Search

Current-induced magnetization switching by spin-orbit torque generated in heavy metals offers an enticing realm for energy-efficient memory and logic devices. The spin Hall efficiency is a key parameter in describing the generation of spin current. Recent findings have reported enhancement of spin Hall efficiency by mechanical strain, but its origin remains elusive. Here, we demonstrate a 45% increase in spin Hall efficiency in the platinum/cobalt (Pt/Co) bilayer, of which 78% of the enhancement was preserved even after the strain was removed. Spin transparency and X-ray magnetic circular dichroism revealed that the enhancement was attributed to a bulk effect in the Pt layer. This was further confirmed by the linear relationship between the spin Hall efficiency and resistivity, which indicates an increase in skew-scattering. These findings shed light on the origin of enhancement and are promising in shaping future utilization of mechanical strain for energy-efficient devices. Agency for Science, Technology and Research (A*STAR) Economic Development Board (EDB) National Research Foundation (NRF) Submitted/Accepted version This work is supported by an Industry-IHL Partnership Program (NRF2015-IIP001-001) and an EDB-IPP (RCA − 17/284) grant. This work is also supported by the RIE2020 ASTAR AME IAF-ICP grant (No. I1801E0030). W.Z. and P.K.J.W. acknowledge financial support by the Fundamental Research Funds for the Central Universities.

Magnetic domain wall memory technology, wherein the information is stored in magnetic domains of multiple magnetic nanowires, is a potential concept proposed to store the large amount of digital data in the near future, which is generated due to the widespread use of social media and computing devices. However, one of the technological challenges which remains to be solved in domain wall memory is the controllable pinning of the domain walls at the nanometer scale. Here, we demonstrate the possibility to stabilize domain walls with nanoscale modification of magnetic properties by using thermal diffusion of elements from crossbar configuration. We have inspected and evaluated the magnetic properties of the nanowires using Kerr microscopy. The pinning field induced by Cr diffusion of our Ni80Fe20 nanowire was estimated to be about 8 kA/m as determined from minor loop (magnetoresistance vs. magnetic field) measurements. The proposed concept can potentially be used in future domain wall memory applications. © 2018 Elsevier B.V. We gratefully acknowledge Nanyang Technological University Start-Up Grant, AcRF-Tier 1 grant RG163/15 of Ministry of Education Singapore, and NTU-JSPS grant offered by Nanyang Technological University, Singapore and Japan Society for the Promotion of Science for the funding and support of this research.

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).

Spin transfer torque magnetoresistive random access memory (STT-MRAM) has been recognized to be the most promising non-volatile memory technology for future technology nodes. STT-MRAM utilizes an array of magnetic tunnel junctions (MTJ) as its storage elements, which in its rudimentary form consists of two ferromagnetic electrodes sandwiching an insulating oxide. Ferromagnetic materials with perpendicular magnetic anisotropy (PMA) are found to offer higher thermal stability, excellent scalability and lower write current requirement as compared to ferromagnetic materials with in-plane anisotropy (IMA). However, the complexity behind creating materials with high PMA often means that compromises have to be made in order to satisfy all requirements simultaneously. Therefore, there remains a need to continue material research to ensure that STT-MRAM is compatible with the complementary metal-oxide-semiconductor backend-of-line (CMOS BEOL) processes. The objective of this thesis is to investigate different materials that are able to induce PMA while remaining relevant to contemporary MRAM applications. In this thesis, we focused on studying three different aspects within the MTJ stack that utilizes materials with PMA (denoted as pMTJ). In Chapter 3, Ho was observed to be a suitable candidate as a seed layer for Co/Pt multilayer as it can achieve hcp structure at 400°C annealing temperature appropriate for fcc-Co/Pt growth. In Chapter 4, amorphous Tb is used to replace Ta as an ultra-thin text-breaking coupling layer owing to its thermal robustness and strong exchange coupling. In Chapter 5, we studied how the limitation of Ta diffusion into the tunnel barrier can improve the thermal stability of the MTJ. Moreover, we observe that at elevated temperatures, the effective anisotropy field, Heff, decays at an increased rate as compared to saturation magnetization, Ms. This offers insight on the temperature dependence of thermal stability at standard MRAM operating conditions as well as optimization strategy beyond the 20nm technology node. Doctor of Philosophy

High temperature studies of spin Hall effect have often been neglected despite its profound significance in real-world devices. In this work, high temperature spin torque ferromagnetic resonance measurement was performed to evaluate the effects of temperature on the Gilbert damping and spin Hall efficiency of PtxCu1−x. When the temperature was varied from 300 K to 407 K, the Gilbert damping was relatively stable with a change of 4% at composition x = 66%. Alloying Pt and Cu improved the spin Hall efficiency of Pt75Cu25/Co/Ta by 29% to a value of 0.31 ± 0.03 at 407 K. However, the critical switching current density is dependent on the ratio between the Gilbert damping and spin Hall efficiency and the smallest value was observed when x = 47%. It was found that at this concentration, the spin transparency was at its highest at 0.85 ± 0.09 hence indicating the importance of interfacial transparency for energy efficient devices at elevated temperature. Agency for Science, Technology and Research (A*STAR) Economic Development Board (EDB) National Research Foundation (NRF) Published 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 EDB-IPP Program (RCA -17/284) and a RIE2020 AME IAF-ICP Grant (No. I1801E0030) are also acknowledged.

