Giant spin pumping at the ferromagnet (permalloy) – organic semiconductor (perylene diimide) interface

Pure spin current based devices have attracted great interest in recent days. Spin current injection into non-magnetic materials is essential for the design and development of such pure spin current based devices. In this context, organic semiconductors (OSCs) can be potential non-magnetic materials over widely explored heavy metals. This is due to the relatively low spin-orbit coupling of OSCs, which is essential to host the spin current with a large spin diffusion length and long spin-relaxation time. This research work demonstrates the harvesting of spin currents at the perylene diimide (PDI)/permalloy (Py) based OSC interface. The observed high linewidth broadening of 2.18 mT from the ferromagnetic resonance spectra indicates the presence of giant spin pumping from Py to PDI. The resultant spin-mixing conductance, 1.54 × 1018 m-2 quantifies the amount of spin current injected from Py to PDI, which is in a similar range to ferromagnet/heavy metals. © The Royal Society of Chemistry

Domain wall dynamics in ferromagnetic cylindrical and planar nanostructures

This thesis presents a comprehensive study on domain wall (DW) dynamics in NiFe cylindrical and planar nanostructures. In cylindrical nanowires of relatively low aspect ratio, a three dimensional helical DW is found to separate two vortices of opposite chirality. The formation of helical DW is controlled by introducing geometrical modulations along the nanowire. The magnetic charge calculation at the helical DW shows an abrupt transition between two opposite charges (positive to negative or vice versa). To verify the micromagnetic simulations, compositionally modulated nanowires are grown by pulsed electrodeposition at two different potentials. Differential etching of the two layers of NiFe with different compositions leads to the formation of constrictions. The presence of the helical DWs in the constricted cylindrical NiFe nanowires is verified by magnetic force microscopy (MFM) imaging. At high aspect ratio, transverse DWs that are found in sub-50 nm cylindrical nanowires are shown to possess an intrinsic oscillatory behavior in the translational motion. Moreover, in the absence of external energies, the oscillations are governed by the energy transfer from the DW rotations. Such oscillations are self-sustained and unique to the transverse DWs in cylindrical nanowires. By setting up a magnetostatically coupled nanowire system, an infinite oscillation is achieved by the application of current to balance the DW coupling. The sustained oscillation is analogous to simple harmonic motion between a compressed and relaxed state of the DW. Solving the simple harmonic equation unfolds the finite the mass associated with the DW in cylindrical nanowires which was assumed to be mass less in previous studies. The transverse DW pinning and depinning mechanisms are studied at the geometrical modulations in the planar and cylindrical nanowires. In planar nanowires, DW pinning potential is found to be chirality dependant at an anti-notch structure. The potential barrier undergoes a transition from smooth and gradual to steep and abrupt shape as the dimensions of the anti-notch are varied. In cylindrical nanowires, the DW pinning has shown contrary behaviors with the application of current and magnetic field. Interestingly, an increase in the notch depth results in lowering the depinning current density. The DW deformation and rotation assist the spin-polarized current in depinning process. The degree of DW deformation is higher at the deeper notch and lowers the depinning current density. The DW pinning at the anti-notch has shown two different phenomenon as the height of the anti-notch is varied. At lower dimensions, the pinning mechanism follows the trend similar to the notch. However, at higher dimensions, the DW transformation from transverse to vortex configuration causes the lowering in the barrier height in the field driven case. The barrier potential rises for the current driven case due to the vortex chirality switching within the anti-notch. In planar NiFe nanostructures, the DW injection methods using local Oersted field and the external magnetic field are presented. The key focus is devoted to the transverse DWs in narrow nanowires. Using patterned nanostructures, techniques to control, detect and rectify the DW chirality are presented by using MFM imaging. The selective motion of the vortex core in the presence of the linear magnetic field assists in DW chirality detection. Two different switching mechanisms of the DW within a slanted rectangular structure result in rectifying the chirality of the transverse DW. Finally, a DW based reconfigurable magnetic logic is demonstrated in which a single structure performs all the basic logic operations. Two underlying principles, transverse DW selective switching and the effect of transverse Oersted field on the DW are verified experimentally by MFM imaging.DOCTOR OF PHILOSOPHY (SPMS

