Adjusting the Neel relaxation time of Fe3O4/ZnxCo1-xFe2O4 core/shell nanoparticles for optimal heat generation in magnetic hyperthermia

In this work it is shown a precise way to optimize the heat generation in high viscosity magnetic colloids, by adjusting the Neel relaxation time in core/shell bimagnetic nanoparticles, for magnetic fluid hyperthermia (MFH) applications. To pursue this goal, Fe3O4/ZnxCo1-xFe2O4 core/shell nanoparticles were synthesized with 8.5 nm mean core diameter, encapsulated in a shell of similar to 1.1 nm of thickness, where the Zn atomic ratio (Zn/(Zn + Co) at%) changes from 33 to 68 at%. The magnetic measurements are consistent with a rigid interface coupling between the core and shell phases, where the effective magnetic anisotropy systematically decreases when the Zn concentration increases, without a significant change of the saturation magnetization. Experiments of MFH of 0.1 wt% of these particles dispersed in water, in Dulbecco modified Eagles minimal essential medium, and a high viscosity butter oil, result in a large specific loss power (SLP), up to 150 W g(-1), when the experiments are performed at 571 kHz and 200 Oe. The SLP was optimized adjusting the shell composition, showing a maximum for intermediate Zn concentration. This study shows a way to maximize the heat generation in viscous media like cytosol, for those biomedical applications that require smaller particle sizes

Remanence plots as a probe of spin disorder in magnetic nanoparticles

Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-FeO nanoparticles isolated by thick SiO shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles

Polarization Analyzed Small Angle Neutron Scattering of Ferrite Nanoparticles

Ferromagnetic nanoparticles offer a range of possible applications in nanotechnology, biomedical practices, and data storage, but important issues exist regarding their true magnetic structure. We have been investigating 9 nm diameter Fe3O4 nanoparticles and 11 nm diameter CoFe2O4 nanoparticles coated with an oleic acid shell. The nanoparticles were synthesized by solution chemistry methods and characterized by X-ray diffraction, SQUID, and vibrating sample magnetometry. Polarization Analyzed Small Angle Neutron Scattering (PASANS) was used under various temperatures and applied magnetic fields to investigate the magnetic structure of the particles. PASANS has revealed the iron oxide particles have a canted magnetic shell in high field that disappears in lower field, while the cobalt iron oxide particles are fully canted. We have developed an energy-based model to explain the origins of this canting and shell formation that agrees with the experimental results

Micromagnetic modelling of thin film media

The magnetic recording industry is continuously trying to increase the density of recording media. There is a continuous need, therefore, to fully understand the magnetic processes that occur in such media. To enable this understanding, theoretical studies are conducted in the form of micromagnetic modelling. This thesis is concerned with the micromagnetic modelling of thin film media. Experiments have shown that thin film media consists of closely packed, irregularly shaped grains with non-magnetic boundaries. To simulate a model of the physical structure, the Voronoi Construction Technique was implemented to give the required irregular structure. The grains were assumed to have uniaxial anisotropy and the magnetisation within the grain was assumed to vary throughout. To achieve non-uniformity, the grains were divided into triangles and the magnetisation within each triangle was assumed to vary linearly. The effect on the magnetisation within the grains due to the influences from an externally applied field, an anisotropy field, a magnetostatic field and an exchange field was observed. The motion of the magnetic moments under these influences was modelled by the Landau-Lifshitz equation. The most time consuming calculation in the modelling process is the magnetostatic field calculation. Therefore, continuous research into more efficient methods of calculating this field is carried out. The model initially uses a dipole approximation to calculate the total contribution from the magnetostatic field. A more accurate magnetostatic calculation, based on volume and surface charges of the triangles, was implemented to calculate the close range magnetostatic effects. The integrals were found to have singularities when the point of evaluation lay on one of the vertices of a triangle to be integrated. The Shift Method was introduced to overcome the problem which translated the problem vertex slightly away from the point of evaluation. Vast differences in the hysteresis loops when using the two methods of calculating the magnetostatic field were seen. The dipole approximation appeared to be too inaccurate in calculating the magnetostatic field. With the introduction of the more accurate method, the model compared well against previous findings

