Shape-engineered ferromagnets and micromagnetic simulation techniques for spin-transfer-torque random access memory

Spin-transfer-torque random access memory (STTRAM) has received great attention as a prospective universal memory due to high speed read and write capabilities, scalability to smaller technology nodes and non-volatile data retention. Two major factors that could limit the performance of large scale STTRAM arrays are the high switching current and the stochastic switching behavior. In this work, possible routes to mitigate these issues have been explored and new techniques have been proposed to estimate the reliability of the write process. Large area of the selection transistor required to support high switching current impacts the bit storage density of an STTRAM memory array. To increase the bit storage density, a multi-state STTRAM cell employing a cross-shaped ferromagnet was proposed previously. Here, the spin-transfer-torque (STT) driven mag-netization dynamics of the cross-shaped ferromagnet is revisited. As a low power alternative, voltage controlled magnetic anisotropy (VCMA) based writing scheme is studied. Trade-offs and limitations of the VCMA-induced switching over STT are also discussed. In the next part of this dissertation, magnetic properties and magnetization process of epitaxial chromium telluride thin films have been studied. Presence of strong perpendicular magnetic anisotropy in this material makes it an attractive choice for device applications. In this work, anisotropy energies of chromium telluride thin films have been estimated from magnetization measurements. The magnetization reversal process is then studied using analytical models as well as micromagnetic simulations. The last part of this work focuses on the write error rates (WER) of STTRAM. The stochastic write process of STTRAM at finite temperatures gives rise to write errors when a bit fails to switch within the duration of the write pulse. Ultra-low WER on the scale of 10⁻⁹ or less are desired for practical applications. Micromagnetic simulations are required to capture spatially-incoherent magnetization dynamics inside a ferromagnet, which may effect the WER. In this work, using the techniques of rare event enhancement, reliable calculation of WERs to 10⁻⁹ is demonstrated while keeping the computational effort to a minimum. Employing rare-event-enhanced micromagnetic simulations, WERs of both perpendicular and in-plane STTRAM bits are calculated and effects of spatially-incoherent excitations on the WER slopes are discussed.Electrical and Computer Engineerin

The 2022 magneto-optics roadmap

Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today's magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton–Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light–matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future

Chirality as Generalized Spin-Orbit Interaction in Spintronics

This review focuses on the chirality observed in the excited states of the magnetic order, dielectrics, and conductors that hold transverse spins when they are evanescent. Even without any relativistic effect, the transverse spin of the evanescent waves are locked to the momentum and the surface normal of their propagation plane. This chirality thereby acts as a generalized spin-orbit interaction, which leads to the discovery of various chiral interactions between magnetic, phononic, electronic, photonic, and plasmonic excitations in spintronics that mediate the excitation of quasiparticles into a single direction, leading to phenomena such as chiral spin and phonon pumping, chiral spin Seebeck, spin skin, magnonic trap, magnon Doppler, and spin diode effects. Intriguing analogies with electric counterparts in the nano-optics and plasmonics exist. After a brief review of the concepts of chirality that characterize the ground state chiral magnetic textures and chirally coupled magnets in spintronics, we turn to the chiral phenomena of excited states. We present a unified electrodynamic picture for dynamical chirality in spintronics in terms of generalized spin-orbit interaction and compare it with that in nano-optics and plasmonics. Based on the general theory, we subsequently review the theoretical progress and experimental evidence of chiral interaction, as well as the near-field transfer of the transverse spins, between various excitations in magnetic, photonic, electronic and phononic nanostructures at GHz time scales. We provide a perspective for future research before concluding this article.Comment: 136 pages, 60 figure

A Comprehensive Study of Magnetic and Magnetotransport Properties of Complex Ferromagnetic/Antiferromagnetic- IrMn-Based Heterostructures

