arXiv subjects

S. N. Piramanayagam

Publications and source records attributed to S. N. Piramanayagam.

At least 19 recordsLinked to original sources

arXiv2026

Graded Orbital Interface Engineering for Enhanced Damping-like Torque Efficiency

Orbitronics has emerged as a promising approach for generating orbital currents using low cost, abundant materials through the orbital Hall effect. Since orbital current cannot directly interact with the local magnetization of a conventional ferromagnet, a non-magnetic layer with strong spin orbit coupling is required for converting orbital current into spin current. However, the propagation of orbital current across the interface between an orbital source and an orbital to spin converter remains poorly understood. Here, we tuned the Ru/Pt orbital interface from an abrupt interface to a graded interface by introducing a controlled RuPt intermixed layer and quantified the resulting damping like torque efficiencies. A small, intermixed thickness of 0.26 nm enhances the damping like torque efficiency by 15% compared with the abrupt Ru/Pt interface and by 70% compared with the Pt reference. Further increasing the intermixing thickness progressively suppresses the torque efficiency. We attribute this nonmonotonic behavior to the competition between enhanced orbital transmission through a gradual crystal field transition and orbital current dephasing due to alloy disorder and scattering. Our results demonstrate that the orbital-source/converter interface is an active element for controlling orbital transport and provides an experimentally accessible approach for optimizing orbital current transmission and torque generation in orbitronic heterostructures.

physics.app-ph↗

arXiv2026

Magnetic Orbital Hall Effect in Altermagnet RuO$_2$

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.

cond-mat.mes-hall↗

arXiv2026

Strain effects in [001] textured Co80Ir20 thin films with negative magnetocrystalline anisotropy

Co80Ir20 ferromagnetic thin films have recently been the focus of intensive research because the negative magnetocrystalline anisotropy adds to the shape anisotropy and favors a strong alignment of the magnetization in the film plane for [001] textured or epitaxial thin films. However, the role of magnetoelastic effects has not been properly considered in most published research. In this work we have performed a detailed analysis of 24 nm Co80Ir20 thin films deposited on Si/SiO2 with different underlayers (Ta, Pt) and overlayers in order to induce [001]-textured growth and different degrees of strain. Using x-ray diffraction measurements we have found that the c-axis lattice parameter depends on the underlayer material (larger negative strain for Ta), but the degree of texture and the average grain size remain essentially constant, except for one of the multilayers. Differences in the magnetic behavior according to the underlayer were also found in room temperature magnetization vs field loops and temperature dependent dc magnetization measurements. Anisotropy was quantified using ferromagnetic resonance which showed that the effective anisotropy field is also dependent on the underlayer. Ta underlayers show an anisotropy close to that expected for shape, while Pt underlayer induces an additional in-plane anisotropy field of the order of 7-9 kOe. A simple model of stress induced anisotropy gives anisotropy field values similar to those observed experimentally. The correlation between observed strain and anisotropy together with the similarity in microstructural properties strongly suggests that stress effects cannot be disregarded when analyzing the magnetic data for the estimation of the magnetocrystalline contribution.

cond-mat.mtrl-sci↗

arXiv2026

Confinement-controlled pathways to complex skyrmionic textures in Co/W/Pt multilayers

Magnetic skyrmions and higher-order topological spin textures offer rich opportunities for multi-level information encoding, yet their deterministic stabilization and transformation under geometric confinement at room temperature remain poorly understood. Here, we demonstrate that geometric confinement acts as a robust and universal control parameter that governs a hierarchical transformation pathway of chiral spin textures in Pt/Co/W multilayer micro-tracks. As the confinement increases, extended labyrinth domains fragment into isolated skyrmions, followed by the systematic suppression of skyrmion pairs and the preferential stabilization of compact higher-order textures. We find that confinement strongly enhances the formation of skyrmioniums via recombination and promotes their subsequent evolution into uniform skyrmion bags by capturing additional skyrmions. Statistical analysis reveals a confinement-driven redistribution of topological populations, with skyrmion bags emerging as the dominant state in the narrowest tracks. Supported by micromagnetic simulations, our results establish geometric confinement as a deterministic selector of complex topological textures and reveal a previously unexplored route for engineering higher-order skyrmionic states at room temperature. These findings provide a scalable materials strategy for multistate skyrmion-based spintronic and memory architectures.