Skyrmion based majority logic gate by voltage controlled magnetic anisotropy in a nanomagnetic device

Magnetic skyrmions are topologically protected spin textures and they are suitable for future logic-in-memory applications for energy-efficient, high-speed information processing and computing technologies. In this work, we have demonstrated skyrmion-based 3 bit majority logic gate using micromagnetic simulations. The skyrmion motion is controlled by introducing a gate that works on voltage controlled magnetic anisotropy. Here, the inhomogeneous magnetic anisotropy behaves as a tunable potential barrier/well that modulates the skyrmion trajectory in the structure for the successful implementation of the majority logic gate. In addition, several other effects such as skyrmion-skyrmion topological repulsion, skyrmion-edge repulsion, spin-orbit torque and skyrmion Hall effect have been shown to govern the logic functionalities. We have systematically presented the robust logic operations by varying the current density, magnetic anisotropy, voltage-controlled gate dimension and geometrical parameters of the logic device. The skyrmion Hall angle is monitored to understand the trajectory and stability of the skyrmion as a function of time in the logic device. The results demonstrate a novel method to achieve majority logic by using voltage controlled magnetic anisotropy which further opens up a new route for skyrmion-based low-power and high-speed computing devices

Skyrmion based 3D low complex runtime reconfigurable architecture design methodology of universal logic gate

In this study, we introduce the area efficient low complex runtime reconfigurable architecture design methodology based on Skyrmion logic for universal logic gate (ULG) i.e. NOR/NAND implementation using micromagnetic simulations. We have modelled the two input 3D device structure using bilayer ferromagnet/heavy metal where the magnetic tunnel junctions inject and detect the input and output skyrmions by exploiting the input reversal mechanism. The implementation of NOR and NAND is performed using this same device where it is reconfigured runtime with enhanced tunability by the ON and OFF state of current passing through a non magnetic metallic gate respectively. This gate acts as a barrier for skyrmion motion (additional control mechanism) to realize the required Skyrmion logic output states. To the best of authors’s knowledge the boolean optimizations and the mapping logic have been presented for the first time to demonstrate the functionalities of the NOR/NAND implementation. This proposed architecture design methodology of ULG leads to reduced device footprint with regard to the number of thin film structures proposed, low complexity in terms of fabrication and also providing runtime reconfigurability to reduce the number of physical designs to achieve all truth table entries (∼75% device footprint reduction). The proposed 3D ULG architecture design benefits from the miniaturization resulting in opening up a new perspective for magneto-logic devices

Skyrmion Dynamics in Concentric and Eccentric Nano-Ring Structures

Skyrmions are found to be promising for next-generation energy-efficient spintronic applications. Moreover, ultrafast skyrmion dynamics in the gigahertz-band offer an excellent opportunity to exploit such topologically protected nanostructures in high-frequency applications. Here, we present a systematic investigation of the microwave properties of the skyrmions in concentric and eccentric ring structures using micromagnetic simulations. Two gyrotropic modes with clockwise and counterclockwise gyration are observed in skyrmion when excited by an in-plane microwave field. The high-frequency response is found to be enhanced by 3.5 GHz by using a small bias field of 40 mT. The skyrmion dynamics are found to be extremely sensitive to the edge repulsions and a remarkably large frequency shift of 2 GHz of the skyrmion resonance modes is observed by simply varying the position of a skyrmion in an eccentric ring structure. The results are substantiated by directly correlating the observations with inertial mass associated with a skyrmion calculated analytically. The results provide additional functionality of the skyrmions based on their tunable microwave properties which may have potential implications in the field of miniaturized reconfigurable microwave devices. © 1965-2012 IEEE

Reconfigurable Logic Operations via Gate Controlled Skyrmion Motion in a Nanomagnetic Device