Tribological Performance of the Head-Disk Interface in Perpendicular Magnetic Recording and Heat-Assisted Magnetic Recording

International Data Corporation (IDC) estimates that hard disk drives will still be the main storage device for storing digital data in the next 10 years, holding approximately 80% of the data inside data centers. To increase the areal density of hard disk drives, the mechanical spacing between the head and disk surface has decreased to approximately 1nm. At such a small spacing, tribology of the head-disk interface, including head-disk contacts, wear, material buildup, and lubricant transfer, become increasingly more important for the reliability of hard disk drives. In addition to small spacing, heat-assisted magnetic recording (HAMR) technology aims to deliver higher areal density recording by heating up the media surface to a few hundred Celsius degrees, facilitating the writing process. High temperature at the head and disk surfaces cause serious reliability issues for the head-disk interface (HDI). Therefore, understanding of the main factors that affect the reliability of the head-disk interface is an essential task. In this dissertation, the effect of bias voltage and helium environment on the tribological performance of the head-disk interface is investigated. To do this, we first simulated the flying characteristics of the slider as a function of bias voltage in air and helium environment. Thereafter, an experimental study was performed using custom built tester located inside a sealed environmental chamber to study the effect of air and helium on wear and lubricant redistribution at the head-disk interface during load-unload. We investigated the effect of bias voltage and relative humidity on wear, material buildup, and nano-corrosion on the slider surface. Finally, we have studied laser current and laser optical power in heat-assisted magnetic recording as a function of operating radius, head-disk clearance, media design, and their effects on the life-time of the head-disk interface. The results of this dissertation provide guidance for the effect of bias voltage, relative humidity, and helium environment on wear, material buildup, corrosion, and lubricant transfer at the head-disk interface. More importantly, our experimental study in heat-assisted magnetic recording leads to a better understanding of the main factors that cause failure of the HAMR head-disk interface. Our results are important for the improvement of the tribological performance and reliability of perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) head-disk interface

