Characterization of magnetic nanostructures by magnetic force microscopy

Práce pojednává o mikroskopii magnetických sil magneticky měkkých nanostruktur, zejména NiFe nanodrátů a různě tvarovaných tenkých vrstev - například disků. Práce se zaměřuje na téměř vše, co s touto mikroskopickou technikou souvisí: přípravu měřicích sond a vzorků, samotná pozorování a mikromagnetické simulace magnetického stavu vzorků. Byla pozorována jádra magnetických vírů, jak s komerčními, tak s námi připravenými sondami. Podařilo se zobrazit i magnetické doménové stěny v nanodrátech o průměru pouhých 50 nm. Připravili jsme fungující sondy s různými magnetickými vrstvami: magneticky tvrdého kobaltu, slitiny CoCr a magneticky měkké slitiny NiFe. Magneticky tvrdé sondy poskytovaly lepší signál, zatímco magneticky měkké byly vhodnější pro pozorování magneticky měkkých vzorků, protože je příliš neovlivňují. Námi připravené sondy jsou přinejmenším srovnatelné se standardními komerčními sondami. Simulace se ve většině případů shodují jak s měřením, tak teorií. Dále představujeme také naše prvotní výsledky modelování interakce vzorku s magnetickou sondou, které mohou složit k simulaci měření pomocí mikroskopie magnetických sil, a to i v případě, kdy sonda ovlivňuje magnetický stav vzorku.The thesis deals with magnetic force microscopy of soft magnetic nanostructures, mainly NiFe nanowires and thin-film elements such as discs. The thesis covers almost all aspects related to this technique - i.e. from preparation of magnetic probes and magnetic nanowires, through the measurement itself to micromagnetic simulations of the investigated samples. We observed the cores of magnetic vortices, tiny objects, both with commercial and our home-coated probes. Even domain walls in nanowires 50 nm in diameter were captured with this technique. We prepared functional probes with various magnetic coatings: hard magnetic Co, CoCr and soft NiFe. Hard probes give better signal, whereas the soft ones are more suitable for the measurement of soft magnetic structures as they do not influence significantly the imaged sample. Our probes are at least comparable with the standard commercial probes. The simulations are in most cases in a good agreement with the measurement and the theory. Further, we present our preliminary results of the probe-sample interaction modelling, which can be exploited for the simulation of magnetic force microscopy image even in the case of probe induced perturbations of the sample.

Convergence of a ferromagnetic film model

International audienceDans cette note, nous présentons un résultat de Gamma-convergence pour les films de matériaux ferromagnétiques. Nous proposons un modèle pour lequel il est possible d'assurer une convergence forte des minimiseurs quand le paramètre d'échange tend vers zéro. Dans ce modèle, l'épaisseur du film est considérée comme constante et l'aimantation est contrainte à rester constante dans l'épaisseur du film

