Fabrication and Simulation of Nanomagnetic Devices for Information Processing

Nanomagnetic devices are highly energy efficient and non-volatile. Because of these two attributes, they are potential replacements to many currently used information processing technologies, and they have already been implemented in many different applications. This dissertation covers a study of nanomagnetic devices and their applications in various technologies for information processing – from simulating and analyzing the mechanisms behind the operation of the devices, to experimental investigations encompassing magnetic film growth for device components to nanomagnetic device fabrication and measurement of their performance. Theoretical sections of this dissertation include simulation-based modeling of perpendicular magnetic anisotropy magnetic tunnel junctions (p-MTJ) and low energy barrier nanomagnets (LBM) – both important devices for magnetic device-based information processing. First, we propose and analyze a precessionally switched p-MTJ based memory cell where data is written without any on-chip magnetic field that dissipates energy as low as 7.1 fJ. Next, probabilistic (p-) bits implemented with low energy barrier nanomagnets (LBMs) are also analyzed through simulations, and plots show that the probability curves are not affected much by reasonable variations in either thickness or lateral dimensions of the magnetic layers. Experimental sections of this dissertation comprise device fabrication aspects from the basics of material deposition to the application-based demonstration of an extreme sub-wavelength electromagnetic antenna. Magnetic tunnel junctions for memory cells and low barrier nanomagnets for probabilistic computing, in particular, require ultrathin ferromagnetic layers of uniform thickness, and non-uniform growth or variations in layer thickness can cause failures or other problems. Considerable attention was focused on developing methodologies for uniform thin film growth. Lastly, micro- and nano-fabrication methods are used to build an extreme sub-wavelength electromagnetic antenna implemented with an array of magnetostrictive nanomagnets elastically coupled to a piezoelectric substrate. The 50 pW signal measured from the approximately 250,000-nanomagnet antenna sample was 10 dB above the noise floor

MODELING AND BENCHMARKING OF SPINTRONIC DEVICES AND THEIR APPLICATIONS

Spintronic devices are promising candidates for low power applications such as logic and memory due to the characteristics of non-volatility, scalability, and fast switching speed. To evaluate the array-level performances of various spintronic memory devices, we have benchmarked spin-transfer torque magnetorestrictive random-access memory (STT-MRAM), spin-orbit torque MRAM (SOT-MRAM), voltage-controlled exchange coupling MRAM (VCEC-MRAM), and magnetoelectric MRAM (ME-MRAM). Among them, electric-field driven devices such as magnetoelectric (ME) device and the VCEC-MRAM can eliminate the joule heating energy thus is potentially more energy efficient than the current-controlled devices. Bismuth ferrite (BiFeO3) is a multiferroic material with the properties of ferroelectricity, antiferromagnetism, and weak ferromagnetism at room temperature. By combining BiFeO3 with a ferromagnet such as CoFe to form a BiFeO3/CoFe heterojunction, one can manipulate the magnetic state of CoFe by applying an external electric field. However, the switching mechanisms of the ferroelectric and the magnetic order of the BiFeO3 and CoFe are less understood which limits the estimation of the delay time and the write energy of the ME device. To evaluate the potential performance of this voltage-controlled BFO/CoFe heterojunction device in memory or logic application, we present a unified micromagnetic/ferroelectric simulation framework that can model the transient response and the switching behaviors of both BFO and CoFe layers. In addition, the important material parameters such as the interface exchange coupling coefficient are extracted from the experiments. Next, we build a physics-based compact model of the BFO/CoFe heterojunction to simulate the ME device in the circuit level. The results from our compact model closely match very well with those from the micromagnetic models when simulating the magnetization dynamics of BFO and CoFe. Using the compact model we developed, the SPICE simulation shows that ME-MRAM can potentially operate with a lower write energy compared to the STT-MRAM, SOT-MRAM or even SRAM when the coercive voltage of the BFO layer is as small as 20mV. Last, we model and benchmark the read and write performances of SOT-MRAM using various SOT materials including heavy metals, alloys, Weyl semi-metals, and topological insulators. The non-ideal factors such as current-shunting effect, current crowding effect, and the variability are included. Our results indicate that spintronic memory devices are prospective candidates in the embedded memory application due to the better energy efficiency and smaller layout area compared to SRAM.Ph.D