自旋波驱动畴壁运动动力学的微磁学研究

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2021. 2. Chan Park.자벽 이동은 오랫동안 차세대 논리 및 메모리 장치를 개발하는 데에 가능한 해결책으로 여겨져 왔다. 자벽 이동을 구동하기 위하여, 최근 스핀파가 새로운 원동력으로 제안되고 있다. 그러나, 자벽이동의 기구와 원리 관련 이해가 부족하며, 스핀파를 이용하여 자벽이동을 정밀하게 제어하는 것은 많은 해결되지 못한 문제를 가지고 있다. 이 논문에서는 자성 나노스트립 （magnetic nanostrip） 에서 스핀파로 인한 자벽 이동의 동역학을 미시 자기 시뮬레이션 (micromagnetic simulation) 을 이용하여 아래와 같이 세 가지 문제를 중심으로 조사하였다. 첫째, 스핀파가 구동된 자벽 이동의 물리적 메커니즘; 둘째, 스핀파로 인한 자벽 이동의 관성 변위; 셋째, 임의의 스핀파 (arbitrary spin waves) 와 여러 종류의 자벽이 포함된 시스템에서의 자벽 이동 거동; 첫 번째 문제와 관련하여, 스핀파의 흡수를 계산하였고 스핀파 펄스를 사용했다는 점에서 기존 연구와 차별화된다. 계산된 스핀파 흡수는 자벽 이동 속도와 동일한 경향을 가지며, 자벽 이동은 spin-transfer torque (STT) 를 제공하기 위하여 스핀파 흡수를 필요로 한다는 것이 확인되었다. 두 번째 문제와 관련하여, 유발된 스핀파 펄스가 자벽 이동을 구동할 수 있는 것과 자벽 이동의 가속과 감속 현상이 관찰되었다. 스핀파 펄스가 가해지면, 자벽이 가속과 감속을 한다는 것을, 1차원 모델을 이용하여, 설명하였다. 특히, 감속 과정은 자벽의 이완 (domain wall relaxation) 의 결과로 발생하는 것을 확인하였다 세 번째 문제와 관련하여, 서로 다른 파형의 스핀파와 다양한 형태의스택형 자벽 구조가 사용되었다. 그리고 임의의 스핀파에 의한 자벽이동을 푸리에 분석을 이용하여 정량화하였으며, 다양한 형태의 자벽이 포함된 자벽이동은 resonant 픽의 움직임이 변형된다는 것이 확인되었다. 이외에, 스택형 자벽 구조의 움직임은 속도 스펙트럼 (velocity spectrum) 에 변화를 나타내는 것을 확인하였다. 이 연구는 스핀파와 자벽 이동의 상호작용에 대한 이해를 높이고 다양한 구조의 자벽이 포함된 시스템에서의 자벽이동을 제어하는 것에 실질적으로 활용될 수 있으며, 자벽 이동을 이용하는 장치의 개발에 큰 도움을 줄 수 있을 것이다.Magnetic domain wall motion has long been considered a feasible solution to developing next-generation logic and memory devices. Recently spin wave has been proposed as a new driving force for the domain wall motion. Due to the unclear physics, however, it is currently still immature to achieve reliable control of domain wall motion using spin wave. In this thesis, the dynamics of spin wave-induced domain wall motion in a magnetic nanostrip is investigated using micromagnetic simulation. Particularly, three important problems are studied: (1) mechanism of spin wave-induced domain wall motion, (2) spin wave-induced domain wall inertial displacements, and (3) domain wall motion in cases with arbitrary spin waves and multiple domain walls. As regards the first problem, spin wave absorption by domain wall is for the first time calculated and is compared with the forward domain wall velocity. The excellent agreement between the two quantities suggests that forward domain wall motion necessarily consumes spin wave absorption for the required magnonic spin-transfer torque. Concerning the second problem, a spin wave pulse is generated to drive domain wall motion. Negligible acceleration and inevitable deceleration are observed. Such inertial displacements can be understood based on a 1-D model developed and used in this study. Particularly, the deceleration process is found to be a result of domain wall relaxation which includes the release of domain wall internal energy and reduction of the out-of-plane tilting of domain wall. Concerning the third problem, spin waves of different waveforms are generated and stacked domain wall structures are formed. It is found that spin wave harmonic is the basic element when interacting with domain wall and an arbitrary spin wave-induced domain wall motion can be quantified based on the Fourier analysis. The motion of the stacked domain walls is shown to exhibit modifications in the velocity spectrum, which can be ascribed to a changed property of spin wave reflection. This thesis aims to shed further light on the interaction between spin waves and domain walls and pave the way for future development of domain wall motion-based applications.Abstract i Acknowledgement ii Lsit of Figures iii List of Tables xv Chapter 1. Introduction 1 1.1 Motivation 1 1.1.1 Novel data storage based on domain wall motion 1 1.1.2 Other applications based on domain wall motion 7 1.2 Background 10 1.2.1 Domain wall 10 1.2.2 Domain wall motion 16 1.2.3 Spin wave-induced domain wall motion 22 1.3 Research objectives 28 1.4 Scope of this thesis 29 Reference 31 Chapter 2. Theoretical fundamentals 36 2.1 Basics of magnetism 36 2.1.1 Magnetic field 38 2.1.2 Magnetic moment 41 2.1.3 Magnetic interactions 52 2.1.4 Magnetic order 65 2.2 Theory of micromagnetism 77 2.2.1 Assumptions in the continuum theory of micromagnetism 79 2.2.2 Thermodynamics in micromagnetism 80 2.2.3 Landau free energy and effective field 81 2.2.4 Static micromagnetism 95 2.2.5 Dynamic micromagnetism 100 2.2.6 Micromagnetic simulation 135 Reference 138 Chapter 3. Mechanism of spin wave-induced domain wall motion 145 3.1 Introduction 145 3.2 Micromagnetic simulation 147 3.3 Results and discussion 147 3.4 Conclusion 153 Reference 155 Chapter 4. Spin wave-induced domain wall inertial displacements 157 4.1 Introduction 157 4.2 Micromagnetic simulation 159 4.3 Results and discussion 160 4.4 Conclusion 172 Reference 173 Chapter 5. Domain wall motion in cases with arbitrary spin wave and multiple domain walls 177 5.1 Introduction 177 5.2 Micromagnetic simulation 179 5.3 Results and discussion 180 5.4 Conclusion 195 Reference 197 Chapter 6. Conclusion and future works 200 6.1 Conclusion 200 6.2 Future works 201 List of Publications 203 Abstract in Korean 204Docto

