Chiral symmetry breaking for deterministic switching of perpendicular magnetization by spin-orbit torque

Symmetry breaking is a characteristic to determine which branch of a bifurcation system follows upon crossing a critical point. Specifically, in spin-orbit torque (SOT) devices, a fundamental question arises: how to break the symmetry of the perpendicular magnetic moment by the in-plane spin polarization? Here, we show that the chiral symmetry breaking by the DMI can induce the deterministic SOT switching of the perpendicular magnetization. By introducing a gradient of saturation magnetization or magnetic anisotropy, non-collinear spin textures are formed by the gradient of effective SOT strength, and thus the chiral symmetry of the SOT-induced spin textures is broken by the DMI, resulting in the deterministic magnetization switching. We introduce a strategy to induce an out-of-plane (z) gradient of magnetic properties, as a practical solution for the wafer-scale manufacture of SOT devices.Comment: 16 pages, 4 figure

Role of dimensional crossover on spin-orbit torque efficiency in magnetic insulator thin films

Magnetic insulators (MIs) attract tremendous interest for spintronic applications due to low Gilbert damping and absence of Ohmic loss. Magnetic order of MIs can be manipulated and even switched by spin-orbit torques (SOTs) generated through spin Hall effect and Rashba-Edelstein effect in heavy metal/MI bilayers. SOTs on MIs are more intriguing than magnetic metals since SOTs cannot be transferred to MIs through direct injection of electron spins. Understanding of SOTs on MIs remains elusive, especially how SOTs scale with the film thickness. Here, we observe the critical role of dimensionality on the SOT efficiency by systematically studying the MI layer thickness dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. We first show that the TmIG thin film evolves from two-dimensional to three-dimensional magnetic phase transitions as the thickness increases, due to the suppression of long-wavelength thermal fluctuation. Then, we report the significant enhancement of the measured SOT efficiency as the thickness increases. We attribute this effect to the increase of the magnetic moment density in concert with the suppression of thermal fluctuations. At last, we demonstrate the current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up to 15 nm. The switching current density is comparable with those of heavy metal/ferromagnetic metal cases. Our findings shed light on the understanding of SOTs in MIs, which is important for the future development of ultrathin MI-based low-power spintronics

A study of spin-orbit torques and deterministic field-free switching of perpendicular magnetization

Spintronic devices provide an energy-efficient platform for implementing non-volatile memory and logic. In spintronic memory devices, the information is stored in the magnetization state of the free magnetic layer, and current, voltage or strain-induced mechanisms are used for switching the magnetization. Among the operation methods, current-induced spin-orbit torque (SOT) is a promising mechanism for magnetization switching with faster dynamics, higher endurance, and potentially higher energy efficiency compared to the conventional spin-transfer torque. For magnetic memory applications, perpendicular magnetic anisotropy is desirable since it enables high memory densities. However, for deterministic SOT switching of a perpendicular magnet, an external magnetic field collinear with the current is required, which hampers the applicability of SOT switching. Although there have been several prior effects aimed at solving this problem, but most involve structural asymmetries or additional layers that are not practical for large wafer scale applications. Consequently, a practical realization of deterministic field-free SOT switching of perpendicular magnetization remains a challenge.In this dissertation, we present two main methods for realizing deterministic field-free SOT switching for practical uses. First, we show that the external in-plane field can be replaced by a built-in exchange bias field using antiferromagnetic materials. We also show that certain antiferromagnets can create the SOTs themselves, serving as the layer providing both the exchange bias and SOTs. As the second approach, we use the concept of structural asymmetry reported previously, and modify the conventional SOT heterostructure by inserting a slightly asymmetric light-metal at the heavy-metal/ferromagnet interface. The broken structural symmetry enables the creation of current-induced out-of-plane effective magnetic fields, which break the symmetry between the up and down states for each current polarity and allow for deterministic SOT switching at zero external magnetic field. We also apply the asymmetry concept to a second material system with a minimal structural asymmetry, resulting in an enhanced magnetic uniformity across large wafer areas. We show that the latter approach for field-free SOT switching has almost all the characteristics of a practical solution that could be used in applications. We also present a better understanding of the deterministic switching process enabled by the structural asymmetry by examining its microscopic origins. In the last part of this dissertation, we discuss several other interesting aspects of the antiferromagnet-based material system (IrMn/CoFeB/MgO) that we primarily developed for the field-free SOT switching. We show that this material system has several unique properties, including the simultaneous presence of Dzyaloshinskii-Moriya interaction, voltage-control of magnetic anisotropy (VCMA), exchange bias field, and spin-orbit torques. We use this material system as a platform for studying magnetic skyrmions, where we can create/annihilate skyrmions using single voltage pulses without any external magnetic fields via the VCMA effect. Furthermore, we use spin-orbit torques to move the skyrmions in the same structure, providing a promising platform for skyrmion-based device applications

A study of spin-orbit torques and deterministic field-free switching of perpendicular magnetization

