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

Domain wall dynamics in ferromagnetic nanostructures

Topological defects of domain walls (DWs) in ferromagnetic materials are of paramount importance as regard to both fundamental physics as well as potential applications in non-volatile memory and spin logic devices. This thesis investigates topological manipulation and dynamics in both in-plane and out-of-plane magnetized nanostructures. For in-plane nanostructures, a technique to control, detect and rectify the topological nature of a transverse DW was proposed and experimentally verified. The topological defects play a crucial role in DWs interaction, leading to mutual annihilation or formation of bound states. This causes stochasticity in the DW generation. A method whereby single DW can be generated deterministically was demonstrated and experimentally verified. Lastly, dynamics of chiral Néel DWs driven by spin orbit torques (SOTs) in perpendicularly magnetized heterostructures were studied. A cross over from the field dominated DW depinning to current dominated DW depinning was established as the magnitude of the current was increased. The SOT in concert with Dzyaloshinskii-Moriya interaction (DMI) was correlated to this anomaly.​Doctor of Philosophy (SPMS

Spintronic heterostructures for artificial intelligence: a materials perspective

With the advent of the Big Data era, neuromorphic computing (NC) (also known as brain-inspired computing) has gained a lot of research interest. Spintronic devices are the emerging candidates for implementing the NC due to their intrinsic nonvolatility, extremely high endurance, low-power consumption, and complementary metal-oxide compatibility. Many research groups have proposed various NC architectures based on spintronic devices. Herein, a collective survey of different spintronic-based approaches is given for NC. The reviewed approaches include the progress of stochastic magnetic tunnel junction (MTJ)devices, spin-torque nano-oscillator, spin-Hall nano-oscillator, domain walls, and skyrmion devices. In all of these approaches, spin–orbit torque (SOT)-based magnetization control, which is achieved via spintronics heterostructures, plays a significant role. Various heterostructures of heavy metal and ferromagnetic layers that have been proposed are reviewed for generating SOT. In addition, the phenomena and materials involved in the generation of orbital torque are summarized due to the orbital Hall effect (OHE), which has recently gained researchers' attention. Finally, an outlook on the opportunities and challenges for spintronic-based NC hardware is provided, shedding light on its great potential for artificial intelligence (AI) applications.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThe authors gratefully acknowledge the funding from the National Research Foundation (NRF), Singapore, for the CRP21 grant (NRF-CRP21-2018-0003). This research is also partially supported by the Ministry of Education, Singapore under its Tier 2 grant MOE-T2EP50122-0023

Ultimate Spin Currents in Commercial Chemical Vapor Deposited Graphene

Establishing ultimate spin current efficiency in graphene over industry-standard substrates can facilitate research and development exploration of spin current functions and spin sensing. At the same time, it can resolve core issues in spin relaxation physics while addressing the skepticism of graphene's practicality for planar spintronic applications. In this work, we reveal an exceptionally long spin communication capability of 45 mu m and highest to date spin diffusion length of 13.6 mu m in graphene on SiO2/Si at room temperature. Employing commercial chemical vapor deposited (CVD) graphene, we show how contact-induced surface charge l transfer doping and device doping contributions, as well as spin relaxation, can be quenched in extremely long spin channels and thereby enable unexpectedly long spin diffusion lengths in polycrystalline CVD graphene. Extensive experiments show enhanced spin transport and precession in multiple longest channels (36 and 45 mu m) that reveal the highest spin lifetime of similar to 2.5-3.5 ns in graphene over SiO2/Si, even under ambient conditions. Such performance, made possible due to our devices approaching the intrinsic spin-orbit coupling of similar to 20 mu eV in graphene, reveals the role of the D'yakonov-Perel' spin relaxation mechanism lin graphene channels as well as contact regions. Our record demonstration, fresh device engineering, and spin relaxation insights unlock the ultimate spin current capabilities of graphene on SiO2/Si, while the robust high performance of commercial CVD graphene can proliferate research and development of innovative spin sensors and spin computing circuits

Experimental advances in charge and spin transport in chemical vapor deposited graphene