physics.app-ph↗

arXiv2026

Spin splitting torque enabled artificial neuron with self-reset via synthetic antiferromagnetic coupling

Spintronic artificial neurons are intriguing building blocks for energy efficient Neuromorphic Computing (NC). Nevertheless, most contemporary implementations rely on symmetry breaking external in plane magnetic fields (H_X) for neuron operation, which limits scalability and hardware practicality. We experimentally demonstrate an altermagnet/Synthetic Antiferromagnetic Coupling (SAF) based spintronic neuron that uses out of plane spin (σ_Z) polarized spin-splitting torque to eliminate the necessity of an external H_X. The neuron device also features intrinsic self-reset function facilitated by built-in exchange coupling. Furthermore, the proposed device is validated for Spiking Neural Network (SNN) applications by achieving test accuracies of 95.99% and 94.36% on the MNIST and N-MNIST datasets, respectively. These results demonstrate the hardware feasibility and compatibility of the proposed spintronic neuron, highlighting its potential for compact, scalable and energy-efficient neuromorphic computing systems.

physics.app-ph↗

arXiv2026

Synergy of fivefold boost SOT efficiency and field-free magnetization switching with broken inversion symmetry: Toward neuromorphic computing

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.

cond-mat.mes-hall↗

arXiv2025

Giant Damping-like Torque Efficiency via Synergistic Spin Hall and enhanced Orbital Hall Effects

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.

physics.app-ph↗

arXiv2024

Electrical Control Grain Dimensionality with Multilevel Magnetic Anisotropy

In alignment with the increasing demand for larger storage capacity and longer data retention, electrical control of magnetic anisotropy has been a research focus in the realm of spintronics. Typically, magnetic anisotropy is determined by grain dimensionality, which is set during the fabrication of magnetic thin films. Despite the intrinsic correlation between magnetic anisotropy and grain dimensionality, there is a lack of experimental evidence for electrically controlling grain dimensionality, thereby impeding the efficiency of magnetic anisotropy modulation. Here, we demonstrate an electric field control of grain dimensionality and prove it as the active mechanism for tuning interfacial magnetism. The reduction in grain dimensionality is associated with a transition from ferromagnetic to superparamagnetic behavior. We achieve a non-volatile and reversible modulation of the coercivity in both the ferromagnetic and superparamagnetic regimes. Subsequent electrical and elemental analysis confirms the variation in grain dimensionality upon the application of gate voltages, revealing a transition from a multidomain to a single-domain state accompanied by a reduction in grain dimensionality. Furthermore, we exploit the influence of grain dimensionality on domain wall motion, extending its applicability to multilevel magnetic memory and synaptic devices. Our results provide a strategy for tuning interfacial magnetism through grain size engineering for advancements in high-performance spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Leaky-Integrate-Fire Neuron via Synthetic Antiferromagnetic Coupling and Spin-Orbit Torque

Neuromorphic computing (NC) is a promising candidate for artificial intelligence applications. To realize NC, electronic analogues of brain components, such as synapses and neurons, must be designed. In spintronics, domain wall (DW) based magnetic tunnel junctions - which offer both synaptic and neuronal functionalities - are one of the promising candidates. An electronic neuron should exhibit leaky-integrate-fire functions similar to their biological counterparts. However, most experimental studies focused only on the integrate-and-fire functions, overlooking the leaky function. Here, we report on a domain wall neuron device that achieves integration using spin-orbit torque-induced domain wall motion and a leaky function via synthetic antiferromagnetic coupling. By fabricating Hall bar devices in a special geometry, we could achieve these two functionalities. During the leaky process, the maximum DW velocity achieved was 2500 μm/s. The proposed design utilizes materials used in STT-MRAM fabrication and is compatible with CMOS fabrication. Therefore, this neuron can be readily integrated into NC.

