Precise Control of 360-degree Magnetic Domain-wall Formation and Their Properties in Geometrically Confined Nanowires

Precise Control of 360-degree Magnetic Domain-wall Formation and Their Properties in Geometrically Confined Nanowires
Title Precise Control of 360-degree Magnetic Domain-wall Formation and Their Properties in Geometrically Confined Nanowires PDF eBook
Author Dan Wang
Publisher
Pages
Release 2019
Genre
ISBN

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For decades, magnetism is widely applied in the industry as technologies such as sensors, memories, motors, generators, and others. Since the invention of the giant magnetoresistance effect (GMR) effect and the resulting magnetic read head, which was awarded the 2007 Nobel Prize in Physics to Albert Fert and Peter Grunberg, the study of magnetic-based technology has developed rapidly. There are many advantages to using magnetic-based devices such as high storage capacity, high reliability, cheaper cost, and non-volatility. Thanks to those advantages, magnetic-based devices for example hard disk drives (HDDs) is now widely used in computer memories even compared with solid state disk drives (SSDs) [1]. However, different from SSDs which store data in microchips, HDDs use a fixed read/write head to read information from the mechanically moved magnetic disk, which is slow and energetically inefficient. Such kind of low speed and high power needed consumption is preventing magnetic based devices from further applications. In my thesis, I will illustrate my study towards resolving these disadvantages, using a newly discovered phenomenon called spintronics. Due to the spin transfer torque between electron spins and lattice in materials such as ferromagnets, the magnetic domains can be driven by injecting a current, via a domain wall (DW) motion. Such property enables the potential applications of DWs in high-speed memory or logic devices. I will first give a summary of the magnetic energy terms which relevant to understanding thin film domain wall behavior. Next, I will give a brief introduction to magnetic energy terms and the motivation and background of my study on magnetic domain walls (DWs). There are two types of transverse DWs, a 180° domain wall (180DW) and a 360° domain wall (360DW). My research will mainly focus on the study of fast and in-situ formation of these two types of DWs, especially 360DWs which have not been well understood previously. In my method, these two types of DWs will be generated by using an external Oersted field, then injecting a current pulse in the transverse current line, and the chirality of DWs is based on the design and control of nanowire geometry. By using this method, not only the reliability is high for application purposes, but also the chirality of the formed 180DW and 360 DW can be well controlled, which is critical in applications as devices. After discussing the results of 180/360DWs formation, I will then talk about their dynamics property under the magnetic field or spin current, and further on how the chirality of 180/360DWs will response to geometry effects of the nanowire. Finally, with a combination of DW chirality and topological effects, I have discovered that the trajectory of the DWs can be controlled by the DWs chirality in a well-controlled Y-shape nanowire, which allows us to design a chirality sorter of 180/360DWs using such devices. My research is implemented mainly by micromagnetic simulations using finite element differentiation methods. The dynamics of magnetization is based on the one-dimensional Landau-Lifshitz-Gilbert (LLG) equations where both magnetic field and spin current will exert torques to magnetic moments. Two different tool kits are used for my simulations, OOMMF and Mumax3. Both of the two tools have their respective advantages and disadvantages and are more appropriate in respective studies, which will be discussed in further detail. I have also compared the results of the two tools. In the last, I will talk about the experimental study of DW behaviors. I have built a magnetoresistance system that can apply a magnetic field and spin current pulses into the samples and detect the change of sample magnetization by measuring the change of sample resistance. I will show the preliminary results for experimental measurements in the thesis and present my plans for future work.

Geometrical Control of Domain Walls and the Study of Domain Wall Properties of Materials with Perpendicular Magnetic Anisotropy

Geometrical Control of Domain Walls and the Study of Domain Wall Properties of Materials with Perpendicular Magnetic Anisotropy
Title Geometrical Control of Domain Walls and the Study of Domain Wall Properties of Materials with Perpendicular Magnetic Anisotropy PDF eBook
Author Jinshuo Zhang (Ph. D.)
Publisher
Pages 168
Release 2017
Genre
ISBN

