Author: Marco Menarini
Publisher:
ISBN:
Category :
Languages : en
Pages : 163
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Book Description
Magnetic materials are vital components of many existing and future applications, from data storage and spin-logic devices, to Terahertz sensors and artificial synapses from neuromorphing computing. Driven by the need to faster responses and high-density storage, the focus of this work is the modeling of thermal and optical excitation of magnetic materials by an external laser source. Many models focus on the use of fields or current as the primary driving force behind the change in magnetization and the models, without taking in account the optical contribution of the light, which has been shown to produce changes in magnetizing on a faster timescale than the ones observed with the use of either current or field. Moreover, granular media are usually modelled using simplistic finite difference approach or numerically intensive finite-elements approach to model every grain. These approach leads to either an unrealistic description of the media (finite-difference) or to an over-sampling of the nodes of the problem, increasing significantly the computational time required to run the simulations. This dissertation improves upon the state of the art of micromagnetic modelling by introducing a Voronoi tessellation model to simulate realistic granular structures at elevated temperature for high anisotropy materials. This approach considers the geometry of the grains for compute the far-field contribution. This approach has been proven effective in modeling realistic media for heat assisted magnetic recording and perpendicular media in general. The model presented it also introduces in the dynamic of the magnetizing the optical contribution and helps to describes complex phenomena like the ultrafast-demagnetization and the helicity dependent optical reversal of magnetic material subjected to an external optical source. While the model provides a qualitative interpretation of the experiments, additional data is required to evaluate the quantitative contribution of the optical excitation and the correctness of the thermal fluctuations. This dissertation is structured as follow. In Chapter 1 and 2 introduce key concepts of magnetism and the basic Micromagnetic model that is going to be used as the basis for the numerical simulations. In Chapter 3 I introduce a micromagnetic code based on Voronoi tessellation and the non-uniform fast Fourier transform (NUFFT) method. The code is capable of efficiently and accurately simulating magnetization dynamics in large and structurally complex granular systems, such as multilayer granular media used for perpendicular magnetic recording, bit patterned media, granular nanowires, and read heads. In these systems the granular microstructure and distributions in grain and interface properties play an important role in device performance. The presented Voronoi simulator allows comprehensive studies to be performed as it accounts for the detailed granular microstructure and distributions that characterize true systems. Simulation time is greatly reduced by a non-uniform fast Fourier transform algorithm and implementation on graphics processing units (GPUs). Simulations of conventional magnetic recording, heat-assisted magnetization reversal, domain wall dynamics in granular nanowires, and particulate tape recording are presented. Chapter 4 explores the generation of electrical field signals in the terahertz frequency (THz) range using antiferromagnets (AFM). Using micromagnetic model simulation, we investigated a potential mechanism for laser-induced THz signals in the AFM phase of FeRh/Pt bilayer films. In the simulations, the FeRh film is modelled as two Fe-sublattices coupled via intra-lattice exchange field and subjected to a sub-picosecond thermal pulse. Our simulation exposes a partial canting between the magnetizations of two Fe-sublattices, within the first picosecond after the excitation. This short-lived state relaxes abruptly into the initial AFM phase, injecting a spin current into the Pt layer via spin pumping, which will eventually be converted into charge current oscillating at THz frequency. Chapters 5 and 6 discuss the phenomenon of all-optical switching of the magnetization in magnetic nanostructures. While all-optical switching of the magnetization in magnetic nanostructures by femtosecond circularly polarized laser pulses without an external magnetic field has been demonstrated in several systems, a theoretical framework that convincingly explain the phenomenon is still missing. In Chapter 5 we propose a theory where the ferromagnetic macrospin ground state is optically excited by the circularly polarized light to a spin reversed state, which is then "Coulomb collapsed" to the magnetization reversed ground state. The optical excitation lasts for the duration of the laser pulse and the system relaxes at a fast rate due to the electron-electron interaction. In Chapter 6 we present a computational model based on this theory. We construct a three-state model for the magnetization dynamics, the Landau-Lifshitz-Lambda (LLL) model, as an ensemble of such states to account for the temperature effects. After the optical excitation lapses, the LLL model reduces to the Landau-Lifshitz-Bloch formulation, allowing to consider the magnetization relaxation dynamics at elevated temperatures. We apply the theory to simulate AOS in FePt films subject to multiple femtosecond circular polarized laser pulses. The simulation results demonstrate characteristic AOS features and agree with recent experiments. Chapter 7 identifies problems in the performance of the established stochastic model in micromagnetics in modeling the thermal fluctuations of longitudinal and transverse components of the magnetization at elevated temperature. A correct estimation of the thermal fluctuation is paramount to develop multiscale atomistic-micromagnetic models. The chapter presents a consistent solution for the diffusion coefficients that satisfy the corresponding Fokker-Planck equation and provide the correct equilibrium magnetization at elevated temperature.
