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Synergy of fivefold boost SOT efficiency and field-free magnetization switching with broken inversion symmetry: Toward neuromorphic computing
Authors:
Badsha Sekh,
Hasibur Rahaman,
Subhakanta Das,
Mitali,
Ramu Maddu,
Kesavan Jawahar,
S. N. Piramanayagam
Abstract:
Non-volatile Neuromorphic Computing (NC) elements utilizing Spin Orbit Torque (SOT) provide a viable solution to alleviate the memory wall bottleneck in contemporary computing systems. However, the two challenges, low SOT efficiency and the need for in plane symmetry breaking field for perpendicular magnetization switching, greatly limit its practical implementation. In this work, the enhanced SOT…
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Non-volatile Neuromorphic Computing (NC) elements utilizing Spin Orbit Torque (SOT) provide a viable solution to alleviate the memory wall bottleneck in contemporary computing systems. However, the two challenges, low SOT efficiency and the need for in plane symmetry breaking field for perpendicular magnetization switching, greatly limit its practical implementation. In this work, the enhanced SOT efficiency of Platinum (Pt) SOT layer and field free perpendicular magnetization switching are achieved by integrating thin Ruthenium Oxide (RuO2) layer in our material stack. The optimal RuO2 thickness (0.5 nm) enhances 5.2 times Damping Like (DL) SOT efficiency compared with pure SOT layer (Pt), as determined by hysteresis loop shift measurements, with a relatively low resistivity (90 micro-Ohm-cm). Moreover, we achieve 3 times reduction of critical magnetization switching current density compared to reference sample. Our experimental findings also demonstrate Rashba-induced substantial field-free magnetization switching in the presence of an emergent built-in interfacial field. Notably, reliable multi resistance synaptic states are achieved by tailoring the synergistic effects of enhanced SOT and interfacial magnetism. The functionality of synaptic states has been further evaluated by implementing an artificial neural network and achieved image recognition accuracies of approximately 95% and 87% on the MNIST and Fashion-MNIST datasets, respectively. This systematic study paves the way to energy-efficient, field-free SOT synapses for practical NC applications.
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Submitted 8 January, 2026;
originally announced January 2026.
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Giant Damping-like Torque Efficiency via Synergistic Spin Hall and enhanced Orbital Hall Effects
Authors:
Subhakanta Das,
Sabpreet Bhatti,
Ramu Maddu,
Bilal Jamshed,
Go Dong Wook,
S. N. Piramanayagam
Abstract:
Current-induced spin-orbit torque (SOT) has emerged as a promising method for achieving energy-efficient magnetisation switching in advanced spintronic devices. Over the past two decades, researchers have primarily focused on enhancing spin current generation through the spin Hall effect, relying predominantly on the spin degree of freedom (DoF) of the electron, while neglecting its orbital counte…
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Current-induced spin-orbit torque (SOT) has emerged as a promising method for achieving energy-efficient magnetisation switching in advanced spintronic devices. Over the past two decades, researchers have primarily focused on enhancing spin current generation through the spin Hall effect, relying predominantly on the spin degree of freedom (DoF) of the electron, while neglecting its orbital counterpart. Orbital Hall effect depends critically on the crystallinity and the interface between the orbital Hall layer and the orbital-to-spin conversion layer. However, most experimental works on orbital Hall effect relied on polycrystalline films with no special attention to improve the crystallographic texture. In this work, we have grown the Ru layer on a NiW seedlayer, which helped to improve the crystallographic texture, thereby enhancing the switching efficiency by over 44%. Such a huge increase in switching efficiency was achieved by (i) improving crystallographic texture and (ii) leveraging both spin and orbital DoFs. Our study underscores the potential for improving the spin-torque efficiency by combining interface engineering, orbital and spin Hall effects to drive next-generation spintronics.
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Submitted 23 July, 2025;
originally announced July 2025.