Tuning the magnetic anisotropy in nanostructured magnetic oxides

Among nanostructured magnetic materials, nanoparticles (NPs) are unique complex physical objects: in these systems a multidomain organization is energetically unfavorable and single-magnetic-domain particles are formed, each one with a huge magnetic moment with comparison to that of single atoms, thus they are often named “supermoment”. The attractive performance of magnetic NPs based materials are appealing for several technological fields ranging from nanomedicine to high-density magneto recording. Thus, understanding the physics of magnetic nanoparticles and controlling their magnetic properties represent hot topics not only for fundamental studies but also for technological applications. The magnetic behavior of such entities is related to the reversal of their magnetization; this can be a thermal or a field activated transition, which is characterized by an energy barrier defined as a magnetic anisotropy energy (MAE), which is influenced by several parameters. Thus, the tuning of the magnetic properties of nanoparticles means control of the MAE. In this work it will be discussed how to tune the MAE at the nanoscale showing the main parameters that can influence the anisotropy itself. It will be investigated the role of particle volume in the effective anisotropy, and its correlation with the surface contribution, exploring its strong effect with particle size below 10 nm. In this framework it will be investigated the role of organic coating, underlining its ability to reduce the magnetic disorder arising from the broken symmetry at particles surface. In addition, in nanoparticle ensemble, the MAE may differ from one particle to another due to particles size and shape distributions. Thus it will be defined a detailed statistical analysis of particles’ morphology, leading to the development of a new instrument to analyze particles morphology, called “aspect maps”. The relation between the physical chemical structures of nanoparticles will be investigated on nickel doped cobalt ferrite samples, demonstrating how to tune the MAE by chemical composition, i.e., controlling magnetocrystalline anisotropy. Furthermore it will be analyzed the evolution of interparticles interactions with respect single particle magnetic anisotropy by means of a modified random anisotropy model. The last part of this work will deal with the design of novel nanostructured composites. La0.67Ca0.33MnO3 and CoFe2O4 will be combined using two different structures, which can be easily extent to other materials, to improve their magnetic interactions in order to obtain tunable magnetotrasport proprieties of the final composites

A NEW SYSTEM TO INCREASE THE LIFE EXPECTANCY OF OPTICAL DISCS. PERFORMANCE ASSESSMENT WITH A DEDICATED EXPERIMENTAL SETUP.