Author: Marko V. Lubarda
Publisher:
ISBN: 9781267835895
Category :
Languages : en
Pages : 305
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Book Description
Magnetic nanostructures are vital components of numerous existing and prospective magnetic devices, including hard disk drives, magnetic sensors, and microwave generators. The ability to examine and predict the behavior of magnetic nanostructures is essential for improving existing devices and exploring new technologies and areas of application. This thesis consists of three parts. In part I, key concepts of magnetism are covered (chapter 1), followed by an introduction to micromagnetics (chapter 2). Key interactions are discussed. The Landau-Lifshitz-Gilbert equation is introduced, and the variational approach of W.F. Brown is presented. Part II is devoted to computational micromagnetics. Interaction energies, fields and torques, introduced in part I, are transcribed from the continuum to their finite element form. The validity of developed models is discussed with reference to physical assumptions and discretization criteria. Chapter 3 introduces finite element modeling, and provides derivations of micromagnetic fields in the linear basis representation. Spin transfer torques are modeled in chapter 4. Thermal effects are included in the computational framework in chapter 5. Chapter 6 discusses an implementation of the nudged elastic band method for the computation of energy barriers. A model accounting for polycrystallinity is developed in chapter 7. The model takes into account the wide variety of distributions and imperfections which characterize true systems. The modeling presented in chapters 3-7 forms a general framework for the computational study of diverse magnetic phenomena in contemporary structures and devices. Chapter 8 concludes part II with an outline of powerful acceleration schemes, which were essential for the large-scale micromagnetic simulations presented in part III. Part III begins with the analysis of the perpendicular magnetic recording system (chapter 9). A simulation study of the recording process with readback analysis is presented. Heat-assisted magnetic recording is considered in chapter 10. The effects of optical spot size and switching rate on signal quality are investigated. Chapter 11 is devoted to bit patterned media (BPM). Reversal modes and thermal stability of Co/Pd multilayer islands are studied. A novel BPM design, called capped BPM, is shown to provide enhanced tunability of thermal stability, writability, switching field distributions, and readback. Chapter 12 discusses spin valve applications. An all-perpendicular composite spin valve is shown to significantly reduce the tradeoff between switching currents and thermal stability. The last chapter 13 covers domain wall (DW) devices. DW motion in magnetically frustrated nanorings is analyzed. Antiferromagnetically coupled nanowires are shown to extend DW velocities beyond the Walker breakdown limit. A crosswire architecture is proposed for Boolean operations and the study of disorder dynamics.
Author: Zhidong Zhao
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
This research work focuses on the geometry and shape effects on submicron magnetic material. A web based micromagnetics program is written to model the hysteresis loop of nano magnetic samples with arbitrary geometry shapes and multiple magnetic materials. Three material samples have been modeled with this program along with nano magnets with a variety of geometric shapes. Shape anisotropy has been introduced to a permalloy ring by adding a cross-tie structure with various widths. The in-plane hysteresis loop and reversal behavior have no notable difference in direction parallel to the cross-tie, but greatly changed in perpendicular and diagonal directions. The switching field distribution is significantly reduced. The two distinct "onion" bit states of the modified ring elements are stabilized in the hysteresis in the diagonal direction. The changes in the modified rings make them better candidates for Magnetic Random Access Memory elements. Two Pac-Man elements, PM I and PM II, geometrically modified from disc and half disc respectively, are modeled. The PM I element undergoes a magnetic reversal through a two-stage mechanism that involves nucleation in the left and right middle areas followed by vortex core formation and vortex core motion in the lower middle area. The reversal process of the PM II element lacks the vortex core formation and motion stage. The switching field of the PM I and PM II elements are the same but the switching field distribution of the PM II elements is much narrower than that of the PM I element. Only the PM II element meets MRAM application requirements. The thickness dependence of the magnetic properties of a core-shell structure has been studied. The nano particles have a cobalt core and a permalloy shell. The nano spheres are the same size but with various shell thickness. Simulations reveal a multi-stage reversal process without the formation of a Bloch wall for thin-shell structure and smooth reversal process with the formation and motion of a Bloch wall for thick-shell structure. Gradual transition of the hysteresis loop patterns has been observed.
