substrate. It is found that the magnetization could not switch in the absence of an external in-plane magnetic field when the current was flowing in the direction of the mirror symmetry high-symmetry axis ([11–2]). While along the direction of the low-symmetry axis ([1–10]) which destroyed the mirror symmetry to the greatest extent, field-free switching could be performed, and the switching ratio was up to 30%. This was mainly attributed to the composition gradient-induced in-plane inversion asymmetry and the low symmetry at the interface of Co/Pt broken out-of-plane inversion. Furthermore, CoPt devices with nanoscale thickness exhibited stable multistate magnetic switching behavior without an external magnetic field, and simultaneously nonlinear synaptic characteristics were obtained. A neural network was established, in which the synaptic weights between the hidden layer and the output layer are updated by using the resistance state of SOT synaptic devices, and the network could achieve about 90.54% recognition accuracy when applied to MNIST’s handwritten digital dataset. This work confirmed that a field-free SOT switch could be realized in single-layer CoPt, which provided theoretical guidance and data support for spin devices.

Symmetry-broken Josephson junctions and superconducting diodes in magic-angle twisted bilayer graphene

The coexistence of gate-tunable superconducting, magnetic and topological orders in magic-angle twisted bilayer graphene provides opportunities for the creation of hybrid Josephson junctions. Here we report the fabrication of gate-defined symmetry-broken Josephson junctions in magic-angle twisted bilayer graphene, where the weak link is gate-tuned close to the correlated insulator state with a moiré filling factor of υ = −2. We observe a phase-shifted and asymmetric Fraunhofer pattern with a pronounced magnetic hysteresis. Our theoretical calculations of the junction weak link—with valley polarization and orbital magnetization—explain most of these unconventional features. The effects persist up to the critical temperature of 3.5 K, with magnetic hysteresis observed below 800 mK. We show how the combination of magnetization and its current-induced magnetization switching allows us to realise a programmable zero-field superconducting diode. Our results represent a major advance towards the creation of future superconducting quantum electronic devices.

Magnetization switching process by dual spin–orbit torque in interlayer exchange-coupled systems

We report current-induced magnetization switching in Pt/Co/Ir/Co/Pt multilayers with different Ir layer thicknesses (tIr), where the perpendicularly magnetized Co layers are coupled ferromagnetically or antiferromagnetically through an interlayer exchange coupling and are sandwiched by the Pt spin Hall layers. The domain structures formed during switching vary depending on the magnetization alignment, i.e., a ferromagnetically coupled or antiferromagnetically coupled configuration. These results clarify the macroscopic picture of switching process for interlayer exchange-coupled systems. The local picture of the switching process is also examined by a numerical calculation based on a macrospin model, which reveals the switching dynamics triggered by dual spin–orbit torques for both antiferromagnetically and ferromagnetically coupled cases. The numerical calculation shows that the dual spin–orbit torques from the two Pt layers effectively act on the two Co layers not only for the antiferromagnetically coupled case but also for the ferromagnetically coupled one. Our findings deepen the understanding of the switching mechanism in a magnetic multilayer and provide an avenue to design spintronic devices with more efficient spin–orbit torque switching.

Odd symmetry planar Hall effect: A method of detecting current-induced in-plane magnetization switching

The robustness of spin–orbit torque (SOT) induced magnetization switching and ease of deposition make type-X devices, where the in-plane easy axis lies perpendicular to the spin polarization, attractive in the field of spintronics. However, universally applicable and straightforward detection of type-X magnetization reversal is still elusive, unlike type-Z switching, which employs DC-based anomalous Hall effect measurement. Here, we demonstrated that the odd planar Hall voltage (O-PHV) signal exhibits an odd symmetry with the application of an external magnetic field which motivates us to develop a reading mechanism for detecting magnetization switching of in-plane magnetized type-X devices. We verified our DC-based reading mechanism in the Pt/Co/NiFe/Pt stack where a thin Co layer is inserted to create dissimilar interfaces about the NiFe layer. Remarkably, the current-induced in-plane fields are found to be significantly large in Pt/Co/NiFe/Pt stack. Furthermore, we employed the O-PHV method to detect the current-induced magnetization switching. The pure DC nature of the writing and reading mechanism of our proposed type-X detection technique through O-PHV makes it the easiest in-plane magnetization detection technique. Moreover, the high repeatability and easy detection of our proposed method will open avenues toward in-plane SOT switching based memory devices and sensors.