Spintronic device modeling and evaluation using modular approach to spintronics

Spintronics technology finds itself in an exciting stage today. Riding on the backs of rapid growth and impressive advances in materials and phenomena, it has started to make headway in the memory industry as solid state magnetic memories (STT-MRAM) and is considered a possible candidate to replace the CMOS when its scaling reaches physical limits. It is necessary to bring all these advances together in a coherent fashion to explore and evaluate the potential of spintronic devices. This work creates a framework for this exploration and evaluation based on Modular Approach to Spintronics, which encapsulate the physics of transport of charge and spin through materials and the phenomenology of magnetic dynamics and interaction in benchmarked elemental modules. These modules can then be combined together to form spin-circuit models of complex spintronic devices and structures which can be simulated using SPICE like circuit simulators. In this work we demonstrate how Modular Approach to Spintronics can be used to build spin-circuit models of functional spintronic devices of all types: memory, logic, and oscillators. We then show how Modular Approach to Spintronics can help identify critical factors behind static and dynamic dissipation in spintronic devices and provide remedies by exploring the use of various alternative materials and phenomena. Lastly, we show the use of Modular Approach to Spintronics in exploring new paradigms of computing enabled by the inherent physics of spintronic devices. We hope that this work will encourage more research and experiments that will establish spintronics as a viable technology for continued advancement of electronics

Spin-orbit effects in asymmetrically sandwiched ferromagnetic thin films

Asymmetrically sandwiched ferromagnetic thin films display a large number of spin-orbit effects, including the Dzyaloschinsii-Moriya interaction (DMI), spin-orbit torques (SOT) and magnetoresistance (MR) effects. Their concurrence promises the implementation of interesting magnetic structures like skyrmions in future memory and logic devices. The complex interplay of various effects originating from the spin-orbit coupling and their dependencies on the microstructural details of the material system mandates a holistic characterization of its properties. In this PhD thesis, a comprehensive study of the spin-orbit effects in a chromium oxide/cobalt/platinum trilayer sample series is presented. The determination of the complete micromagnetic parameter set is based on a developed measurement routine that utilizes quasistatic methods. The unambiguous quantification of all relevant constants is crucial for the modeling of the details of magnetic structures in the system. In this context the necessity of a strict distinction of magnetic objects, that are stabilized by magnetostatics or the DMI, was revealed. Furthermore, a sample layout was developed to allow for the simultaneous quantification of the magnitudes of SOTs and MR effects from nonlinear magnetotransport measurements. In conjunction with a structural characterization, the dominating dependence of the effect magnitudes on microstructural details of the systems is concluded. Precisely characterized systems establish a solid groundwork for further investigations that are needed for viable skyrmion-based devices.:1 Introduction 2 Fundamentals 2.1 Towards new devices 2.2 Spin-orbit effects 2.2.1 Spin-current sources 2.2.2 Magnetoresistanceeffects 2.2.3 Spin-orbit torques 2.2.4 Harmonic analysis 2.3 Micromagnetic model 2.3.1 Dzyaloshinskii-Moriya interaction (DMI) 2.3.2 Consequences of the DMI for magnetic structures 2.3.3 Interface-induced DMI in asymmetrically stacked ferromagnets 2.3.4 Quantification of the interface-induced DMI 2.3.5 Levy-Fert three-site model including roughness 3 The CrOx/Co/Pt sample system 3.1 Experimental techniques 3.2 Structural characterization 4 Complete micromagnetic characterization 4.1 Magnetometry 4.1.1 Static investigation 4.1.2 Ferromagnetic resonance 4.2 DMI quantification 4.2.1 Field-driven domain wall creep motion 4.2.2 Asymmetric domain growth 4.2.3 Winding pair stability 4.3 Determination of the exchange parameter 4.3.1 Generation of circular magnetic objects 4.3.2 Homochiral magnetic bubble domains 4.4 Results 5 Magnetotransport measurements 5.1 Measurement setup 5.2 Magnetoresistance effects 5.3 Spin-orbit torque quantification 5.4 Results 6 Discussion 6.1 Structural predomination of the DMI strength 6.2 Ultra-thin limit exchange parameter reduction 6.3 Magnetotransport properties 6.4 Magneticstructures in //CrOx/Co/Pttrilayers 7 Conclusion and Outlook A Appendix A.1 Calculation of the skyrmion diameter A.2 Micromagnetic simulation of the winding pair stability Bibliography Acknowledgement

Bismuth

Bismuth—a wonder metal with unique features—plays an important role in the bismuth-related optoelectronic materials. The innovative development of bismuth optoelectronic materials will undoubtedly drive the social development and economic growth in the world towards a glorious future