Spin-wave generation and transport in magnetic microstructures

Generating, miniaturizing and controlling spin waves on the nanometer scale is of great interest for magnonics. For instance, this holds the prospect of exploring wave-based logic concepts and reduced Joule heating, by avoiding charge transport, in spin-wave circuitry. In this work, a novel approach is for the first time confirmed experimentally, which allows confining spin-wave transport to nanometre-wide channels defined by magnetic domain walls. This is investigated for different domain wall types( 90deg and180deg Néel walls) in two material systems of polycrystalline Ni81Fe19 and epitaxial Fe. The study covers the thermal, linear and non-linear regime utilizing micro- focused Brillouin light scattering microscopy complemented by micromagnetic simulations. An initially linear dispersion dominated by dipolar interactions is found for the guided spin waves. These are transversally confined to sub-wavelength wide beams with a well-defined wave vector along the domain wall channel. In the non-linear regime, higher harmonic generation of additional spin-wave beams at the sides of the domain wall channel is observed. Furthermore, the possibility to shift the position of the domain wall over several microns by small magnetic fields is demonstrated, while maintaining its spin-wave channeling functionality. Additionally, spin-wave transmittance along domain walls, which change direction at the edges of the structure as well as between interconnected walls of identical and different type is studied. Characterization of spin-wave transmission through interconnected domain walls is an important step towards the development of magnonic circuitry based on domain wall(-networks). With respect to developing flexible and scalable spin-wave sources, the second part of this thesis addresses auto-oscillations in spin Hall oscillators (based on a Pt / Ni81Fe19 bilayer) of tapered nanowire geometry. In these systems, a simultaneous formation of two separate spin-wave bullets of distinct localization and frequency has been indicated. This spin-wave bullet formation is con- firmed experimentally and investigated for different driving currents. Subsequently, control over these bullets by injecting external microwave signals of varying frequency and power is demon- strated, switching the oscillator into single-mode operation. Three synchronized auto-oscillatory states are observed, which can be selected by the frequency of the externally imprinted signal. This synchronization results in linewidth reduction and frequency-locking of the individual bullet modes. Simultaneously the bullet-amplitude is amplified and is found to scale as P2/3 with the injected microwave power P. This amplification and control over position and frequency of the spin-wave bullets is promising for the development of microwave amplifiers/detectors and spin- wave sources on the nanoscale based on spin Hall oscillators.:1 Introduction 1 2 Theoretical background 4 2.1 Energy density of thin film ferromagnets and domain(wall) formation 2.2 Magnetizationdynamicsinthinfilmferromagnets 11 2.2.1 Spin-wavedispersioninthelinearregime 13 2.2.2 Magnetizationdynamicsinthenon-linearregime 17 2.3 SpinHallOscillators 21 2.3.1 Spin Hall effect and spin transfer torque in a ferromagnet/heavy-metal bi- layersystem 21 2.3.2 Characteristics of magnetization auto-oscillations 25 2.3.3 Improvement of monochromaticity, coherence and output power by injec- tionlocking 28 3 Materials and Methods 31 3.1 ElectronBeamLithography,EBL 31 3.2 Ni81Fe19 microstructures 32 3.3 Femicrostructures 34 3.4 TaperedspinHalloscillators 35 3.5 Micro-focused Brillouin Light Scattering Spectroscopy, μBLS 36 3.5.1 μBLSspatialresolution 40 4 Experimental results 43 4.1 Spin-wave dynamics in multi-domain magnetic configurations 43 4.1.1 Spin-wave dynamics of 180◦ Néel walls in rectangular elements 44 4.1.2 Spin-wave dynamics of 90◦ Néel walls in square elements 63 4.1.3 Spin-wave dynamics of interconnected Néel walls in Fe wires 76 4.2 Auto-oscillationintaperedwiregeometries 88 4.2.1 Initial static magnetic configuration and effective field 89 4.2.2 Thermally excited dynamics and spectral properties 91 4.2.3 Direct microwave excitation of spin-wave dynamics 93 4.2.4 Auto-oscillatoryresponse 96 4.2.5 Microwaveamplificationandinjectionlocking 104 5 Summary and outlook 114 Own publications 118 Bibliography 120 Acknowledgement 141 A Appendix 143 A.1 Splitting process in magnetic domains confined by domain walls 143 A.2 reconfigurable remanent states in square structures stabilized by local ion irradiation 144 A.3 Domain wall displacements induced by a scanning laser beam 145 A.4 Magnetic Force Microscopy investigation of the domain wall type and width 147 A.5 Micromagnetic simulations: problem definition and analysis 149 A.6 Current dependence of auto-oscillations in the tapered SHO 152 A.7 Fabrication of Ni81Fe19 microstructures for spin waves in domain walls 15

Characterisation of MFM tip stray fields using Lorentz electron tomography

The work presented in this thesis is a study of the magnetic properties of various magnetic force microscopy (MFM) tips using Lorentz electron microscopy and tomography. The implementation of tomography and differential phase contrast (DPC) microscopy allows the stray field distribution in the half space in front of MFM tips to be measured with a spatial resolution of <30 nm and a field resolution of <2 mT. This information will allow the development of better models for MFM imaging performance and, potentially, the quantification of MFM images