physics.app-ph↗

arXiv2023

Stability and Character of Zero Field Skyrmionic States in Hybrid Magnetic Multilayer Nanodots

Ambient magnetic skyrmions stabilized in multilayer nanostructures are of immense interest due to their relevance to magnetic tunnel junction (MTJ) devices for memory and unconventional computing applications. However, existing skyrmionic nanostructures built using conventional metallic or oxide multilayer nanodots are unable to concurrently fulfill the requirements of nanoscale skyrmion stability and feasibility of all-electrical readout and manipulation. Here, we develop a few-repeat hybrid multilayer platform consisting of metallic [Pt/CoB/Ir]3 and oxide [Pt/CoB/MgO] components that are coupled to evolve together as a single, composite stack. Zero-field (ZF) skyrmions with sizes as small as 50 nm are stabilized in the hybrid multilayer nanodots, which are smoothly modulated by up to 2.5x by varying CoB thickness and dot sizes. Meanwhile, skyrmion multiplets are also stabilized by small bias fields. Crucially, we observe higher order 'target' skyrmions with varying magnetization rotations in moderately-sized, low anisotropy nanodots. These results provide a viable route to realize long-sought skyrmionic MTJ devices and new possibilities for multi-state skyrmionic device concepts.

cond-mat.mtrl-sci↗

arXiv2023

Dynamics of interacting skyrmions in magnetic nano-track

Controlling multiple skyrmions in nanowires is important for their implementation in racetrack memory or neuromorphic computing. Here, we report on the dynamical behavior of two interacting skyrmions in confined devices with a comparison to a single skyrmion case. Although the two skyrmions shrink near the edges and follow a helical path, their behavior is different. Because the leading skyrmion is between the edge and the trailing one, its size is reduced further and collapses at a lower current density compared to the single skyrmion case. For higher current density, both skyrmions are annihilated with a core-collapse mechanism for the leading one followed by a bubble-collapse mechanism for the trailing one.

physics.app-ph↗

arXiv2022

Emulation of Neuron and Synaptic Functions in Spin-Orbit Torque Domain Wall Devices

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.

cond-mat.mes-hall↗

arXiv2022

Synaptic modulation of conductivity and magnetism in a CoPt-based electrochemical transistor

Among various types of neuromorphic devices towards artificial intelligence, the electrochemical synaptic transistor emerges, in which the channel conductance is modulated by the insertion of ions according to the history of gate voltage across the electrolyte. Despite the striking progress in exploring novel channel materials, few studies report on the ferromagnetic metal-based synaptic transistors, limiting the development of spin-based neuromorphic devices. Here, we present synaptic modulation of both conductivity as well as magnetism based on an electrochemical transistor with a metallic channel of ferromagnetic CoPt alloy. We first demonstrate its essential synaptic functionalities in the transistor, including depression and potentiation of synaptic weight, and paired-pulse facilitation. Then, we show a short- to long-term plasticity transition induced by different gate parameters, such as amplitude, duration, and frequency. Furthermore, the device presents multilevel and reversible nonvolatile states in both conductivity and coercivity. The results demonstrate simultaneous modulation of conductivity and magnetism, paving the way for building future spin-based multifunctional synaptic devices.

physics.app-ph↗

arXiv2022

Diode Like Attributes in Magnetic Domain Wall Devices via Geometrical Pinning for Neuromorphic Computing

Neuromorphic computing (NC) is considered as a potential vehicle for implementing energy-efficient artificial intelligence (AI). To realize NC, several materials systems are being investigated. Among them, the spin-orbit torque (SOT) -driven domain wall (DW) devices are one of the potential candidates. To implement these devices as neurons and synapses, the building blocks of NC, researchers have proposed different device designs. However, the experimental realization of DW device-based NC is only at the primeval stage. In this study, we have proposed and investigated pine-tree-shaped DW devices, based on the Laplace force on the elastic DWs, for achieving the synaptic functionalities. We have successfully observed multiple magnetization states when the DW was driven by the SOT current. The key observation is the asymmetric pinning strength of the device when DW moves in two opposite directions (defined as, xhard and xeasy). This shows the potential of these DW devices as DW diodes. We have used micromagnetic simulations to understand the experimental findings and to estimate the Laplace pressure for various design parameters. The study leads to the path of device fabrication, where synaptic properties are achieved with asymmetric pinning potential.

