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Skyrmions are found to be promising for next generation energy efficient spintronic applications. Moreover, ultrafast skyrmion dynamics in GHz-band offer an excellent opportunity to exploit such topologically protected nanostructures in high frequency applications. Here, we present a systematic investigation on the microwave properties of the skyrmions in concentric and eccentric ring structures using micromagnetic simulations. Two gyrotropic modes with clockwise and counter-clockwise 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 an additional functionality of the skyrmions based on their tunable microwave properties which may have potential implications in the field of miniaturized reconfigurable microwave devices.

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.

Heavy metal–ferromagnet bilayer structures have attracted great research interest for charge-to-spin interconversion. In this work, we investigated the effect of the permalloy (Py) seed layer on the tantalum (Ta) polycrystalline phase and its spin Hall angle. Interestingly, for the same deposition rates the crystalline phase of Ta deposited on the Py seed layer strongly depends on the thickness of the seed layer. We observed a phase transition from α-Ta to (α + β)-Ta while increasing the Py seed layer thickness. The observed phase transition is attributed to the strain at the interface between the Py and Ta layers. Ferromagnetic resonance-based spin pumping studies reveal that the spin-mixing conductance in the (α + β)-Ta is relatively higher as compared to the α-Ta. Spin Hall angles of α-Ta and (α + β)-Ta are obtained from the inverse spin Hall effect (ISHE) measurements. The spin Hall angle of (α + β)-Ta is estimated to be θSH = −0.15 ± 0.009 which is relatively higher than that of the α-Ta. Our systematic results connecting the phase of Ta with the seed layer and its effect on the efficiency of spin to charge conversion might resolve ambiguities across various literature and open up new functionalities based on the growth process for emerging spintronic devices.

The investigation of the gigahertz dynamics of a skyrmion—a topologically protected chiral spin texture, offers several applications in high-frequency magnonic devices. In this work, we have investigated the motion and microwave resonant dynamics of skyrmions in an engineered nanostructure using micromagnetic simulations. The structure is a tapered ferromagnetic/heavy metal bilayer with notches to stabilize the skyrmion for dynamic studies. The skyrmion is moved along the nanostructure via current-induced spin–orbit torques. Multiple stable skyrmion states were demonstrated in the structure, where a remarkable tunability of 6 GHz is observed between the initial and the final state. This large shift in the resonant frequency has been attributed to the variation in the size of the skyrmion at various locations in the engineered nanostructure. The dependency of the skyrmion velocity and their microwave responses on the DzyalosnDzyaloshinskii–Moriya interaction strength and the perpendicular magnetic anisotropy is described. The calculation of the skyrmion Hall angle shed light on the motion and stability of the skyrmion in the structure.

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.

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.

Researchers present a method of phase modulation and phase-shift keying for spin-torque nano-oscillators (STNOs), and demonstrate transmission of a voice signal (telephony) with their STNO-based gear. The key elements of the proposed method are the ability of an STNO to be synchronized by an external signal, and to shift its phase relative to the source's phase according to an additional control signal. These properties allow not only control of the phase, but also reduction of phase noise, which is a main issue for all oscillators with nanoscale dimensions. The results comprise a significant advance for wireless data and signal communication with nano-oscillators.

Traditional micro-electronics work by controlling the flow of electron charge through transistor switches. Spintronics, which exploits the spin degree of freedom of electron, can lead to devices that outstrip the performance of traditional semiconductor technology. The spin moment in magnetic nanostructures by virtue of their inherent non-volatility can potentially offer the opportunity of ultra-low power and high speed devices. This chapter describes proof-of-concept devices that utilize magnetic domain walls in nanowire network to perform logical operations. A magnetic reconfigurable logic device is discussed, whose operation is mediated via domain wall motion. The logical operation can be programmed at run-time and the whole booelan logic family, from simple NOT to complex operations such as NAND can be obtained. The second part of the chapter explores the feasibility of encoding logical bit within the internal state of domain wall, leading to mobile data bits. Simple single to double bits logical operations are demonstrated.

The production of high entropy alloy-based nanocomposites is an exciting yet challenging area in terms of its scalability and industrial applications. Here we developed graphene oxide (GO) reinforced FeCoNiCuZn high entropy alloy (HEA) nanocomposites through an electrochemical approach using aqueous medium in a single step. Transmission electron microscopy observations confirmed uniformly distributed nanocrystalline dual FCC phase quinary alloy nanoparticles throughout the GO layers. On the other hand, the presence of GO affects the electrochemical reduction of multiple elements during alloy formation in the deposition process, which often leads to dual phases with slight deviations in alloy composition, unlike the pure metal-GO composites. Additionally, incorporation of GO has not shown any effect on the ferromagnetic nature of FeCoNiCuZn HEA with saturation magnetization (M s ) ∼ 43.5 emu g−1. The obtained saturation magnetization is relatively higher compared to the existing reported magnetic nanoparticles with GO. Hence, this technique shows its potential applicability and provides an old technique yet a new approach for synthesizing GO-HEA nanocomposites for various magnetic applications.