Manipulation of ferromagnetic (FM) spins (and spin textures) using an antiferromagnet (AFM) as an active element in exchange coupled AFM/FM heterostructures is a promising branch of spintronics. Recent ground-breaking experimental demonstrations, such as electrical manipulation of the interfacial exchange coupling and FM spins, as well as ultrafast control of the interfacial exchange-coupling torque in AFM/FM heterostructures, have paved the way towards ultrafast spintronic devices for data storage and neuromorphic computing device applications.[5,6] To achieve electrical manipulation of FM spins, AFMs offer an efficient alternative to passive heavy metal electrodes (e.g., Pt, Pd, W, and Ta) for converting charge current to pure spin current. However, AFM thin films are often integrated into complex heterostructured thin film architectures resulting in chemical, structural, and magnetic disorder. The structural and magnetic disorder in AFM/FM-based spintronic devices can lead to highly undesirable properties, namely thermal dependence of the AFM anisotropy energy barrier, fluctuations in the magnetoresistance, non-linear operation, interfacial spin memory loss, extrinsic contributions to the effective magnetic damping in the adjacent FM, decrease in the effective spin Hall angle, atypical magnetotransport phenomena and distorted interfacial spin structure. Therefore, controlling the magnetic order down to the nanoscale in exchange coupled AFM/FM-based heterostructures is of fundamental importance. However, the impact of fractional variation in the magnetic order at the nanoscale on the magnetization reversal, magnetization dynamics, interfacial spin transport, and the interfacial domain structure of AFM/FM-based heterostructures remains a critical barrier. To address the aforementioned challenges, we conduct a comprehensive experimental investigation of chemical, structural, magnetization reversal (integral and element-specific), magnetization dynamics, and magnetotransport properties, combined with high-resolution magnetic imaging of the exchange coupled Ni3Fe/IrMn3-based heterostructures. Initially, we study the chemical, structural, electrical, and magnetic properties of epitaxially textured MgO(001)/IrMn3(0-35 nm)/Ni3Fe(15 nm)/Al2O3(2.0 nm) heterostructures. We reveal the impact of magnetic field annealing on the interdiffusion at the IrMn3/Ni3Fe interface, electrical resistivity, and magnetic properties of the heterostructures. We further present an AFM IrMn3 film thickness dependence of the exchange bias field, coercive field, magnetization reversal, and magnetization dynamics of the exchange coupled heterostructures. These experiments reveal a strong correlation between the chemical, structural and magnetic properties of the IrMn3-based heterostructures. We find a significant decrease in the spin-mixing conductance of the chemically-disordered IrMn3/Ni3Fe interface compared to the chemically-ordered counterpart. Independent of the AFM film thickness, we unveil that thermally disordered AFM grains exist in all the samples (measured up to 35-nm-thick IrMn3 films). We develop an iterative magnetic field cooling procedure to systematically manipulate the orientation of the thermally disordered and reversible AFM moments and thus, achieve tunable magnetic, and magnetotransport properties of exchange coupled AFM-based heterostructures. Subsequently, we investigate the impact of fractional variation in the AFM order on the magnetization reversal and magnetotransport properties of the epitaxially textured ɣ-phase IrMn3/Ni3Fe, Ni3Fe/IrMn3/Ni3Fe, and Ni3Fe/IrMn3/Ni3Fe/CoO heterostructures. We probe the element-specific (FM: Ni and Co, and AFM: Mn) magnetization reversal properties of the exchange coupled Ni3Fe/IrMn3/Ni3Fe/Co/CoO heterostructures in various magnetic field cooled states. We present a detailed procedure for separating the spin and orbital moment contributions for magnetic elements using the XMCD sum rule. We address whether Mauri-type domain walls can develop at the (polycrystalline) exchange coupled Ni3Fe/IrMn3/Ni3Fe interfaces. We further study the impact of magnetic field cooling on the AFM Mn (near L2,3-edges) X-ray absorption spectra. Finally, we employ a combination of in-field high-resolution magnetic force microscopy, magnetooptical Kerr effect magnetometry with micro-focused beam, and micromagnetic simulations to study the magnetic vortex structures in exchange coupled FM/AFM and AFM/FM/AFM disk structures. We examine the magnetic vortex annihilation mechanism mediated by the emergence and subsequent annihilation of the vortex-antivortex (V-AV) pairs in simple FM and exchange coupled FM/AFM as well as AFM/FM/AFM disk structures. We image the distorted magnetic vortex structures in exchange coupled FM/AFM disks proposed by Gilbert and coworkers. We further emphasize crucial magnetic vortex properties, such as handedness, effective vortex core radius, core displacement at remanence, nucleation field, annihilation field, and exchange bias field. Our experimental inquiry offers profound insight into the interfacial exchange interaction, magnetization reversal, magnetization dynamics, and interfacial spin transport of the AFM/FM-based heterostructures. Moreover, our results pave the way towards nanoscale control of the magnetic properties in AFM-based heterostructures and point towards future opportunities in the field of AFM spintronic devices.:1. Introduction 2. Magnetic Interactions and Exchange Bias Effect 3. Materials 4. Experimental Methods 5. Structural, Electrical, and Magnetization Reversal Properties of Epitaxially Textured ɣ-IrMn3/ Ni3Fe Heterostructures 6. Magnetization Dynamics of MgO(001)/IrMn3/Ni3Fe Heterostructures in the Frequency Domain 7. Tunable Magnetic and Magnetotransport Properties of MgO(001)/Ni3Fe/IrMn3/Ni3Fe/ CoO/Pt Heterostructures 8. Element-Specific XMCD Study of the Exchange Couple Ni3Fe/IrMn3/Ni3Fe/Co/CoO Heterostructures 9. Distorted Vortex Structure and Magnetic Vortex Reversal Processes in Exchange Coupled Ni3Fe/IrMn3 Disk Structures 10. Conclusions and Outlook Addendum Acronyms Symbols Publication List Author Information Acknowledgments Statement of Authorshi