Negli ultimi decenni si \ue8 registrata una crescita esponenziale per quel che riguarda l'uso di supporti digitali per l\u2019archiviazione dei dati informativi. Tuttavia, l\u2019aspettativa di vita di questi supporti \ue8 inadeguata rispetto alle necessit\ue0 delle istituzioni che si occupano di archiviare e preservare, ad esempio, il patrimonio artistico, storico, e culturale. Sulla base di questa problematica, pi\uf9 volte sollevata dall\u2019UNESCO [1,35], proponiamo un approccio innovativo che la degradazione chimico-fisica dei dischi ottici in modo da aumentarne l\u2019aspettativa di vita. Gli obiettivi di questa tesi sono quindi la progettazione e la messa in atto di una nuova strategia intelligente, volta ad aumentare l'aspettativa di vita dei dischi ottici. Diversi apparati sperimentali sono stati sviluppati al fine di studiare i meccanismi della degradazione chimico-fisica dei dischi tramite test di invecchiamento accelerato che simulano le condizioni operative di utilizzo del disco. Nei dischi sono state identificate aree critiche, nelle quali la degradazione cresce statisticamente pi\uf9 velocemente della media, e delle aree sicure, dove la degradazione cresce invece pi\uf9 lentamente. Due apparati sperimentali sono stati costruiti. Il primo \ue8 una camera climatica in grado di indurre invecchiamento artificiale sui dischi. Il secondo \ue8 un sistema robotico in grado di rilevare la quantit\ue0 di errori in ogni blocco di dati, prima della fase di correzione degli errori effettuata dal codice "Reed Solomon" (prima e dopo la fase di invecchiamento artificiale, senza polveri, e in un ambiente con controllo di temperatura e umidit\ue0). L'analisi dei dati cos\uec raccolti ha permesso di identificare le gi\ue0 citate aree fisiche in cui i blocchi di dati hanno un numero di errori che si avvicina o addirittura supera la capacit\ue0 di correzione dei dati propria del codice Reed Solomon standard. I risultati di queste analisi hanno portato allo sviluppo di un "Codice Reed Solomon Adattivo" (codice A-RS), che consente di proteggere le informazioni memorizzate all'interno delle aree critiche. Infatti, il codice A-RS utilizza un algoritmo di redistribuzione che viene applicato ai simboli di parit\ue0. La ridistribuzione della ridondanza \ue8 stata calcolata tramite una funzione di degrado, a sua volta ottenuta fittando gli errori rilevati nella fase sperimentale. L'algoritmo di redistribuzione sposta i simboli di parit\ue0 dai blocchi di dati nelle aree sicure ai blocchi di dati nelle aree critiche. Inoltre, \ue8 interessante sottolineare come questo processo non diminuisca la capacit\ue0 di memoria dei Digital Versatile Discs (DVDs) e dei Blu-ray Discs (BDs). Questa strategia consente quindi di evitare la dismissione anticipata dei dischi ottici, dovuta alle possibili perdite di dati.The past decade has witnessed an exponential growth in the use of digital supports for big data archiving. However, the expected lifespan of these supports is inadequate with respect to the actual needs of heritage institutions. Stemming from the issues raised by UNESCO [1, 2], we address the problem of alleviating the effects of aging on optical discs. To achieve this purpose, we propose a novel logical approach that is able to conteract the physical and chemical degradation of different types of optical discs, increasing their life expectancy. In other words, the objectives of this thesis are the design and the implementation of a new intelligent strategy aimed at increasing the life expectancy of optical discs. An experimental setup has been developed in order to investigate the physical and chemical degradation processes of discs, by means of accelerated aging tests that simulated the operational disc-use conditions. Critical areas are identified where disc degradation is statistically faster than average, while in safe areas the degradation is relatively slow. To collect the needed data, two experimental devices have been built. The first is a climatic chamber, which is able to induce artificial disc aging. The second is a robotic device, which is able to detect the amount of errors in each data block prior to the \u201cReed Solomon\u201d error correction stage (before and after the accelerate aging stage, without dust and in an environment with controlled temperature and humidity). The analysis of the data allows to identify the aforementioned physical areas where data blocks have a number of errors that approaches or exceeds the data correction capability of the standard Reed Solomon code. The results of these analyses have led to develop an \u201cAdaptive Reed Solomon Code\u201d (A-RS code) that allows to protect the information stored within the critical areas. The A-RS code uses a redistribution algorithm that is applied to parity symbols. It is calculated from the fitting of the experimental errors obtained through the \u201cdegradation function\u201d. The redistribution algorithm shifts a certain number of parity symbols from Data Blocks in safe areas to Data Blocks in critical areas. Interestingly, these processes do not diminish the memory capability of Digital Versatile discs (DVDs) and of Blu-Ray discs (BDs). This strategy therefore avoids the early dismission of optical discs, due to possible losses of even minimal parts of the recorded information