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Magnetic based devices such as hard disk drives (HDDs) are widely used in the computer industry because of their high memory capacity, non-volatility and low cost compared to semiconductor-based solid state disk drives (SSDs). However, they also suffer from low energy efficiency and low speed, due to the requirement for mechanical motion in order to access the data. In my thesis, I will first give a brief introduction to the motivation and background in the study of magnetic domain walls (DWs), which have attracted great attention due to their ability to be moved by field and/or current and corresponding potential applications in high speed memory or logic devices. I will then discuss how to geometrically control the behaviors of DWs in a ferromagnetic nanowire. I will first discuss how natural geometry distortions such as edge tapering from sputtering on an undercut resist profile and wire width variation from the patterning process would affect DW behavior, including static configurations, stability and dynamics under current pulsing. I will then discuss how similar geometrical effects will affect the properties of materials with perpendicular magnetic anisotropy (PMA). The same geometry modulation will have different effects depending on the origin of the PMA. Such results are confirmed by observing the magnetic reversal process. Besides the study on 180DWs, we will then discuss the field and current effects on 360 degree DWs (360DWs), which have many unique properties compared to 180DWs and are an alternative candidate for DW based devices. I will then discuss control of 360DW behavior by designing a geometrical heterostructure. We have found that by utilizing the asymmetric Oersted field originated from the heterostructure, we are able to control the 360DWs depending on their chirality. The structure can function as a 360DW chirality filter, which provides extra freedom in DW-based applications. These studies were conducted by a combination of micromagnetic simulations and experimental implementations. Techniques being used including OOMMF micromagnetic simulations, Comsolfinite element simulations, electrical measurements, magnetic force microscopy and other characterization techniques.

Magnetic Domain Wall Formation in Ferromagnetic Nanowires

Magnetic Domain Wall Formation in Ferromagnetic Nanowires
Title Magnetic Domain Wall Formation in Ferromagnetic Nanowires PDF eBook
Author
Publisher
Pages 150
Release 2009
Genre
ISBN

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Nucleation and Propagation of Magnetic Domain Walls in Cylindrical Nanowires with Diameter Modulations

Nucleation and Propagation of Magnetic Domain Walls in Cylindrical Nanowires with Diameter Modulations
Title Nucleation and Propagation of Magnetic Domain Walls in Cylindrical Nanowires with Diameter Modulations PDF eBook
Author Beatrix Trapp
Publisher
Pages 0
Release 2018
Genre
ISBN

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In all current data storage devices, the information bits are stored in form of domain walls in a thin film or in patterned media on a two-dimensional surface . Within the next decade, further increase of the storage density in these devices is expected to come to a halt due to several fundamental and technological issues. Thus there have recently been efforts to develop three-dimensional devices combining the versatility of solid state RAM with the cost efficiency of common hard disk drives.A particularly interesting theoretical concept for a three-dimensional magnetic memory has been proposed in 2004 by S. Parkin et al. . Their racetrack memory consists of a vertical array of magnetic nanowires with either cylindrical or rectangular cross section. The bits are encoded in a series of up to 100 domain walls per wire. Using nanosecond spin polarized current pulses these walls are shifted past an integrated read head.Magnetic domain walls in cylindrical nanowires have raised the interest of the scientific community due to their possible application in a functional device as well as due to exciting new properties which arise from the geometric confinement. Up to date, only a few pioneering experimental studies on such domain walls exist. They indicate strong pinning effects preventing a deterministic domain wall propagation. So far the microscopic origin of this pinning has only partially been understood. It is expected however that beside the wire geometry the material microstructure may play a considerable role.Situated within the framework of the European FP 7 project m3D, the objective of my work has been to investigate the domain wall propagation in cylindrical nanowires with diameter modulations by means of magnetic force microscopy and micromagnetic simulation. As the domain wall energy increases with the wire diameter, protrusions (resp. notches) are expected to act as an artificial energy barrier (resp. well). Consequently, a deterministic domain wall propagation controlled via the wire geometry seems possible.A first part of my work concerns material optimization. For this, NiCo alloy wires (100-200nm diameter and multiple tens of micrometers in length) with two distinct geometries have been fabricated by template assisted electrodeposition (Chemist collaborators at Univ. Erlangen, Prof. J.Bachmann). I have then explored the impact of the alloy composition as well as of possible post-fabrication annealing on the material microstructure. Subsequently, domain wall propagation in individual nanowires has been investigated under the influence of either a quasistatic magnetic field or a nanosecond magnetic field pulse. In addition I have performed complementary micromagnetic simulations to study the effect of the modulation geometry on the domain wall pinning.

Domain Wall Behaviour in Ferromagnetic Nanowires with Interfacial and Geometrical Structuring

Domain Wall Behaviour in Ferromagnetic Nanowires with Interfacial and Geometrical Structuring
Title Domain Wall Behaviour in Ferromagnetic Nanowires with Interfacial and Geometrical Structuring PDF eBook
Author David Michael Burn
Publisher
Pages
Release 2013
Genre
ISBN