A study of complex magnetic configurations using magnetic force microscopy

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física la Materia Condensada y Nanotecnología. Fecha de lectura:15-06-2018Esta tesis tiene embargado el acceso al texto completo hasta el 15-12-2019The irruption of nanomagnetism in industry has brought remarkable advances in data storage technologies. In addition, further development in this field is expected to revolutionize traditional medicine by addressing diagnosis and disease treatment from a localized approach. Applications require a deep fundamental knowledge on the magnetic behavior of nanostructures. This thesis is framed on the study of non-trivial magnetic configurations and magnetization reversal processes of different nano-objects. The magnetic configuration and magnetization reversal process of cylindrical shaped magnets (nanowires and nanodots) have been studied using Magnetic Force Microscopy. Different mechanisms have been studied to obtain well-defined pinning sites in nanowires with axial magnetization. The magnetization reversal process of nanowires of complex magnetic configuration, due to strong magnetocrystalline anisotropy, has been studied. Finally, results obtained in cylindrical nanodots are presented, where topologically protected spin textures have been unveile

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

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 < 10 nm, and tends toward zero for t < 7 nm. It is suggested that this is due to strain at the SiO2/Ni81Fe19 interface, which changes the magnetostriction and is related to the AMR by spin-orbit effects. The structure and pinning behaviour of DWs has been systematically investigated as a function of nanowire width, thickness and notch geometry. Although the wall structure is sensitive to the nanowire cross-sectional area, the DW depinning behaviour is relatively insensitive to notch geometry and instead is highly sensitive to wall type and chirality. A detailed model has been developed to make predictions for the AMR of individual DWs in nanowires. The model incorporates experimentally derived thickness dependent resistivity parameters and detailed DW spin structures from micromagnetic simulations. The magnitude of DW resistance is sensitive to wire width and the AMR ratio, and is found to be extremely sensitive to the magnitude of the magnetoresistance

Studies in The Characterisation of Magnetic Force Microscope Probe

A magnetically coated tip is a fundamental part of the MFM instrument. These tips’ are bought commercially and/or individually manufactured in various shapes and sizes and with various material coatings and thicknesses. The sheer extent of possible combinations and the lack of a truly standard and reproducible tip is perhaps, one of the major contributing factors that prevent a complete understanding of the instrument and its characteristics and a full comprehension of how the tip interacts with a sample. While the MFM instrument is capable of generating qualitative images, a full metrological characterisation of its magnetic probe is one of the major concerns. In this research project, the practical implications of a diagnostic sample in the form of a simple geometrical wire structure have been demonstrated. With the aid of mathematical modelling, the understanding of the interaction between the tip and the sample is improved. In addition, this research explored the effects of systematic reduction of a tip’s magnetic volume and its resulting images. It highlighted the significance of magnetic volume in image capture and provided a comprehensive quantitative insight in image type, reproducibility and quality. This project thus represents a further step towards the characterisation of MFM probes, which has the potential for ultimately benefitting the nano-magneto-electronic and data storage industry

Ferrimagnetic rare-earth-transition-metal heterostructures: implications for future data storage, sensors, and unconventional computing