Despite structural and processing-induced imperfections, wafer-scale chemical vapor deposited (CVD) graphene today is commercially available and has emerged as a versatile form that can be readily transferred to desired substrates for various nanoelectronic and spintronic applications. In particular, over the past decade, significant advancements in CVD graphene synthesis methods and experiments realizing high-quality charge and spin transport have been achieved. These include growth of large-grain graphene, new processing methods, high-quality electrical transport with high-carrier mobility, micron-scale ballistic transport, observations of quantum and fractional quantum Hall effect, as well as the spintronic performance of extremely long spin communication over tens of micrometers at room temperature with robust spin diffusion lengths and spin lifetimes. In this short review, we discuss the progress in recent years in the synthesis of high-quality, large-scale CVD graphene and improvement of the electrical and spin transport performance, particularly towards achieving ballistic and long-distance spin transport that show exceptional promise for next-generation graphene electronic and spintronic applications

Bi-directional high speed domain wall motion in perpendicular magnetic anisotropy Co/Pt double stack structures

We report bi-directional domain wall (DW) motion along and against current flow direction in Co/Pt double stack wires with Ta capping. The bi-directionality is achieved by application of hard-axis magnetic field favoring and opposing the Dzyloshinskii-Moriya interaction (DMI), respectively. The speed obtained is enhanced when the hard-axis field favors the DMI and is along the current flow direction. Co/Pt double stack is a modification proposed for the high spin-orbit torque strength Pt/Co/Ta stack, to improve its thermal stability and perpendicular magnetic anisotropy (PMA). The velocity obtained reduces with increase in Pt spacer thickness due to reduction in DMI and enhances on increasing the Ta capping thickness due to higher SOT strength. The velocity obtained is as high as 530 m/s at a reasonable current density of 1 × 1012 A/m2 for device applications. The low anisotropy of the device coupled with the application of hard-axis field aids the velocity enhancement by preventing Walker breakdown.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Diode characteristics in magnetic domain wall devices via geometrical pinning for neuromorphic computing

Neuromorphic computing (NC) is considered a potential vehicle for implementing energy-efficient artificial intelligence. To realize NC, several technologies are being investigated. Among them, the spin−orbit torque (SOT)- driven domain wall (DW) devices are one of the potential candidates. Researchers have proposed different device designs to achieve neurons and synapses, the building blocks of NC. However, the experimental realization of DW device-based NC is only at the primeval stage. Here, we have studied pine-tree DW devices, based on the Laplace pressure on the elastic DWs, for achieving synaptic functionalities and diode-like characteristics. We demonstrate an asymmetric pinning strength for DW motion in two opposite directions to show the potential of these 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 provides a strategy to fabricate a multifunctional DW device, exhibiting synaptic properties and diode characteristics.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)Submitted/Accepted versionThe authors gratefully acknowledge the National Research Foundation (NRF), Singapore, for the CRP21 grant (NRFCRP21-2018-0003). They also acknowledge the support provided by Agency for Science, Technology and Research, A*STAR RIE2020 AME Grant No. A18A6b0057 for this work. H.R. thanks the NTU research scholarship for carrying out research at NTU

Combined Bottom-Up and Top-Down Approach for Highly Ordered One-Dimensional Composite Nanostructures for Spin Insulatronics

Engineering magnetic proximity effects-based devices requires developing efficient magnetic insulators. In particular, insulators, where magnetic phases show dramatic changes in texture on the nanometric level, could allow us to tune the proximity-induced exchange splitting at such distances. In this paper, we report the fabrication and characterization of highly ordered two-dimensional arrays of LaFeO3 (LFO)-CoFe2O4 (CFO) biphasic magnetic nanowires, grown on silicon substrates using a unique combination of bottom-up and top-down synthesis approaches. The regularity of the patterns was confirmed using atomic force microscopy and scanning electron microscopy techniques, whereas magnetic force microscopy images established the magnetic homogeneity of the patterned nanowires and absence of any magnetic debris between the wires. Transmission electron microscopy shows a close spatial correlation between the LFO and CFO phases, indicating strong grain-to-grain interfacial coupling, intrinsically different from the usual core-shell structures. Magnetic hysteresis loops reveal the ferrimagnetic nature of the composites up to room temperature and the presence of a strong magnetic coupling between the two phases, and electrical transport measurements demonstrate the strong insulating behavior of the LFO-CFO composite, which is found to be governed by Mottvariable range hopping conduction mechanisms. A shift in the Raman modes in the composite sample compared to those of pure CFO suggests the existence of strain-mediated elastic coupling between the two phases in the composite sample. Our work offers ordered composite nanowires with strong interfacial coupling between the two phases that can be directly integrated for developing multiphase spin insulatronic devices and emergent magnetic interfaces