Spintronic devices provide an energy-efficient platform for implementing non-volatile memory and logic. In spintronic memory devices, the information is stored in the magnetization state of the free magnetic layer, and current, voltage or strain-induced mechanisms are used for switching the magnetization. Among the operation methods, current-induced spin-orbit torque (SOT) is a promising mechanism for magnetization switching with faster dynamics, higher endurance, and potentially higher energy efficiency compared to the conventional spin-transfer torque. For magnetic memory applications, perpendicular magnetic anisotropy is desirable since it enables high memory densities. However, for deterministic SOT switching of a perpendicular magnet, an external magnetic field collinear with the current is required, which hampers the applicability of SOT switching. Although there have been several prior effects aimed at solving this problem, but most involve structural asymmetries or additional layers that are not practical for large wafer scale applications. Consequently, a practical realization of deterministic field-free SOT switching of perpendicular magnetization remains a challenge.In this dissertation, we present two main methods for realizing deterministic field-free SOT switching for practical uses. First, we show that the external in-plane field can be replaced by a built-in exchange bias field using antiferromagnetic materials. We also show that certain antiferromagnets can create the SOTs themselves, serving as the layer providing both the exchange bias and SOTs. As the second approach, we use the concept of structural asymmetry reported previously, and modify the conventional SOT heterostructure by inserting a slightly asymmetric light-metal at the heavy-metal/ferromagnet interface. The broken structural symmetry enables the creation of current-induced out-of-plane effective magnetic fields, which break the symmetry between the up and down states for each current polarity and allow for deterministic SOT switching at zero external magnetic field. We also apply the asymmetry concept to a second material system with a minimal structural asymmetry, resulting in an enhanced magnetic uniformity across large wafer areas. We show that the latter approach for field-free SOT switching has almost all the characteristics of a practical solution that could be used in applications. We also present a better understanding of the deterministic switching process enabled by the structural asymmetry by examining its microscopic origins. In the last part of this dissertation, we discuss several other interesting aspects of the antiferromagnet-based material system (IrMn/CoFeB/MgO) that we primarily developed for the field-free SOT switching. We show that this material system has several unique properties, including the simultaneous presence of Dzyaloshinskii-Moriya interaction, voltage-control of magnetic anisotropy (VCMA), exchange bias field, and spin-orbit torques. We use this material system as a platform for studying magnetic skyrmions, where we can create/annihilate skyrmions using single voltage pulses without any external magnetic fields via the VCMA effect. Furthermore, we use spin-orbit torques to move the skyrmions in the same structure, providing a promising platform for skyrmion-based device applications

Flood susceptibility mapping using multi-temporal SAR imagery and novel integration of nature-inspired algorithms into support vector regression

Flood has long been known as one of the most catastrophic natural hazards worldwide. Mapping flood-prone areas is an important part of flood disaster management. In this study, a flood susceptibility mapping framework was developed based on a novel integration of nature-inspired algorithms into support vector regression (SVR). To this end, various remote sensing (RS) and geographic information system (GIS) datasets were applied to the hybridized SVR models to map flood susceptibility in Ahwaz township, Iran. The proposed framework has two main steps: 1) updating the flood inventory (historical flooded locations) using the proposed RS-based flood detection method developed within the google earth engine (GEE) platform. The mosaicked images of multi-temporal Sentinel-1 synthetic aperture radar (SAR) data have been used in this step; 2) producing flood susceptibility map using the standalone SVR and hybridized model of SVR. The hybridized methods were derived from a novel integration of SVR with meta-heuristic algorithms, hence forming the SVR-bat algorithm (SVR-BA), SVR-invasive weed optimization (SVR-IWO), and SVR-firefly algorithm (SVR-FA). A spatial database of flood locations and 11 conditioning factors (altitude, slope angle, aspect, topographic wetness index, stream power index, normalized difference vegetation index (NDVI), distance to stream, curvature, rainfall, soil type, and land use/cover) were built for the susceptibility modelling. The accuracy of the proposed model was evaluated using the statistical and sensitivity indices, such as root mean square error (RMSE), receiver operating characteristic (ROC) and area under the ROC curve (AUROC) index. The results indicated that all hybridized models outperformed the standalone SVR. According to AUROC values, the predictive power of the SVR-FA was the highest with the value of 0.81, followed by SVR-IWO, SVR-BA, and SVR with values of 0.80, 0.79, and 0.77, respectively.</p

Flood susceptibility mapping using multi-temporal SAR imagery and novel integration of nature-inspired algorithms into support vector regression