Materials Optimization and GHz Spin Dynamics of Metallic Ferromagnetic Thin Film Heterostructures

Metallic ferromagnetic (FM) thin film heterostructures play an important role in emerging magnetoelectronic devices, which introduce the spin degree of freedom of electrons into conventional charge-based electronic devices. As the majority of magnetoelectronic devices operate in the GHz frequency range, it is critical to understand the high-frequency magnetization dynamics in these structures. In this thesis, we start with the static magnetic properties of FM thin films and their optimization via the field-sputtering process incorporating a specially designed in-situ electromagnet. We focus on the origins of anisotropy and hysteresis/coercivity in soft magnetic thin films, which are most relevant to magentic susceptibility and power dissipation in applications in the sub-GHz frequency regime, such as magnetic-core integrated inductors. Next we explore GHz magnetization dynamics in thin-film heterostructures, both in semi-infinite samples and confined geometries. All investigations are rooted in the Landau-Lifshitz-Gilbert (LLG) equation, the equation of motion for magnetization. The phenomenological Gilbert damping parameter in the LLG equation has been interpreted, since the 1970's, in terms of the electrical resistivity. We present the first interpretation of the size effect in Gilbert damping in single metallic FM films based on this electron theory of damping. The LLG equation is intrinsically nonlinear, which provides possibilities for rf signal processing. We analyze the frequency doubling effect at small-angle magnetization precession from the first-order expansion of the LLG equation, and demonstrate second harmonic generation from Ni81 Fe19 (Permalloy) thin film under ferromagnetic resonance (FMR), three orders of magnitude more efficient than in ferrites traditionally used in rf devices. Though the efficiency is less than in semiconductor devices, we provide field- and frequency-selectivity in the second harmonic generation. To address further the relationship between the rf excitation and the magnetization dynamics in systems with higher complexity, such as multilayered thin films consisting of nonmagnetic (NM) and FM layers, we employ the powerful time-resolved x-ray magnetic circular dichroism (TR-XMCD) spectroscopy. Soft x-rays have element-specific absorption, leading to layer-specific magnetization detection provided the FM layers have distinctive compositions. We discovered that in contrast to what has been routinely assumed, for layer thicknesses well below the skin depth of the EM wave, a significant phase difference exists between the rf magnetic fields Hrf in different FM layers separated by a Cu spacer layer. We propose an analysis based on the distribution of the EM waves in the film stack and substrate to interpret this striking observation. For confined geometries with lateral dimensions in the sub-micron regime, there has been a critical absence of experimental techniques which can image small-amplitude dynamics of these structures. We extend the TR-XMCD technique to scanning transmission x-ray microscopy (STXM), to observe directly the local magnetization dynamics in nanoscale FM thin-film elements, demonstrated at picosecond temporal, 40 nm spatial and less than 6° angular resolution. The experimental data are compared with our micromagnetic simulations based on the finite element analysis of the time-dependent LLG equation. We resolve standing spin wave modes in nanoscale Ni81 Fe19 thin film ellipses (1000 nm × 500 nm × 20 nm) with clear phase information to distinguish between degenerate eigenmodes with different symmetries for the first time. With the element-specific imaging capability of soft x-rays, spatial resolution up to 15 nm with improved optics, we see great potential for this technique to investigate functional devices with multiple FM layers, and provide insight into the studies of spin injection, manipulation and detection