Current induced magnetization switching in ferrimagnetic insulators with perpendicular magnetic anisotropy: A macrospin modeling

Spin-orbit torque (SOT) assisted magnetization switching by in-plane current in heavy-metal (HM)/ferrimagnetic insulators (FMIs) heterostructure with perpendicular magnetic anisotropy (PMA) has attracted more attention than in HM/FM (ferromagnetic) heterostructure due to it’s low magnetic losses and electric insulation. Here, macrospin modeling was employed to study the current-induced magnetization switching in Bi doped Y3Fe5O12 (BiYIG), Tm3Fe5O12 (TmIG) and Eu3Fe5O12 (EuIG) FMIs layers with heavy metal on to explore promising FMIs materials for SOT-based fast and efficient devices. The magnetization switching phase diagram (MSPD) of BiYIG, TmIG, EuIG1 and EuIG2 was calculated. It is found that the distribution of MSPD is strongly dependent both on the PMA field Hk and Gilbert damping parameter α. The larger the PMA field Hk is, the greater the damping-like effective field HDL (proportional to the current density) and in-plane magnetic field Hx required to realize magnetization switching in HM/FMIs. Furthermore, switching trajectories and diagrams of BiYIG, TmIG, EuIG1 and EuIG2 with variable Hx and HDL were calculated. The results show that compared with spin transfer torque (STT) and SOT-assisted magnetization switching in all-metallic heterostructures, the switching speed in HM/FMIs system has great advantages. Besides, presence of an external field-like effective field (HFL) can improve switching reliability for low Gilbert damping heterostructure like HM/BiYIG. It is noteworthy that although small damping coefficient is not conducive to the stability of magnetization switching, a huge advantage of smaller damping coefficient is that it can reduce the critical switching current density, which is of great significance to the application of SOT devices. In the end, the SOT-assisted magnetization switching loops of four kinds of FMIs were calculated. This study provides a new perspective for further exploring the current-induced magnetization switching in HM/FMIs system.

Magnetization switching of a nearly compensated ferrimagnet by enhanced spin-orbit torque

Spin–orbit torques (SOTs) provide an efficient way to electrically manipulate the magnetic order in spintronic devices. Compared with conventional ferromagnetic materials, ferrimagnetic materials have the advantages of antiferromagnetically coupled sublattices and induced ultrafast spin dynamics. In this paper, we study the current-induced magnetization switching in the ferrimagnetic Ta/GdFeCo/MgO system. Robust SOT-induced magnetization switching can be achieved at the magnetic compensation temperature point of around 70 K, where the magnetization is nearly zero and the coercivity can reach almost 3 T. The temperature dependence of the SOT efficiency is quantified by the second harmonic method, and the enhanced SOT efficiency near the magnetic compensation temperature is attributed to the negative exchange coupling between the two sublattices of CoFe and Gd. This work demonstrates the SOT switching of the nearly compensated ferrimagnet, with great potential for future magnetic interaction-free and ultrafast ferrimagnetic applications.