Owing to the topological protection and the ease of efficient manipulation, skyrmions have emerged as potential candidates for carrying information in future memory and logic devices. Here, we have proposed a reconfigurable skyrmion based two-input logic device architecture. Using micromagnetic simulations, we have demonstrated that the device is capable of performing both OR and AND logic gate functionalities in a reconfigurable manner. Different logic functionality of the device is selected by using a current through a nonmagnetic metallic gate, and the resultant Oersted field controls the trajectory of the skyrmion, which in turn determines the logic states. The logic functions are implemented on a ferromagnet/heavy metal bilayer device structure by virtue of several physical effects, such as the spin-orbit torque, skyrmion-edge repulsion, skyrmion-skyrmion topological repulsion, and skyrmion Hall effect. The skyrmion trajectory has been characterized by estimating the skyrmion Hall angle. A wide range of operations by varying the current density, skyrmion velocity, Dzyaloshinskii-Moriya interaction, magnetic anisotropy, and geometrical parameters have been presented in detail. We believe that our spin orbit torque driven logic design will have potential implications for a high-speed and low-power skyrmion based computing architecture. © 2022 American Chemical Society

Large Spin Pumping and Inverse Spin Hall Effect in Ta/Py Bilayer Structures

Spin mixing conductance at the ferromagnet (FM)/heavy metal (HM) interface is a key parameter to estimate the spin current injection into heavy metal. The electrical detection of such injected spin current in heavy metal is possible via inverse spin Hall effect (ISHE). Herein, a large spin mixing conductance in the range of (4.95 (Formula presented.) 6.52) (Formula presented.) 1018m−2 in Ta/permalloy(Py) bilayer structures is reported. In-plane angular-dependent ISHE voltage measurements in Ta/Py samples reveal the dominance of spin pumping voltage component over the other spin rectification contributions. Voltage contribution due to spin pumping and anisotropic magnetoresistance (AMR) increases with the Py thickness due to the enhancement in spin pumping and increase in spin-dependent scattering, respectively. Anomalous Hall effect (AHE) contribution in Py is found to be relatively small. © 2022 Wiley-VCH GmbH

Skyrmion racetrack memory with an antidot

Skyrmion racetrack memory has a lot of potential in future non-volatile solid state devices. By application of current in such devices, both spin-orbit torque and spin-transfer torques are proven to be useful to nucleate and propagate skyrmions. However, the current applied during nucleation of successive skyrmions may have unwanted perturbation viz. Joule heating and the skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore, new methodology is desired to decouple the generation and propagation of skyrmions. Here, we present a novel route via micromagnetic simulations for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local Oersted field. Antidots are holes in a magnetic nanoelement. Multiple skyrmions can be simultaneously generated by incorporating a greater number of antidots. Controlled skyrmion injection can be achieved by tuning the separation between the antidots that are placed at either end of the nanotrack. Here, we propose a novel design to realise skyrmionic racetrcak memory, where one can individually generate and manipulate the skyrmions within the nanotrack. © 2020 IOP Publishing Ltd

Reconfigurable logic via gate controlled domain wall trajectory in magnetic network structure

An all-magnetic logic scheme has the advantages of being non-volatile and energy efficient over the conventional transistor based logic devices. In this work, we present a reconfigurable magnetic logic device which is capable of performing all basic logic operations in a single device. The device exploits the deterministic trajectory of domain wall (DW) in ferromagnetic asymmetric branch structure for obtaining different output combinations. The programmability of the device is achieved by using a current-controlled magnetic gate, which generates a local Oersted field. The field generated at the magnetic gate influences the trajectory of the DW within the structure by exploiting its inherent transverse charge distribution. DW transformation from vortex to transverse configuration close to the output branch plays a pivotal role in governing the DW chirality and hence the output. By simply switching the current direction through the magnetic gate, two universal logic gate functionalities can be obtained in this device. Using magnetic force microscopy imaging and magnetoresistance measurements, all basic logic functionalities are demonstrated.NRF (Natl Research Foundation, S’pore)Published versio