Author: Rok Dittrich
Publisher:
ISBN:
Category :
Languages : en
Pages : 7
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Book Description
A path finding method and a stochastic time integration scheme for the simulation of thermally activated magnetization processes are introduced. The minimum energy path and the saddle points for the thermally induced transitions between the ground states of NiFe magnetic nanoelements are calculated.
Author: Ram K. Gupta
Publisher: CRC Press
ISBN: 1000640175
Category : Science
Languages : en
Pages : 380
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Book Description
A low-dimensional magnet is a key to the next generation of electronic devices. In some respects, low-dimensional magnets refer to nanomagnets (nanostructured magnets) or single-molecule magnets (molecular nanomagnets). They also include the group of magnetic nanoparticles, which have been widely used in biomedicine, technology, industries, and environmental remediation. Low-dimensional magnetic materials can be used effectively in the future in powerful computers (hard drives, magnetic random-access memory, ultra-low power consumption switches, etc.). The properties of these materials largely depend on the doping level, phase, defects, and morphology. This book covers various nanomagnets and magnetic materials. The basic concepts, various synthetic approaches, characterizations, and mathematical understanding of nanomaterials are provided. Some fundamental applications of 1D, 2D, and 3D materials are covered. This book provides the fundamentals of low-dimensional magnets along with synthesis, theories, structure-property relations, and applications of ferromagnetic nanomaterials. This book broadens our fundamental understanding of ferromagnetism and mechanisms for realization and advancement in devices with improved energy efficiency and high storage capacity.
Author:
Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 788
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Book Description
Author: Nicholas J. Darton
Publisher: Cambridge University Press
ISBN: 1107031095
Category : Medical
Languages : en
Pages : 317
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Book Description
Drawing together topics from a wide range of disciplines, and featuring up-to-date examples of clinical usage and research applications, this text provides a comprehensive insight into the fundamentals of magnetic biosensors and the applications of magnetic nanoparticles in medicine.
Author: Manuel Vázquez
Publisher: Woodhead Publishing
ISBN: 0081001819
Category : Technology & Engineering
Languages : en
Pages : 847
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Book Description
Magnetic nanowires and microwires are key tools in the development ofenhanced devices for information technology (memory and data processing) andsensing. Offering the combined characteristics of high density, high speed, andnon-volatility, they facilitate reliable control of the motion of magnetic domainwalls; a key requirement for the development of novel classes of logic and storagedevices. Part One introduces the design and synthesis of magnetic nanowires andmicrowires, reviewing the growth and processing of nanowires and nanowireheterostructures using such methods as sol-gel and electrodepositioncombinations, focused-electron/ion-beam-induced deposition, chemicalvapour transport, quenching and drawing and magnetic interactions. Magneticand transport properties, alongside domain walls, in nano- and microwiresare then explored in Part Two, before Part Three goes on to explore a widerange of applications for magnetic nano- and microwire devices, includingmemory, microwave and electrochemical applications, in addition to thermalspin polarization and configuration, magnetocalorific effects and Bloch pointdynamics. Detailed coverage of multiple key techniques for the growth and processing of nanowires and microwires Reviews the principles and difficulties involved in applying magnetic nano- and microwires to a wide range of applications Combines the expertise of specialists from around the globe to give a broad overview of current and future trends
Author: Hari Singh Nalwa
Publisher:
ISBN: 9781588830005
Category : Science
Languages : en
Pages : 0
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Book Description
Twelve contributions comprise a reference source that is a coherent presentation of the state of the art in this fast growing area of nanotechnology research. Magnetic nanostructures are important for their phenomenal potential for storage; their great commercial value will come from applications in
Author: Olle Eriksson
Publisher: Oxford University Press
ISBN: 0198788665
Category : Science
Languages : en
Pages : 265
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Book Description
Several large experimental facilities that focus on detection and probing magnetization dynamics have been realized in Europe, USA and Japan. This book covers theoretical and practical aspects of the vibrant and emerging research field of magnetization dynamics.