Large Spin-Orbit-Torque Efficiency and Room-Temperature Magnetization Switching in SrIrO3/ Co-Fe-B Heterostructures

Spin-orbit-torque (SOT) switching is a potentially energy-efficient mechanism by which electrical current can control magnetization. We demonstrate room-temperature SOT switching in nanometer-thick ${\mathrm{Co}\mathrm{Fe}\mathrm{B}/\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_{3}$ bilayers despite the amorphous nature of CoFeB and the interface. This behavior is attributed to the large spin Hall angle in ${\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_{3}$ as well as high spin transmission at the interface that contribute to the SOT efficiency ${\ensuremath{\xi}}_{\mathrm{SOT}}$. Our bilayers exhibit large ${\ensuremath{\xi}}_{\mathrm{SOT}}$ of up to 1.4 and effective spin Hall conductivity of $0.9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ ($\ensuremath{\hbar}/$2e)${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\mathrm{m}}^{\ensuremath{-}1}$. In our bilayers, we observe unidirectional magnetoresistance and current-induced magnetization switching with a low critical current density of $1.38\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.2em}{0ex}}{\mathrm{A}/\mathrm{m}}^{2}$ at room temperature. Our results are promising for heterostructures combining technologically relevant CoFeB with high spin-orbit-coupling oxides.

Origin of the anomalous Hall effect at the magnetic insulator/heavy metals interface

Ferrimagnetic insulators (FIMIs) are considered to be promising candidates in spin–orbit torque (SOT) devices due to their ability to propagate a spin current by magnons without Ohmic losses owing to the absence of electronic scattering. Moreover, any electrical current shunt is avoided in magnetic insulating materials. On the other hand, SOT-induced magnetization switching is generally measured through the anomalous Hall effect (AHE) in FIMI/heavy metal (HM) systems. However, the origin of AHE in FIMI/HM remains elusive since charges flow only in the HM. Here, we experimentally demonstrate that the AHE has the same origin as the spin Hall magnetoresistance (SMR). To this end, we have studied two bilayer heterostructures, Tm3Fe5O12(TmIG)/W and TmIG/Pt, where we ensure opposite spin Hall effect (SHE) signs for two heavy metals (W and Pt). The magnitudes of AHE and SMR are found to be larger for TmIG/W than TmIG/Pt. We have also evidenced the identical polarity of AHE hysteresis in both systems revealing a square dependency on the spin Hall angle whereas the current-induced magnetization switching polarity in TmIG/W is opposite to that of TmIG/Pt as expected for opposite spin Hall angle signs. Our results establish that the AHE and the spin-Hall magnetoresistance in TmIG insulating ferromagnets and heavy metal bilayers originate from the same mechanism.

Field-free spin-orbit torque switching of GdCo ferrimagnet with broken lateral symmetry by He ion irradiation

Current-induced magnetization switching by spin-orbit torque (SOT) is of great importance for the energy-efficient operation of spin-based memory and logic devices. However, the requirement of an external in-plane magnetic field to deterministically switch the perpendicular magnetization of a device is a bottleneck for device application. There have been many efforts to realize field-free SOT switching using interlayer/exchange coupling, the spin valve structure, or materials with lateral symmetry breaking. However, limitations of material selection or layer structure modification hinder the application of these methods in practice. Here, we demonstrate the field-free SOT switching of a GdCo ferrimagnet with lateral symmetry breaking by He ion irradiation. Local control of the magnetic property with different He ion irradiation conditions induces a lateral magnetic gradient orthogonal to the current flow direction in the ferrimagnet. We also observe out-of-plane-SOT generation due to lateral symmetry breaking, which is essential for field-free switching. Since the He ion irradiation technique is utilized for the fabrication of complementary−metal−oxide−semiconductors, and resolution can reach the nanometer level, our findings have the potential to serve as the basis for new developments in the fabrication of wafer-scale spintronic memory and logic devices with high energy efficiency and high density.