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.

A magnetic skyrmion is a topological object having particle-spin like configuration, which can be stabilized in ferromagnetic systems with Dzyaloshinskii-Moriya interaction. In this report, we investigate tunable microwave properties of magnetic skyrmions in a circular nanodisk using micromagnetic simulations. An in-plane ac magnetic field excites gyrotropic dynamics with a clockwise and a counter-clockwise mode. We have shown the dependence of microwave responses on the size of the skyrmion by tuning the Dzyaloshinskii-Moriya interaction, magnetic anisotropy, and out-of-plane external bias magnetic field. A remarkably large frequency shift of 4.2 GHz has been observed for a change of 40 mT bias field. Furthermore, a broad microwave tunability of 10.8 GHz has been shown with a variation of magnetic anisotropy of 1 × 105 J/m3. The manifestation of inertial mass of the skyrmion, its dependence on the resonance responses and the sizes of the skyrmion are explained by using Thiele’s equation. The inertial mass of the skyrmion is estimated to be in the order of 10−23 kg. The results open up the potential use of the skyrmions in reconfigurable nano-magnonic devices with large tunability.

We demonstrate spin-orbit torque (SOT) driven multi-state magnetization switching in Co/Pt Hall crosses in the presence of varying externally applied in-plane fields from room temperature (RT = 295 K) to 360 K. In-situ Kerr imaging at various resistance states reveal the evolution of up and down magnetic domain expansion due to current-induced SOT switching. The control of magnetization states in the Hall cross is shown to be attributed to the non-uniform current-density and the resultant effective out-of-plane field due to the device geometry. The critical switching current density scales inversely with device temperature, and the SOT-driven change in Hall resistance varies across device temperatures and the applied in-plane fields. Additionally, the current-induced SOT efficiency, χsat, and the Dzyaloshinskii-Moriya interaction effective field, HDMI, at RT and 360 K are determined using the chiral domain wall model and current-induced loop-shift method. The χsat and HDMI values are found to decrease by ~15% and ~26%, respectively, with increasing device temperature. These results demonstrate the thermal sensitivity of current-induced SOT magnetization switching in multi-state devices. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Accepted version The work was supported by the Singapore National Research Foundation, under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011-01), and an Industry-IHL Partnership Program (NRF2015- IIP001-001). The support from an RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) is also acknowledged.

While ``spin Hall'' heavy metals like Pt have been widely studied for generating spin-orbit torque (SOT) in spintronics, alternatives are also sought. Rare-earth metals are thought to have potential here, but their efficiency remains uncertain. Thus the authors use an innovative technique to measure the effective fields and SOT switching efficiency generated in rare-earth-bearing magnetic multilayers. They find significant enhancement of both dampinglike and fieldlike torques, for enhanced switching efficiency with increasing thickness of a Tb layer, plus a strong angular dependence of both torques. This enhancement indicates that rare-earth metals are promising for spin-orbitronic devices.

We demonstrate the modulation of spin-orbit torque (SOT) fields in synthetic antiferromagnetic (SAF) structures sandwiched between two heavy metals of opposite spin Hall angles. The SOTs fields that are measured by using harmonic Hall voltage technique, increase with net areal magnetization of the Pt/SAF/Ta structures. The result suggests an antiparallel orientation of the SOT fields in the two ferromagnetic layers. The antiparallel alignment of the SOT fields switches the magnetization of SAF structures with a current density as low as 2.3 × 1011 A/m2. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) This work was supported by the Singapore National Research Foundation, Prime Minister’s Office under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011-01), and an Industry-IHL Partnership Program (NRF2015- IIP001-001). The support from an RIE2020 AME-Programmatic Grant (No. A1687b0033) is also acknowledged. WSL is also a member of the Singapore Spintronics Consortium (SG-SPIN).

In the last two decades Spintronics has been an important driver for Information Technologies with the introduction in 1996 of magneto-resistive field sensors in data storage and in 2006 of magnetic random access memories. Spintronics can also bring alternative solutions for rf components used in Information-Communication Technologies. By virtue of their specific spin polarized transport properties spintronic devices become active, multifunctional, compact, low power, low cost, CMOS compatible rf components that combine functions of microwave signal generation (DC-to-RF converter), modulation and frequency selective microwave signal detection (RF-to-DC converter). In this presentation an overview will be provided for the realization of rf spintronic components and their corresponding rf properties with respect to different functions such as signal generation, modulation and detection. The results reported here were obtained in a collaborative effort within the FP7 project MOSAIC [1] including different laboratories SPINTEC-UGA/CEA/CNRS (F); CEA-LETI (F); INL, Braga (Pt); TUD (G); UMPhy CNRS/THALES (F); THALES TRT (F)

A dc current flowing through a magnetic tunnel junction (MTJ) can produce auto-oscillations in microwave range when the current density is relatively high due to spin transfer torque (STT) phenomenon [1].