Modelling data storage in nano-island magnetic materials

Data storage in current hard disk drives is limited by three factors. These are thermal stability of recorded data, the ability to store data, and the ability to read back the stored data. An attempt to alleviate one factor can affect others. This ultimately limits magnetic recording densities that can be achieved using traditional forms of data storage. In order to advance magnetic recording and postpone these inhibiting factors, new approaches are required. One approach is recording on Bit Patterned Media (BPM) where the medium is patterned into nanometer-sized magnetic islands where each stores a binary digit.This thesis presents a statistical model of write errors in BPM composed of single domain islands. The model includes thermal activation in a calculation of write errors without resorting to time consuming micromagnetic simulations of huge populations of islands. The model incorporates distributions of position, magnetic and geometric properties of islands. In order to study the impact of island geometry variations on the recording performance of BPM systems, the magnetometric demagnetising factors for a truncated elliptic cone, a generalised geometry that reasonably describe most proposed island shapes, were derived analytically.The inclusion of thermal activation was enabled by an analytic derivation of the energy barrier for a single domain island. The energy barrier is used in a calculation of transition rates that enable the calculation of error rates. The model has been used to study write-error performance of BPM systems having distributions of position, geometric and magnetic property variations. Results showed that island intrinsic anisotropy and position variations have a larger impact on write-error performance than geometric variations.The model was also used to study thermally activated Adjacent Track Erasure (ATE) for a specific write head. The write head had a rectangular main pole of 13 by 40 nm (cross-track x down-track) with pole trailing shield gap of 5 nm and pole side shield gap of 10 nm. The distance from the pole to the top surface of the medium was 5 nm, the medium was 10 nm thick and there was a 2 nm interlayer between the soft underlayer (SUL) and the medium, making a total SUL to pole spacing of 17 nm. The results showed that ATE would be a major problem and that cross-track head field gradients need to be more tightly controlled than down-track. With the write head used, recording at 1 Tb/in² would be possible on single domain islands.EThOS - Electronic Theses Online ServiceEPSRCSchool of Computer ScienceGBUnited Kingdo

Domain Wall Motion and Magnetization Switching Driven by Magnetic/Laser Fields

The work presented in this thesis mainly focus on the domain wall motion in planar Permalloy nanowires and magnetization switching in ferromagnetic Co/Pt thin films studied with both static Kerr imaging and dynamic pump and probe time-resolved magneto-optical Kerr effect (TR-MOKE) measurement techniques. The aim is to provide insightful support for the future novel data storage devices based on magnetic nanostructures. A spatially resolved wide-field magneto-optical Kerr imaging system has been successfully built during this project. The dependence of the coercivity of the Permalloy nanowires on different notch depth and geometries have been investigated systematically. The results show that the depinning field is strongly dependent on the detailed notch geometry. We have found that even the notches with same triangular feature, but different orientations will have a large depinning field difference. Our micromagnetic simulations support the experimental observations and show the correlation between the notch geometry, and the domain wall pinning and depinning processes. The laser-induced precession dynamics in Co/Pt thin films with multiple layers have been investigated by pump and probe TR-MOKE. By fitting the experimental curves via phenomenological formula, the effective Gilbert damping constants are retrieved. The results show that Gilbert damping constant has a significant external field and layer repetition number dependences. The more Co/Pt layer repeats number, the higher value of constant. This enhancement of the Gilbert damping constant value with more Co/Pt layer repeats could be due to the interfacial effect or decoherence. As the lattice mismatching in the Co/Pt structures increases with the thickness, the increased dislocation may promote electron hopping between two different sites which enhances the intrinsic damping in the thicker films. Combing the TR-MOKE system with the Kerr imaging, the magneto-optical responses of the ferromagnetic Co/Pt films with different Co thicknesses under the action of femtosecond laser beam have been explored systematically. We have developed a new approach to study the helicity dependence in the all optical switching (AOS) by varying the degree of helicity. Our results demonstrate unambiguously that the laser helicity plays an essential role in the AOS. We have further established a detailed relationship between domain switching percentage and the degree of the pumping laser helicity