Continuous modulation of charge-spin conversion by electric field in Pt/Co2FeSi/Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3 heterostructures

The half-metallic Heusler alloy Co2FeSi is an ideal material in spintronic devices due to its higher spin polarization, higher Curie temperature and lower damping parameters. In this work, the dynamic magnetism of Co2FeSi is modulated by electric field and it is demonstrated that the charge-spin conversion efficiency ξ is continuous and controllable by the electric field. We further find an extremely high ξ in ferromagnetic/ferroelectric (FM/FE) heterostructures, which could be ascribed to interfacial effect in FM/FE interface. Moreover, we investigate that the charge-spin conversion efficiency varies with the electric field in a butterfly-like behavior, which accords with the S–E curve of Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3(PMN-PT) and could be attributed to strain effect. The modulation of charge-spin conversion efficiency in FM/FE heterostructures via electric field presents a profound potential for next-generation spintronic devices and applications of current-induced magnetization switching.

Enhanced Spin–Orbit Torque Performances in PtRu–Co–PtRu–Ni–Ru-Based Spintronic Devices

The Pt80Ru20 alloy has been testified to be a satisfying heavy metal layer (HM) in spin–orbit torque (SOT)-induced spintronic devices with a relatively high-spin Hall angle (θDL) and a low critical current density. In this work, the Pt80Ru20–Co–Ni (xNi nm)–Ru multilayers are first prepared to find the field-free SOT-induced switching by the direct coupling of out-of-plane and in-plane ferromagnetic (FM) layers. A low in-plane assist field of −20 Oe and some optimized SOT performances are observed in the devices with xNi = 0.4 and 0.8 nm, respectively. To further enhance the performances of the devices, a 0.6 nm thin Pt80Ru20 layer is inserted between two FM layers. In the Pt80Ru20–Co–Pt80Ru20–Ni (tNi nm)–Ru multilayers, the energy consumption decreases dramatically and a higher θDL of 0.156 is obtained in the device with tNi = 0.8 nm. More importantly, the field-free current-induced magnetization switching is realized in the device with tNi = 1.4 nm. Furthermore, the interfacial Dzyaloshinskii–Moyria Interaction (DMI) can be weakened by the thin insertion as well. Our results reveal that the PtRu-based spintronic devices with enhanced SOT performances and field-free SOT-driven switching can be a promising candidate in the application of SOT-induced spintronic devices.

Current switching of the antiferromagnetic Néel vector in Pd/CoO/MgO(001)

Recently, the electrical switching of antiferromagnetic (AFM) order has been intensively investigated because of its application potential in data storage technology. Herein, we report the current switching of the AFM N\'eel vector in epitaxial Pd/CoO films as a function of temperature. Using combined measurements of Hall resistance (HR) and x-ray magnetic linear dichroism (XMLD) below and above the AFM N\'eel temperature, we unambiguously identified both magnetic and nonmagnetic contributions to the current-induced HR change. Through magnetic field-induced HR measurements, we quantitatively determined the percentage of current-induced CoO spin switching. Further, we showed that the thermal effect dominated the CoO magnetic switching more in samples with a thinner Pd layer and that samples with a thicker CoO layer required higher thermal activation for current-induced magnetic switching. These results provide a clear and comprehensive picture of current-induced AFM spin switching across the AFM N\'eel temperature.

Emergence of considerable thermoelectric effect due to the addition of an underlayer in Pt/Co/Pt stack and its application in detecting field free magnetization switching

Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of an adjacent ferromagnetic layer through spin–orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Generally, thin Ta is used as an underlayer to the HM layer for better adhesion and smoothness of the HM layer. Here, we show that Ta addition to the asymmetric stack Pt/Co/Pt gives rise to several compelling effects, viz., thermoelectric effects [particularly, anomalous Nernst effect (ANE)], and enhanced perpendicular magnetic anisotropy which was negligible in a Pt/Co/Pt stack. For this Ta/Pt/Co/Pt stack, the antidamping-SOT values are evaluated after carefully removing the contribution from the ANE and it is found to match the AD-SOT of the Pt/Co/Pt stack. We have observed current-induced field-free magnetization switching Ta/Pt/Co/Pt stack with Co thickness gradient. Furthermore, we have utilized the thermoelectric effects to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method.