Magnetic multi-component alloys (MCAs) are the new class of multicomponent alloys where, combination of magnetic properties can be tuned with the composition. Due to their significant performance, MCAs have been proposed as the most appropriate choice of materials for several magnetic based sensor applications such as magnetoresistive sensors, etc. At times, for each application, multilayers with specific combination of magnetic properties or morphology throughout the material is required. In such cases, composition of alloying elements plays a significant role in the magnetic behaviour. Electrochemical approach in aqueous media is the most cost-effective method and favours better control over the composition with the help of pulse parameters. Therefore, major focus of the present work is to replace the non-aqueous/organic electrolytes with aqueous electrolytes for synthesizing the MCAs with five and/or six elements through electrodeposition. To the best of our knowledge, there haven’t been any reports on deposition of five principle alloying elements by an electrochemical approach in a single step using an aqueous media. The present study reveals the details on electrochemical deposition of multicomponent alloy thin films (Fe, Co, Ni, X, Y) for magnetic sensor applications. Firstly, a single step approach to develop nanocrystalline Fe-Co-Ni-X-Y (X, Y: Transition elements) MCA thin films is presented. Subsequently, the control over composition of MCA thin films and preliminary studies on their magnetic behaviour will be discussed. Finally, tuning of the magnetic properties of MCAs, such as saturation magnetization, magnetic permeability, and coercivity with respect to composition will be discussed. Based on the performance, these MCA thin films will be categorized for specific sensor applications. As it is important to understand the microstructure and composition of these MCA thin films, preliminary characterisation has been carried out using X-ray diffraction, scanning electron microscopy. For transmission electron microscopy (TEM), specimens have been prepared using focused ion beam (FIB) and analytical scanning/transmission electron microscopy including energy dispersive spectroscopy and electron energy loss spectroscopy have been carried out. The magnetization reversal curves for the MCA thin films were acquired initially using Kerr microscopy and detailed magnetic characterization will be carried out using physical property measurement system (PPMS) with a vibrating sample magnetometer. All the comprehensive details of this study will be discussed during the presentation.

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 version

Controlling the domain wall (DW) trajectory in magnetic network structures is crucial for spin-based device related applications. The understanding of DW dynamics in network structures is also important for study of fundamental properties like observation of magnetic monopoles at room temperature in artificial spin ice lattice. The trajectory of DW in magnetic network structures has been shown to be chirality dependent. However, the DW chirality periodically oscillates as it propagates a distance longer than its fidelity length due to Walker breakdown phenomenon. This leads to a stochastic behavior in the DW propagation through the network structure. In this study, we show that the DW trajectory can be deterministically controlled in the magnetic network structures irrespective of its chirality by introducing a potential barrier. The DW propagation in the network structure is governed by the geometrically induced potential barrier and pinning strength against the propagation. This technique can be extended for controlling the trajectory of magnetic charge carriers in an artificial spin ice lattice. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

Fundamental properties like observation of magnetic monopoles in artificial spin ice lattice [1] depends on the underlying dynamics of magnetic domain walls (DWs) in geometrically frustrated structures. It has been reported previously that the DW follows a chirality dependent path in magnetic network structures [2-4]. This concept has been applied to generate 1D Dirac strings in artificial Kagome type lattice [3]. Thus the deterministic trajectory of magnetic charge carriers in spinice structure and observation of magnetic monopoles depends on the robustness of the chirality of the injected DW. However above a critical field (Walker field), the DW changes its chirality after propagating a distance larger that its fidelity length [5]. This leads to inherent stochasticity in the trajectory of the DW. To overcome this issue, we introduce an asymmetry in the magnetic network structure. We demonstrate with the aid of magnetic force microscopy (MFM) imaging that the DW is constrained to move along one of the two branches irrespective of its chirality. This study can be extended to define trajectory of magnetic charge carriers in artificial spin ice lattice.