FILM DEPOSITION AND MICROFABRICATION OF MAGNETIC TUNNEL JUNCTIONS WITH AN MgO BARRIER

Magnetic tunnel junctions (MTJs), which consist of a thin insulation layer sandwiched by two ferromagnetic (FM) layers, are among the key devices of spintronics that have promising technological applications for computer hard disk drives, magnetic random access memory (MRAM) and other future spintronic devices. The work presented here is related to the development of relevant techniques for the preparation and characterization of magnetic films, exchanged biased systems and MTJs. The fabrication and characterization of PtMn/CoFe exchange biased systems and MTJs with Al-O barriers were undertaken when the new Aviza StratIon fxP ion beam deposition tool was developed by the project consortium funded by DTI MNT. After the Nordiko 9550 spintronic deposition tool was installed at Plymouth, the work focused on the development of MTJ multilayer stacks with layer structures of CoFeB/MgO/CoFe/IrMn and IrMn/CoFeB/MgO/CoFeB to achieve coherent tunneling with a crystalline MgO barrier. The film deposition, microfabrication, magnetic field annealing, microstructural and nano-scale characterization, magnetic and magneto-transport measurement for these devices have been systematically studied to achieve smooth interfaces and desired crystallographic textures and magnetic properties of layer stacks. Magnetoresistance (MR) of up to 200% was obtained from MTJs with a layer structure of Ta/CuN/Ta/CoFeB/MgO/CoFe/IrMn/Ta and a CuN bottom electrode. Enhanced exchange anisotropy from the bottom pinned IrMn/CoFeB stacks has been obtained, which demonstrated the possibility of fabricating MTJs with CoFeB as both the top and bottom FM electrodes with strong exchange bias. The origin of the enhanced exchange bias field was studied by employing high resolution transmission electron microscopy (HRTEM) and x-ray magnetic circular dichroism (XMCD) to examine the mmicrostructure properties and element specific magnetic properties of the stacks. Results demonstrate that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins. MTJs with layered structures of IrMn/CoFeB/MgO/CoFeB were prepared based on this exchange bias system. However, further work is required for the optimisation of the (001) crystallographic textures of the CoFeB/MgO/CoFeB stack to achieve coherent tunneling