In this work, different ferrimagnetic rare-earth-transition-metal heterostructures are investigated. The findings provide implications for future data storage, sensor, and unconventional computing devices. In the first part, ferri- and ferromagnetic films are exchange-coupled and studied as potential composite media for magnetic recording technologies. For this, the underlying individual layers are examined, too. Within this study, the influence of Pd and Pt insertion layers in ferromagnetic Co/Ni multilayers is investigated. In these systems, the maximum effective magnetic anisotropy is more than doubled by the introduced insertion layers, while the initial saturation magnetization and Curie temperature are reduced. Further, amorphous Tb-FeCo alloys and multilayers are studied as the second building block of the desired composite medium. In particular, the structural and magnetic properties are analyzed upon post-annealing. At temperatures above 400 K, irreversible effects on the structural properties are found, which also influence the magnetic properties. It is shown that these changes in properties cannot be prevented by tuning the composition or by a multilayer structure of the film. However, key insights on the structural and magnetic properties upon annealing are provided for future high-temperature devices. Afterward, the exchange-coupled ferrimagnetic/ferromagnetic bilayer is studied. Measurements on the dependency on temperature, the ferrimagnetic composition, and the thickness of the ferromagnet are carried out. Two distinct magnetic reversal mechanisms are revealed. The reversal characteristics depend critically on the thickness of the ferromagnetic layer. The underlying microscopic origin is revealed by high-resolution magnetic force microscopy. Above a certain thickness of the ferromagnet, the switching process is driven by in-plane domain wall propagation. In contrast, thinner ferromagnetic layers exhibit a nucleation-dominated reversal due to grain-to-grain variations in magnetic anisotropy. Although the realization of an exchange-coupled composite medium for magnetic recording can not be achieved, insights for the future realization of sub micron high energy density permanent magnets and spintronic devices are gained. In the second part of this work, topologically protected spin structures, including skyrmions and antiskyrmions, are investigated in Fe/Gd-based multilayers. Particularly in coexisting phases, different topologically protected magnetic quasi-particles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials, until now, antiskyrmions have been exclusive to materials with D2d or S4 symmetry. In this work, first and second-order antiskyrmions are stabilized for the first time by magnetic dipole-dipole interaction. Using Lorentz transmission electron microscopy imaging, coexisting first- and second-order antiskyrmions, Bloch skyrmions, and type-2 bubbles are observed, and the range of material properties and magnetic fields where the different spin objects form and dissipate is determined. The discovered phase pocket of metastable antiskyrmions for low saturation magnetization and uniaxial magnetic anisotropy values is confirmed by micromagnetic simulations and represents a recipe, which has to be satisfied for the stabilization of antiskyrmions by dipole-dipole interaction in other material systems. Furthermore, the nucleation process of the spin objects and the influence of an in-plane magnetic field are studied. Additionally, post-deposition techniques are employed to locally change the anisotropy of the samples and influence the nucleation and stability range of the spin objects. The gained knowledge significantly simplifies future investigations of antiskyrmions. Moreover, the coexisting phases of different topologically protected spin objects and their controlled nucleation provide great potential for further studies on magnetic quasi-particle interactions, spin dynamics, as well as for possible future applications in spintronics, namely the racetrack memory, skyrmionic interconnections, skyrmion-based unconventional computing, and sensor devices

Controlling magnetic domain wall pinning using atomic force microscope tip-based nanomachining

In recent decades, atomic force microscope (AFM) tip-based nanomachining has gained the increased attention of researchers as a technique capable of creating nanoscale features on the surfaces of materials. In principle, it represents a potentially low-cost alternative to other more expensive methods for nanoscale fabrication, particularly at the prototyping stage of device development. However, currently the numbers of practical applications that take advantage of this technique are limited. At the same time, magnetic domains at nanoscales have also shown to be of interest in recent decades with the potential for use in various applications. An example of which includes racetrack memory, a possible future non-volatile data storage system that would have higher densities and faster read/write speeds [1]. A key issue in successfully developing these systems relates to the ability to accurately control the motion of the magnetic domain boundaries known as domain walls (DWs). In this context, this thesis presents a study attempting to combine these issues. AFM tip-based nanomachining is used to create vertical nanotrenches cut along the top surface across the width of magnetic nanowires with the intention of pinning DWs at them. A computational study was conducted focusing on the effects of the shape and geometry of vertical nanotrenches on their pinning strength. This was inspired by the fact that nanotrenches created by AFM tip-based nanomaching have a tendency to be triangular in shape due to the pyramidal tips. Simulations were carried out using the Object-Oriented MicroMagnetic Framework (OOMMF). It was found that triangular nanotrenches pin both transverse and vortex DWs more weakly than their square counterparts. The depth of both shape nanotrenches appear to have approximately linear relationships with the pinning strength. It was found that whilst square nanotrenches retain their pinning strength as their length is increased, triangular nanotrenches reduce in pinning strength beyond a significant length relative to its depth. This was shown to be related to the angle between the triangular nanotrench wall and the x-y plane. It is found that the depinning fields drastically reduce when the angle is reduced below 10 ° corresponding to relatively shallow and long nanotrenches. Experiments are carried out to test the viability of AFM tip-based nanomachining for pinning DWs. Anisotropic Magnetoresistance measurements were used to detect the IV presence of DWs and measure the resulting depinning fields. In addition to this, magnetic full-field transmission soft x-ray microscopy was utilised to directly image DWs pinned at the created sites. It was found to be a viable option as multiple ferromagnetic nanowires are machined and DWs are pinned at the resulting nanotrenches. Further work was conducted to verify the computational results regarding nanotrench depth and pinning strength. Experimental data gathered agrees with the computational simulations and show an approximately linear increase in depinning fields with an increase in nanotrench depth