Flood has long been known as one of the most catastrophic natural hazards worldwide. Mapping flood-prone areas is an important part of flood disaster management. In this study, a flood susceptibility mapping framework was developed based on a novel integration of nature-inspired algorithms into support vector regression (SVR). To this end, various remote sensing (RS) and geographic information system (GIS) datasets were applied to the hybridized SVR models to map flood susceptibility in Ahwaz township, Iran. The proposed framework has two main steps: 1) updating the flood inventory (historical flooded locations) using the proposed RS-based flood detection method developed within the google earth engine (GEE) platform. The mosaicked images of multi-temporal Sentinel-1 synthetic aperture radar (SAR) data have been used in this step; 2) producing flood susceptibility map using the standalone SVR and hybridized model of SVR. The hybridized methods were derived from a novel integration of SVR with meta-heuristic algorithms, hence forming the SVR-bat algorithm (SVR-BA), SVR-invasive weed optimization (SVR-IWO), and SVR-firefly algorithm (SVR-FA). A spatial database of flood locations and 11 conditioning factors (altitude, slope angle, aspect, topographic wetness index, stream power index, normalized difference vegetation index (NDVI), distance to stream, curvature, rainfall, soil type, and land use/cover) were built for the susceptibility modelling. The accuracy of the proposed model was evaluated using the statistical and sensitivity indices, such as root mean square error (RMSE), receiver operating characteristic (ROC) and area under the ROC curve (AUROC) index. The results indicated that all hybridized models outperformed the standalone SVR. According to AUROC values, the predictive power of the SVR-FA was the highest with the value of 0.81, followed by SVR-IWO, SVR-BA, and SVR with values of 0.80, 0.79, and 0.77, respectively.Geo-engineerin

Spin‐Orbit Torque Switching of a Nearly Compensated Ferrimagnet by Topological Surface States

International audiencecharge-spin conversion and ii) the speed of SOT switching. Generally, SOT in a magnetic layer originates from the spin current injection from the adjacent layer with strong spin-orbit coupling (SOC). The charge-spin conversion efficiency is vital and can be quantified as the θ = / SHE s 3D e 3D J J (dimensionless) or q J J t θ = = / / ICS s 3D e 2D SHE s , where s 3D J represents the 3D spin current density; e 3D J and e 2D J represent the 3D and 2D electric (charge) current density, respectively; and t s represents the effective SOC thickness. In the conventional SOC materials such as HMs, in principle, the θ SHE should be much less than 1, which limits their potential applications in the ultralow power magnetization manipulation. [4] In topological insulators (TIs), SOC from topologically protected surface states, where the spin and orbital angular momenta are locked (spin-momentum locking [5-7]), gives rise to a very large θ SHE [8-12] (or q ICS) and the resulting ultralow switching current density [13-15] at low temperature. Recently, several works have reported the room-temperature SOT switching by TIs, [16-19] which opens the door for the applications of topological insulators. However, there are fundamental limitations of FMs: the low switching speed (≈ns) and the stray-field interaction, which limit the operation speed and the density of magnetic memory, respectively. Antiferromagnets (AFMs) can afford the THz ultrafast spin dynamics; [20] however, AFMs produce zero stray field and zero spin polarization because of the opposite coupled spin lattices from the same element, which makes it difficult to detect the antiferromagnetic order efficiently. [21] Ferrimagnets have two antiferromagnetically coupled spin sublattices, and the contribution of each spin sublattice to their properties can be tuned by the composition or temperature. At the magnetic compensation point, ferrimagnets show similar properties of AFMs, such as ultrafast spin dynamics, [22,23] while the detection is still feasible because of the different responses to the optical or electrical excitations from two spin sublattices. [23-25] Here, we combine TIs [Bi 2 Se 3 and (BiSb) 2 Te 3 ] with nearly compensated ferrimagnets [Gd x (FeCo) 1−x ], and investigate the room-temperature SOT in TI/Gd x (FeCo) 1−x systems. By changing the composition of Gd x (FeCo) 1−x , we can tune the net magnetic moment and the dominated spin sublattice (CoFe-rich and Gd-rich). The robust room-temperature SOT switching Utilizing spin-orbit torque (SOT) to switch a magnetic moment provides a promising route for low-power-dissipation spintronic devices. Here, the SOT switching of a nearly compensated ferrimagnet Gd x (FeCo) 1−x by the topological insulator [Bi 2 Se 3 and (BiSb) 2 Te 3 ] is investigated at room temperature. The switching current density of (BiSb) 2 Te 3 (1.20 × 10 5 A cm −2) is more than one order of magnitude smaller than that in conventional heavy-metal-based structures, which indicates the ultrahigh efficiency of charge-spin conversion (>1) in topological surface states. By tuning the net magnetic moment of Gd x (FeCo) 1−x via changing the composition, the SOT efficiency has a significant enhancement (6.5 times) near the magnetic compensation point, and at the same time the switching speed can be as fast as several picoseconds. Combining the topological surface states and the nearly compensated ferrimagnets provides a promising route for practical energy-efficient and high-speed spintronic devices. Topological Spintronics Spintronic devices have been considered as one of the promising candidates for the next-generation memory and logic devices, and spin-orbit torque (SOT) provides an efficient way to manipulate the magnetic moment by electrical method with ultralow power dissipation and ultrafast operating speed. [1-3] Beyond previous studies of SOT switching based on conventional heavy metal/ferromagnet (HM/FM) heterostructures, two crucial issues need to be resolved: improving: i) the efficiency o

Spin-Torque Ferromagnetic Resonance in W / Co − Fe − B / W / Co − Fe − B / Mg O Stacks

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