Engineering Magnetic and Topological Properties in Epitaxial Heusler Compounds

Commercially viable spintronic devices require magnetic contacts with high electrical conductivity, high spin polarization, low Gilbert damping, and perpendicular magnetic anisotropy. The contact must also be amenable to thin film growth techniques to allow device scalability. Until now, this combination of properties had yet to be obtained in a single material. The exquisite control over crystal growth conditions and elemental composition imparted by molecular beam epitaxy can be leveraged to tune magnetic and electronic material properties closer to the ideal set desired by device researchers.Ferromagnetic metals composed of elements with low atomic weight are commonly used for spintronics, but the industry standard CoFeB does not possess high spin polarization, and its perpendicular magnetic anisotropy depends on film thickness, limiting its versatility. On the other hand, Heusler compounds are a class of over 1000 ternary intermetallic materials with highly variable magnetic and electronic properties. The Heusler compound Co2MnSi is well known as a half-metal with 100% spin polarization at the Fermi level, making it an ideal source of spin-polarized current. However, Co2MnSi does not possess perpendicular magnetic anisotropy. In this work, the magnetic anisotropy of Heusler compounds is engineered by breaking their cubic crystal symmetry. This can be accomplished by growing tetragonal crystal structures with the unique axis aligned out-of-plane, or by engineering superlattices composed of alternating layers of dissimilar Heusler compounds. In both cases, the resulting perpendicular magnetic anisotropy does not depend on film thickness, making the materials attractive for a broad range of spintronic device applications. Additionally, the Heusler compound superlattices studied here are composed of Co2MnAl and Fe2MnAl, which combine their electronic structures to produce 95% spin polarization as measured by spin-resolved photoemission spectroscopy. This combines two important magnetic properties never before seen in a single material system. The growth, structural, electronic and magnetic properties of the engineered films will be presented.Finally, Co2TiGe is explored as a candidate of an exotic class of topological materials known as Weyl semimetals. These systems possess a unique band structure that arises due to broken time-reversal symmetry resulting from the internal magnetization. Electrons with energy and momentum near so-called Weyl points have zero effective mass and a discrete chiral charge, making them analogous to the elusive Weyl fermion. The signatures of Weyl semimetallicity in Co2TiGe are probed using magnetotransport and synchrotron-based angle-resolved photoemission spectroscopy

Domain walls in spin-valve nanotracks: characterisation and applications

Magnetic systems based on the manipulation of domain walls (DWs) in nanometre-scaled tracks have been shown to store data at high density, perform complex logic operations, and even mechanically manipulate magnetic beads. The magnetic nano-track has also been an indispensable model system to study fundamental magnetic and magneto-electronic phenomena, such as field induced DW propagation, spin-transfer torque, and other micromagnetic properties. Its value to fundamental research and the breath of potentially useful applications have made this class of systems the focus of wide research in the area of nanomagnetism and spintronics. This thesis focuses on DW manipulation and DW-based devices in spin-valve nanotracks. The spin-valve is a metallic multi-layered spintronic structure, wherein the electrical resistance varies greatly with the magnetisation of its layers. In comparison to monolayer tracks, the spin-valve track enables more sensitive and versatile measurements, as well as demonstrating electronic output of DW-based devices, an achievement of crucial interest to technological applications. However, these multi-layered tracks introduce new, potentially disruptive magnetic interactions, as well as fabrication challenges. In this thesis, the DW propagation in spin-valve nanotracks of different compositions was studied, and a system with DW propagation properties comparable to the state-of-the-art in monolayer tracks was demonstrated, down to an unprecedented lateral size of 33nm. Several DW logic devices of variable complexity were demonstrated and studied, namely a turn-counting DW spiral, a DW gate, multiple DW logic NOT gates, and a DW-DW interactor. It was found that, where the comparison was possible, the overall magnetic behaviour of these devices was analogous to that of monolayer structures, and the device performance, as defined by the range of field wherein they function desirably, was found to be comparable, albeit inferior, to that of their monolayer counterparts. The interaction between DWs in adjacent tracks was studied and new phenomena were observed and characterised, such as DW depinning induced by a static or travelling adjacent DW. The contribution of different physical mechanisms to electrical current induced depinning were quantified, and it was found that the Oersted field, typically negligible in monolayer tracks, was responsible for large variations in depinning field in SV tracks, and that the strength of spin-transfer effect was similar in magnitude to that reported in monolayer tracks. Finally, current induced ferromagnetic resonance was measured, and the domain uniform resonant mode was observed, in very good agreement to Kittel theory and simulations