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Magnetic Orbital Hall Effect in Altermagnet RuO$_2$
Authors:
Badsha Sekh,
Hasibur Rahaman,
Shilei Ding,
Pinkesh Kumar Mishra,
Ramu Maddu,
Tianli Jin,
Subhakanta Das,
S. N. Piramanayagam
Abstract:
Orbital angular momentum provides an alternative channel for current-induced magnetization switching beyond conventional spin--orbit coupling. While orbital Hall effects have been observed in several nonmagnetic materials, their manifestation in symmetry-compensated magnetic systems remains unexplored. Here, we report experimental evidence for a magnetic orbital Hall effect in RuO$_2$. In RuO$_2$(…
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Orbital angular momentum provides an alternative channel for current-induced magnetization switching beyond conventional spin--orbit coupling. While orbital Hall effects have been observed in several nonmagnetic materials, their manifestation in symmetry-compensated magnetic systems remains unexplored. Here, we report experimental evidence for a magnetic orbital Hall effect in RuO$_2$. In RuO$_2$(101)/Pt/Co heterostructures, we observe a pronounced unconventional torque characterized by a large out-of-plane component, strong crystalline anisotropy, and deterministic field-free switching of a perpendicular ferromagnet over a wide range of RuO$_2$ thicknesses. The torque exhibits a non-monotonic dependence on Pt thickness, reaching a maximum at 1.5~nm, and displays a long-range RuO$_2$ thickness ($t_{\mathrm{RuO}_2}$) dependence that saturates for $t_{\mathrm{RuO}_2}>100~\mathrm{nm}$. These features cannot be reconciled with conventional spin-current mechanisms. Rather, they indicate a magnetic orbital Hall effect in RuO$_2$ that could originate from exchange-induced momentum-dependent band splitting and its interplay with spin--orbit and crystal-field coupling, with the generated orbital current converted into torque in Pt. Our findings establish altermagnets as intrinsic sources of orbital currents and extend orbitronics to symmetry-compensated magnetic systems.
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Submitted 13 July, 2026; v1 submitted 21 January, 2025;
originally announced January 2025.
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Zero Field Antiferromagnetically Coupled Skyrmions and their Field-Driven Uncoupling in Composite Chiral Multilayers
Authors:
May Inn Sim,
Dickson Thian,
Ramu Maddu,
Xiaoye Chen,
Hang Khume Tan,
Chao Li,
Pin Ho,
Anjan Soumyanarayanan
Abstract:
Antiferromagnetic (AF) skyrmions are topological spin structures with fully compensated, net-zero magnetization. Compared to their ferromagnetic (FM) skyrmion counterparts, their reduced stray field and enhanced electrical response can enable linear, high-throughput current-driven motion. However, their bubble-like character in conventional bilayer AFs limits their stability to fluctuations, leadi…
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Antiferromagnetic (AF) skyrmions are topological spin structures with fully compensated, net-zero magnetization. Compared to their ferromagnetic (FM) skyrmion counterparts, their reduced stray field and enhanced electrical response can enable linear, high-throughput current-driven motion. However, their bubble-like character in conventional bilayer AFs limits their stability to fluctuations, leading to deformation and annihilation. Here we present the engineering of a composite AF chiral multilayer, wherein the interplay of AF and FM interlayer couplings generates compensated skyrmions with compact structures. High-resolution magnetic imaging and micromagnetic simulations show that the internal exchange field stabilizes AF skyrmions at zero external field with characteristics comparable to FM counterparts at 100 mT. Quantitative analyses establish their decoupling above the exchange field, yielding independent, spatially segregated textures in constituent chiral layers. This work provides a foundation to develop AF spin-textures with enhanced immunity, compatible with efficient readout and manipulation, with relevance to unconventional computing.
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Submitted 7 January, 2025;
originally announced January 2025.
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Emulation of Neuron and Synaptic Functions in Spin-Orbit Torque Domain Wall Devices
Authors:
Durgesh Kumar,
Ramu Maddu,
Hong Jing Chung,
Hasibur Rahaman,
Tianli Jin,
Sabpreet Bhatti,
Sze Ter Lim,
Rachid Sbiaa,
S. N. Piramanayagam
Abstract:
Neuromorphic computing (NC) architecture has shown its suitability for energy-efficient computation. Amongst several systems, spin-orbit torque (SOT) based domain wall (DW) devices are one of the most energy-efficient contenders for NC. To realize spin-based NC architecture, the computing elements such as synthetic neurons and synapses need to be developed. However, there are very few experimental…
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Neuromorphic computing (NC) architecture has shown its suitability for energy-efficient computation. Amongst several systems, spin-orbit torque (SOT) based domain wall (DW) devices are one of the most energy-efficient contenders for NC. To realize spin-based NC architecture, the computing elements such as synthetic neurons and synapses need to be developed. However, there are very few experimental investigations on DW neurons and synapses. The present study demonstrates the energy-efficient operations of neurons and synapses by using novel reading and writing strategies. We have used a W/CoFeB-based energy-efficient SOT mechanism to drive the DWs at low current densities. We have used the concept of meander devices for achieving synaptic functions. By doing this, we have achieved 9 different resistive states in experiments. We have experimentally demonstrated the functional spike and step neurons. Additionally, we have engineered the anomalous Hall bars by incorporating several pairs, in comparison to conventional Hall crosses, to increase the sensitivity as well as signal-to-noise ratio (SNR). We performed micromagnetic simulations and transport measurements to demonstrate the above-mentioned functionalities.
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Submitted 15 December, 2022;
originally announced December 2022.