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The magnetic behaviour in nanoscale structures is of great interest for the fundamental understanding of magnetisation processes and also has importance for wide ranging technological applications. This thesis examines mechanisms for the enhanced control of domain walls in these structures via focussed ion beam modifications to magnetic nanowires and through the inclusion of periodic geometrical modifications to the nanowires geometry. A detailed investigation into the effect of focussed ion beam irradiation on the structure of NiFe/Au bilayers was performed through x-ray reflectivity and fluorescence techniques. This analysis revealed the development of interfacial intermixing with low dose irradiation. This is associated with complex changes of the magnetic behaviour including a rapid decrease, followed by a recovery of the saturation magnetisation with low dose irradiation. This behaviour is attributed to changes in the local environment of the atoms at the interface; resulting in modifications to the magnetic moment on Ni and Fe. The development of an induced moment on Au and a change in the spin-orbit interaction is also suggested. Localised control of the magnetic properties in nanowires demonstrates the ability to manipulate domain walls in these structures. Here, irradiated regions provide pinning sites where the width and dose of the irradiated region give control over the pinning potential. The inclusion edge modulation to nanowires geometry provides additional control over their magnetic behaviour. The direct magnetisation reversal field of these structures is explained by an analytical model based on the torque on the spins following the modulated wire geometry. This model is scalable for different modulation parameters and combines with the effect of localised regions of orthogonal anisotropy along the wire; explaining the reversal behaviour over the entire parameter space. Domain wall mediated reversal in modulated wires was also investigated in these structures. The inclusion of modulation shows an improvement in dynamic properties by the suppression of Walker breakdown. This is due to the relationship between geometrical modulations and the periodicity of micromagnetic domain wall structural changes during the Walker breakdown process. The combination of this work shows a route to the optimisation of the dynamic properties whilst minimising the detrimental increase in the de-pinning field from the modulation.

Magnetic Nano- and Microwires

Magnetic Nano- and Microwires
Title Magnetic Nano- and Microwires PDF eBook
Author Manuel Vázquez
Publisher Woodhead Publishing
Pages 1012
Release 2020-04-01
Genre Technology & Engineering
ISBN 0081028334

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Magnetic Nano-and Microwires: Design, Synthesis, Properties and Applications, Second Edition, reviews the growth and processing of nanowires and nanowire heterostructures using such methods as sol-gel and electrodeposition, focused-electron/ion-beam-induced deposition, epitaxial growth by chemical vapor transport, and more. Other sections cover engineering nanoporous anodic alumina, discuss magnetic and transport properties, domains, domain walls in nano-and microwires. and provide updates on skyrmions, domain walls, magnetism and transport, and the latest techniques to characterize and analyze these effects. Final sections cover applications, both current and emerging, and new chapters on memory, sensor, thermoelectric and nanorobotics applications. This book will be an ideal resource for academics and industry professionals working in the disciplines of materials science, physics, chemistry, electrical and electronic engineering and nanoscience. Details the multiple key techniques for the growth, processing and characterization of nanowires and microwires Reviews the principles and difficulties involved in applying magnetic nano- and microwires to a wide range of applications, also including biomedical and sensing applications Discusses magnetism and transport in nanowires, skyrmions and domain walls in nanowires and the latest innovations in magnetic imaging

An Investigation of the Structure, Pinning and Magnetoresistance of Domain Walls in Ni81Fe19 Planar Nanowires

An Investigation of the Structure, Pinning and Magnetoresistance of Domain Walls in Ni81Fe19 Planar Nanowires
Title An Investigation of the Structure, Pinning and Magnetoresistance of Domain Walls in Ni81Fe19 Planar Nanowires PDF eBook
Author Lara Katrina Bogart
Publisher
Pages 247
Release 2010
Genre Nanowires
ISBN

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Abstract: The research and development of Ni81Fe19 thin films and planar nanowire structures has attracted considerable interest in recent years; in terms of improving the fundamental understanding of the basic physical processes and also for the development of potential applications. Example applications include sensors and the data storage devices. The optimisation of such devices requires detailed knowledge of the thickness dependence and microstructural influences on the magnetic and magnetoresistance properties, along with a thorough understanding of the effect of geometrical confinement on domain wall (DW) structure and pinning behaviour in nanowire structures. The out-of-plane structural properties of thermally evaporated Ni81Fe19 thin films on pre-oxidised silicon substrates have been investigated using x-ray scattering techniques and transmission electron microscopy (TEM). These techniques have been used to provide information on the out-of-plane lattice parameter, the presence and degree of texture and also to quantify the width of the SiO2/Ni81Fe19 interface. Magneto-optical Kerr effect (MOKE) magnetometry, differential phase contrast TEM imaging, micromagnetic simulations and anisotropic magnetoresistance measurements (AMR) have been used to make a detailed study of the thickness dependence of the magnetic behaviour of both thin films and nanowire structures. The resistivity of thin films produced in this study is found to exhibit a higher value and lower mean free path than has previously been reported in the literature, which is attributed to the presence of a microstructure characterised by a small crystallite grain structure. The AMR is strongly thickness dependent for t