Simultaneous determination of effective spin-orbit torque fields in magnetic structures with in-plane anisotropy

The strength of spin-orbit torque in ferromagnetic structures is characterized by fieldlike and dampinglike effective fields. Conventionally, two distinct measurement approaches are employed to quantify the magnitude of the respective effective fields in structures with in-plane magnetic anisotropy. Here, we propose and demonstrate a self-validating method, which enables simultaneous quantification of both the fieldlike and dampinglike terms in structures with in-plane magnetic anisotropy. An analytical expression is derived and validated by harmonic Hall resistance measurement. Both the fieldlike and dampinglike effective fields are extracted from a single measurement using the derived fitting functions for the second harmonic Hall resistance. The first harmonic Hall resistance acts as a reference to confirm that the experimental parameters are consistent with the derived equations

Gated electric field control on multi-layered structures with perpendicular magnetic anisotropy

The ability to control the properties of ferro-metallic materials after device fabrication is a highly desirable feature for fundamental studies and technological advancements. In semiconductor-based devices, electric field control using a gate voltage to vary the conductivity is well established in commercialized devices. In comparison, the electric field control of spintronic devices is still in its infancy and there is a great deal of research interest surrounding this phenomenon. This thesis presents a comprehensive study and experimental implementation of electric field control on ferro-metallic based spintronic device. Despite the electric field screening effect of metals, electric field control was realized on a multilayer ferro-metallic structure. This modulation of anisotropy energy with electric field broadens the application of electric field control to include thicker multi-layered magnetic films. This allows spintronic devices to have a larger magnetic volume, thereby enhancing the thermal stability of the device. Another investigation on electric field control focuses on the dynamics of the magneto-ionic effect using a polymer as the insulating layer. The results show that thinner polymer layers lead to a faster magneto-ionic effect. This allows the possibility of flexible spintronics devices with high fabrication throughput. In addition, electric field control was employed to assist in the injection of domain walls in a geometrically constrained Hall cross structure. The reduction in the required current density for domain wall injection illustrates the wide range of application for electric field control on spintronic devices. Such applications also include novel techniques proposed in this thesis which manipulates domain walls using electric field control. The proposed technique propagates domain walls deterministically at high velocities without relying on any intrinsic or permanent extrinsic pinning features.Doctor of Philosoph

Investigation of spin-orbit torque performance with W/Cu-multilayers as spin current source

We study the W/Cu multilayers as a spin current source and the coherent spin-orbit torques in a Fe layer using the spin-torque ferromagnetic resonance (STFMR) technique. With increasing numbers of layers, the line shape of the STFMR signals changes from predominantly antisymmetric to predominantly symmetric. When using [W(0.5)/Cu(0.5)]5 as a spin current source, the symmetric amplitude increases by a factor of 5 compared to a single W layer. Simultaneously, the effective damping parameter also increases slightly due to enhanced spin pumping. Along with an increasing trend in the damping-like torque efficiency, this suggests that the extrinsic spin Hall effect is enhanced. Concurrently, the antisymmetric amplitude decreases significantly by a factor of 27, which indicates an increase in the field-like torque when multilayers are used as a spin current source.Agency for Science, Technology and Research (A*STAR)Published versionThis work was supported by the RIE2020 ASTAR AME IAF-ICP grant via Grant No. I1801E0030

Oxide-based RRAM materials for neuromorphic computing

In this review, a comprehensive survey of different oxide-based resistive random-access memories (RRAMs) for neuromorphic computing is provided. We begin with the history of RRAM development, physical mechanism of conduction, fundamental of neuromorphic computing, followed by a review of a variety of RRAM oxide materials (PCMO, HfOx, TaOx, TiOx, NiOx, etc.) with a focus on their application for neuromorphic computing. Our goal is to give a broad review of oxide-based RRAM materials that can be adapted to neuromorphic computing and to help further ongoing research in the field.NRF (Natl Research Foundation, S’pore

Cyclic resistance change in perpendicularly magnetized Co/Ni nanowire induced by alternating current pulse injection

We report on cyclic anisotropic magnetoresistance change induced by current pulse injection in perpendicularly magnetized Co/Ni nanowire. By alternating the polarity of the injection pulse, domain walls (DWs) can be deterministically created and annihilated within the nanowire. The injection induces a combined effect of spin transfer torque and Oersted field that leads to simultaneous creation and driving of DWs in the nanowire. DW created by single pulse injection exhibits a fixed depinning field. For multi-pulse injection, the depinning field increases and this is ascribed to the formation of DWs with opposite chirality.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore

Synaptic element for neuromorphic computing using a magnetic domain wall device with synthetic pinning sites

The ability to make devices that mimic the human brain has been a subject of great interest in scientific research in recent years. Current artificial intelligence algorithms are primarily executed on the von Neumann hardware. This causes a bottleneck in processing speeds and is not energy efficient. In this work, we have demonstrated a synaptic element based on a magnetic domain wall device. The domain wall motion was controlled with the use of synthetic pinning sites, which were introduced by boron (B+) ion-implantation for local modification of the magnetic properties. The magnetization switching process of a Co/Pd multilayer structure with perpendicular magnetic anisotropy was observed by using MagVision Kerr microscopy system. The B+ implantation depth was controlled by varying the thickness of a Ta overcoat layer. The Kerr microscopy results correlate with the electrical measurements of the wire which show multiple resistive states. The control of the domain wall motion with the synthetic pinning sites is demonstrated to be a reliable technique for neuromorphic applications.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Spin reflection-induced field-free magnetization switching in perpendicularly magnetized Mgo/Pt/Co heterostructures

Field-free magnetization switching is critical towards practical, integrated spin-orbit torque (SOT)-driven magnetic random-access memory with perpendicular magnetic anisotropy. Our work proposes a technique to modulate the spin reflection and spin density of states within a heavy-metal Pt through interfacing with a dielectric MgO layer. We demonstrate tunability of the effective out-of-plane spin torque acting on the ferromagnetic Co layer, enabling current-induced SOT magnetization switching without the assistance of an external magnetic field. The influence of the MgO layer thickness on effective SOT efficiency shows saturation at 4 nm, while up to 80% of field-free magnetization switching ratio is achieved with the MgO between 5 and 8 nm. We analyze and attribute the complex interaction to spin reflection at the dielectric/heavy metal interface and spin scattering within the dielectric medium due to interfacial electric fields. Further, through substituting the dielectric with Ti or Pt, we confirm that the MgO layer is indeed responsible for the observed field-free magnetization switching mechanism.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)National Research Foundation (NRF)Submitted/Accepted versionThis work was supported by an Industry-IHL Partnership Program (no. NRF2015-IIP001-001) and an EDB-IPP (grant no. RCA-2019-1376). This work was also supported by the RIE2020 ASTAR AME IAF-ICP (grant no. I1801E0030)

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

Tuning the spin-orbit torque effective fields by varying Pt insertion layer thickness in perpendicularly magnetized Pt/Co/Pt(t)/Ta structures

We experimentally determine that a Pt layer insertion between the Co/Ta interface can effectively negate the field-like spin-orbit torque (SOT) in perpendicularly magnetized Pt/Co/Pt(t)/Ta structures, as the Rashba effects at the Pt/Co and Co/Pt interfaces counteract each other. The damping-like term can be tuned by varying the thickness of the Pt insertion layer to change both the sign and magnitude of the SOT effective fields. An asymmetric SOT contribution from the Pt/Co and Co/Pt interfaces was found, leading to zero damping-like SOT being obtained at different inserted Pt and bottom Pt layer thickness. The ability to change the sign of damping-like SOT allows control over the magnetic switching direction in SOT devices.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 AMEProgrammatic Grant (No. A1687b0033) is also acknowledged. WSL is also a member of the Singapore Spintronics Consortium (SG-SPIN)

Electric field control for energy efficient domain wall injection

Domain wall injection by electric means is an energy exhausting process. This process is conventionally carried out by sending a current pulse through a stripline which generates an Oersted field to locally switch the magnetization in a magnetic wire. In this work, the magnetic properties of the device were modulated by electric control to lower the required current density for DW injection. The proposed DW injection device employs a Hall cross structure which simplifies the device fabrication process and allows a larger Oersted field to be generated at the domain wall injection region. Electrical pulses of 50 ns were sent through the Hall bar to inject domain walls. The formation of the resulting domain walls was detected electrically using the Hall resistance and optically by Kerr microscopy. The results show that the required current density for injection of domain walls is reduced by ∼20% with an applied electric field of +250 MV/m on the Hall cross structure.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)The 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 supports from a RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) and an ASTAR AME Programmatic Grant (No. A1687b0033) is also acknowledged. WSL is a member of the Singapore Spintronics Consortium (SG-SPIN)