Domain walls (DWs) in magnetic nanowire have been recognised to have the potential to be used in ultra-high density storage and non-volatile logic devices. For actual applications, the creation of DW on the nanosecond timescale and perfect control of its dynamics is critical. The existing method for creating DW in which local Oersted field is used, two DWs are always nucleated; a head-to-head (HH) and a tail-to-tail (TT) DWs. Due to the intrinsic characteristics of these DWs, they either annihilate or form a coupled DW(1-3). This leads to the stochastic behaviour of DW generation(1). It has been previously reported that the minimum separation of two DWs for their stable existence is approximately 2.6μm(4). Therefore, for strip line width less than 2.6μm there is a very high probability for mutual annihilation of the two generated DWs if no external magnetic field is applied to drive them apart.

The anisotropic magnetoresistance of symmetrical Py (Ni 81 Fe 19 ) nanowire network reflects the local magnetization dynamics via the bifurcated dual-branch.[1] The magnetoresistance (MR) would be useful for the development of logic devices utilizing magneto-electric binary bits.[2] Furthermore, the unique magnetization dynamics is able to assist in the understanding of artificial spin ice structures to be a complex integrated magneto-resistive circuit comprising of the bifurcated dual-branch bases.[3]

Domain wall (DW) based logic and memory devices require precise control and manipulation of DW in nanowire conduits. The topological defects of Transverse DWs (TDW) are of paramount importance as regards to the deterministic pinning and movement of DW within complex networks of conduits. In-situ control of the DW topological defects in nanowire conduits may pave the way for novel DW logic applications. In this work, we present a geometrical modulation along a nanowire conduit, which allows for the topological rectification/inversion of TDW in nanowires. This is achieved by exploiting the controlled relaxation of the TDW within an angled rectangle. Direct evidence of the logical operation is obtained via magnetic force microscopy measurement.

International audience; Spin torque nano-oscillators are nanoscopic microwave frequency generators which excel due totheir large frequency tuning range and agility for amplitude and frequency modulation. Due to theircompactness, they are regarded as suitable candidates for applications in wireless communications,where cost-effective and complementary metal-oxide semiconductor-compatible standalone devicesare required. In this work, we study the ability of a magnetic-tunnel-junction based spin torquenano-oscillator to respond to a binary input sequence encoded in a square-shaped current pulse forits application as a frequency-shift-keying (FSK) based emitter. We demonstrate that below thelimit imposed by the spin torque nano-oscillator intrinsic relaxation frequency, an agile variationbetween discrete oscillator states is possible. For this kind of devices, we demonstrate FSK up to datarates of 400 Mbps, which is well suited for the application of such oscillators in wireless networks

Hall cross structures in magnetic nanowires are commonly used for electrical detection of magnetization reversal in which a domain wall (DW) is conventionally nucleated by a local Oersted field. In this letter, we demonstrate DW nucleation in Co/Ni perpendicular magnetic anisotropy nanowire at the magnetic Hall cross junction. The DWs are nucleated by applying an in-plane pulsed current through the nanowire without the need of a local Oersted field. The change in Hall resistance, detected using anomalous Hall effect, is governed by the magnetic volume switched at the Hall junction, which can be tuned by varying the magnitude of the applied current density and pulse width. The nucleated DWs are driven simultaneously under the spin transfer torque effect when the applied current density is above a threshold. The possibility of multiple DW generation and variation in magnetic volume switched makes nucleation process stochastic in nature. The in-plane current induced stochastic nature of DW generation may find applications in random number generation.

We report on transverse domain wall (DW) dynamics in two closely spaced cylindrical nanowires. The magnetostatically coupled DWs are shown to undergo an intrinsic oscillatory motion along the nanowire length in addition to their default rotational motion. In the absence of external forces, the amplitude of the DW oscillation is governed by the change in the frequency of the DW rotation. It is possible to sustain the DW oscillations by applying spin-polarized current to the nanowires to balance the repulsive magnetostatic coupling. The current density required to sustain the DW oscillation is found to be in the order of 105 A/cm2. Morover, our analysis of the oscillation reveals that the DWs in cylindrical nanowires possess a finite mass. Published version

We report on the magnetization dynamics at a bifurcation in a dual-branch magnetic network structure. When a transverse domain wall (DW) propagates through the network, interaction with an edge defect at the bifurcation leads to the transformation of the DW from transverse to vortex. The topological charge is conserved as the DW moves through the bifurcation, and this charge conservation is intrinsically linked to a −1/2 topological defect in the system. Magnetic force microscopy (MFM) imaging enables the direct observation of defect displacement during DW transformation, which induces a selective switching in the branch of the network structure.

We present a method to drive multiple domain walls in the absence of direct current application in a coupled nanowire system. The domain walls were driven by a combination of remote coupling and exchange repulsion force from the domain wall compressions. The domain walls were compressed as they were unable to annihilate each other due to having similar topological charges. The compressions are present between the subsequent domain walls, which allow them to be driven as a group in the coupled nanowire system. Published version