cond-mat.mes-hall↗

arXiv2021

Emulation of Synaptic Plasticity on Cobalt based Synaptic Transistor for Neuromorphic Computing

Neuromorphic Computing (NC), which emulates neural activities of the human brain, is considered for low-power implementation of artificial intelligence. Towards realizing NC, fabrication, and investigations of hardware elements such as synaptic devices and neurons are essential. Electrolyte gating has been widely used for conductance modulation by massive carrier injections and has proven to be an effective way of emulating biological synapses. Synaptic devices, in the form of synaptic transistors, have been studied using a wide variety of materials. However, studies on metallic channel based synaptic transistors remain vastly unexplored. Here, we have demonstrated a three-terminal cobalt-based synaptic transistor to emulate biological synapse. We realized gating controlled multilevel, nonvolatile conducting states in the proposed device. The device could successfully emulate essential synaptic functions demonstrating short-term and long-term plasticity. A transition from short-term memory to long-term memory has been realized by tuning gate pulse amplitude and duration. The crucial cognitive behavior viz., learning, forgetting, and relearning, has been emulated, showing resemblance to the human brain. Along with learning and memory, the device showed dynamic filtering behavior. These results provide an insight into the design of metallic channel based synaptic transistors for neuromorphic computing.

cond-mat.mtrl-sci↗

arXiv2020

Ultra-low Power Domain Wall Device for Spin-based Neuromorphic Computing

Neuromorphic computing (NC) is gaining wide acceptance as a potential technology to achieve low-power intelligent devices. To realize NC, researchers investigate various types of synthetic neurons and synaptic devices such as memristors and spintronic domain wall (DW) devices. In comparison, DW-based neurons and synapses have potentially higher endurance. However, for realizing low-power devices, DW motion at low energies - typically below pJ/bit - are needed. Here, we demonstrate domain wall motion at current densities as low as 1E6 A/m2 by tailoring the beta-W spin Hall material. With our design, we achieve ultra-low pinning fields and current density reduction by a factor of 10000. The energy required to move the domain wall by a distance of about 20 micrometers is 0.4 fJ, which translates into energy consumption of 0.4 aJ/bit for a bit-length of 20 nm. With a meander domain wall device configuration, we have established a controlled DW motion for synapse applications and have shown the direction to make ultra-low energy spin-based neuromorphic elements.

physics.app-ph↗

arXiv2019

Controlled spin-torque driven domain wall motion using staggered magnetic nanowires

The growing demand for storage, due to big data applications, cannot be met by hard disk drives. Domain wall (DW) memory devices such as racetrack memory offer an alternative route to achieve high capacity storage. In DW memory, control of domain wall positions and their motion using spin-transfer torque are important challenges. In this paper, we demonstrate controlled domain wall motion using spin-transfer torque in staggered magnetic nanowires. The devices, fabricated using electron-beam lithography, were tested using a magneto-optical Kerr microscopy and electrical transport measurements. The depinning current, pinning potential and thermal stability were found to depend on the device dimensions of the staggering nanowires. Thus, the proposed staggering configuration helps to fine-tune the properties of domain wall devices for memory applications.

physics.app-ph↗

arXiv2019

High Frequency Domain Wall Oscillations in Ferromagnetic Nanowire with a Nanoscale Dzyaloshinskii Moriya Interaction (DMI) Region

The Dzyaloshinskii Moriya Interaction (DMI) has laid the foundation for many novel chiral structures such as Skyrmions. In most of the studies so far, the DMI is present in the whole of the magnetic layer. Here, we report our investigations on a ferromagnetic nanowire where DMI is confined to a nanoscale region. We observe that the local modulation of magnetic properties causes oscillation of domain walls under the influence of spin-transfer torque. The oscillation frequency is tunable within a few GHz, making this observation potentially useful for applications in neuromorphic computing.

cond-mat.mes-hall↗