We propose the use of mechanical strain and mild annealing to achieve reversible modulation of spin-orbit torque (SOT) and Gilbert damping parameter. X-ray diffraction results show that the residual spin-orbit torque enhancement and Gilbert damping reduction, due to the post-mechanical strain treatment, can be reset using mild annealing to alleviate the internal strain. The spin Hall efficiency of the heat- and strain-treated Pt/Co bilayer was characterized through spin-torque ferromagnetic resonance, and it was found that the device could switch between the strain enhanced SOT and the pristine state. The Gilbert damping parameter behaves inversely with the spin Hall efficiency, and therefore, strain can be used to easily tune the device switching current density by a factor of ∼2 from its pristine state. Furthermore, the resonance frequency of the Pt/Co bilayer could be tuned using purely mechanical strain, and from the endurance test, the Pt/Co device can be reversibly manipulated over 104 cycles demonstrating its robustness as a flexible device. Agency for Science, Technology and Research (A*STAR) Economic Development Board (EDB) Published version This work was supported by an Industry-IHL Partnership Program (No. NRF2015-IIP001-001) and an EDB-IPP (Grant No. RCA-17/284). This work was also supported by the RIE2020 ASTAR AME IAF-ICP Grant No. I1801E0030.

Precise control of domain wall displacement in nanowires is essential for application in domain wall based memory and logic devices. Currently, domain walls are pinned by creating topographical notches fabricated by lithography. In this paper, we propose localized diffusion of non-magnetic metal into ferromagnetic nanowires by annealing induced mixing as a non-topographical approach to form pinning sites. As a first step to prove this new approach, magnetodynamic properties of permalloy (Ni80Fe20) films coated with different capping layers such as Ta, Cr, Cu and Ru were investigated. Ferromagnetic resonance (FMR), and anisotropy magnetoresistance (AMR) measurements were carried out after annealing the samples at different temperatures (T an ). The saturation magnetization of Ni80Fe20 film decreased, and damping constant increased with T an . X-Ray photoelectron spectroscopy results confirmed increased diffusion of Cr into the middle of Ni80Fe20 layers with T an . The resistance vs magnetic field measurements on nanowires showed intriguing results.

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.

In this study, we report that perpendicular magnetic anisotropy (PMA) in Co/Pt multilayers can be achieved by using hcp-Ho(1 0 0) as the seed layer after annealing at 400 °C. M-H hysteresis loops show that the annealing duration required to achieve optimal PMA in Co/Pt multilayers increases monotonically with the Ho seed layer thickness. XRD measurements reveal that Ho transits from amorphous state to hcp structure after annealing at 400 °C, leading to the formation of fcc-Co/Pt(1 1 1). A larger retention of saturation magnetization is also observed when Ho is used as the seed layer as compared to Ru or Pt, which is ascribed to the suppression of interlayer diffusion. This can be attributed to the large grain size of Ho based on the full-width-half-maximum (FWHM) of the Ho peak from XRD results. Synthetic antiferromagnetic (SAF) structures using Ho as a seed layer also demonstrated an exchange coupling strength of Jₑₓ ≈ 1.05 erg/cm² when the thickness of the Ru coupling layer is 0.4 nm. 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 support from an RIE2020 AME-Programmatic Grant (No. A1687b0033) and an IAF-ICP Grant (No. I1801E0030) is also acknowledged. WSL is also a member of the Singapore Spintronics Consortium (SG-SPIN).

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.

A combination of the harmonic measurement and in situ Kerr imaging was used to experimentally determine the spin-orbit (SO) effective fields in a MgO/CoFeB/Ta structure. Here, we evaluate the SO effective fields through an analytical energy approach by transforming the anomalous Hall effect and planar Hall effect (PHE) voltage into a field dependency while imaging the magnetisation behaviour by differential Kerr microscopy. The analytical fitting to the measurement data indicates the significant coexistence of both a transverse field, HT, and longitudinal field, HL, in the longitudinal (H L = -12 Oe, H T = 8 Oe per 106 A cm-2) and transverse (H L = -12 Oe, H T = -17 Oe per 106 A cm-2) measurement schemes, respectively, due to the PHE. Additionally, dendritic-like domains, indicating the influence of the interfacial Dzyaloshinskii-Moriya interaction (DMI) at the CoFeB/Ta interface, were observed by in situ Kerr imaging. Micromagnetic simulations confirm the dendritic domain formation and edge tilting of the magnetisation, as being due to the DMI. 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).

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. MOE (Min. of Education, S’pore) NRF (Natl Research Foundation, S’pore) Published version