Patterned ferrimagnetic thin films of spinel ferrites obtained directly by laser irradiation

Some spinel ferrites can be oxidized or transformed at moderate temperatures. Such modifications werecarried out on thin films of mixed cobalt copper ferrites and maghemite, by heating small regions with alow-power laser spot applied for about 100 ns. The very simple laser heating process, which can be donedirectly with a conventional photolithographic machine, made it possible to generate two-dimensionalmagnetization heterogeneities in ferrimagnetic films. Such periodic structures could display the specificproperties of magneto-photonic or magnonic crystals

Characterisation of magnetic nanostructures for spintronic applications by electron microscopy

The work presented in this PhD thesis concerns the characterisation of the physical structure, composition and domain structure of advanced magnetic materials by electron microscopy within the FP6 European Research Training Network "Spinswitch". In particular the investigations concerned MgO/CoFeB/MgO multilayers to be employed in magnetic sensors (this work was done in collaboration with INESC-MN Lisbon-Portugal); Ni80Fe20/Cu electrodeposited nanowires to be employed as spin transfer torque devices (this work was done in collaboration with NIRDTP Iasi-Romania and University of Salamanca); multilayers with perpendicular anisotropy which represent potential candidates to be employed in the next generation of MRAMs (this work was done in collaboration with Spintec-CEA-Grenoble). Chapter 1 will provide an overview of the physics behind the topics treated during this work and a description of the general motivations of the research carried out. Chapter 2 will provide an overview of all the experimental techniques employed for the fabrication and characterisation of the samples investigated for this research. Chapter 3 aims to present an investigation using conventional transmission electron microscopy (CTEM) and Lorentz microscopy (LTEM) to characterise respectively the physical microstructure and the domain structure of the CoFeB free layer, embedded in a multilayer composed by SiN/MgO(50)/CoFeB(t)/MgO(15), with t from 30 Å down to 14 Å. We carried out first the investigation of the physical structure performed by selected area diffraction and bright field imaging of planar samples and physically the plan view sections show the structure of the films appears similar. The magnetization reversal behaviour observed during Lorentz TEM experiments are found to vary considerably with the CoFeB thickness, with both domain wall formation and magnetisation rotation seen. In the thicker film the behaviour was characteristic of a typical soft magnetic material with uniaxial anisotropy. However the magnetic reversal of the thinner film was more complex. A particular characteristic of the 14 Å CoFeB layer is the variation of domain wall angle seen when varying the orientation of the applied field This wall asymmetry suggests the presence of a unidirectional anisotropic energy term. To assist in the interpretation of these experimental results a modified Stoner–Wohlfarth model has been constructed and calculations have been carried out by using a MATLAB code. The purpose of the project presented in Chapter 4 was the advanced characterisation of multilayered electrodeposited NiFe/Cu nanowires grown in alumina and polycarbonate templates. In particular the objective was the characterisation of the structure and local chemistry of the nanowires by TEM and the classification of nanowire switching deduced by Lorentz microscopy experiments, which are challenging for this specific material system. In order to perform TEM studies on single nanowires, they should be extracted from their template. The chemical etching used to remove the nanowires from the template in addition to issues related to the deposition of Cu, led to nanowires with edge and compositional irregularities, detrimental for their magnetic properties. Indeed, we were not able to classify the nanowire switching and investigate domain walls forming during the reversal process, but we could only observe a change in the magnetising state. A lot of the work described in this chapter deals with the difficulties associated with imaging these challenging nanowires. Issues were discovered that may have resulted from deposition and/or etching for TEM preparation, therefore we do rely heavily on simulations and calculations. The research presented in Chapter 5 will describe the investigation of the reorientation process of the easy axis for two different multilayer systems magnetised out of plane, and the evolution of their domain structure for increasing temperature, and trying to understand the role of the insertion of a Co/Pt/Ni/Pt multilayer from a microscopic point of view. The two multilayers represent the free layer of a perpendicular MTJ (pMTJ) and this study represents a state of development of materials for pMTJs. Experiments were performed by MOKE magnetometry in polar configuration and Lorentz Microscopy in Fresnel mode. Materials were prepared in Spintec-CEA, Grenoble (France) where the MOKE experiments were also carried out, and Lorentz Microscopy experiments were performed in Glasgow. For the first multilayer (with Co/Pt/Ni/Pt) we found that for lower temperatures (25°C - 220°C) the specimen appears to have a strong perpendicular anisotropy. We observed a small scale random domain structure that we can ascribe to perpendicularly magnetised domains. For higher temperatures (220°C - 300°C) we found a behaviour typical of a soft magnetic material magnetised in plane with low anisotropy and high susceptibility. For the second multilayer (without Co/Pt/Ni/Pt), for instrumental reasons, we were not able to investigation of the magnetic behaviour of the specimen for temperatures above 110°C. The magnetisation is out of plane for all the temperatures investigated. The sample develops a different domain structure when the sample is heated below 100°C or above 100°C. In the first case isotropic serpentine domain structure is visible, with a large periodicity, whereas in the second case, an anisotropic stripe domain structure is visible with a small periodicity

The Micromagnetism of Video Recording Tracks Written on CoNi Metal Evaporated Tape (MET)

The main objective of the work described in this thesis was to study the micromagnetic structure of 8mm video recording tracks. We attempted to quantify the written magnetisation patterns and correlate the results with the bulk magnetic characteristics of each of the different samples analysed. We studied the micromagnetics as a function of the physical and magnetic characteristics of the films and also the wavelength of the recorded signal. The basic principles of ferromagnetism are outlined in chapter 1, and then in chapter 2 we describe the way in which the technology of magnetic recording harnesses these properties. Chapter 2 also provides details of the recording format used in video 8. This section concludes with the demands now placed on the physical and magnetic characteristics of the materials which these high storage density applications require. The high resolution required to examine the written tracks necessitates the use of electron microscopy and chapter 3 describes the basic beam specimen interactions and illustrates the ways in which contrast can be produced which relates to the magnetisation within the sample. Chapter 4 details the specific requirements of the Differential Phase Contrast (DPC) mode of Lorentz Electron Microscopy which have to be met if quantitative interpretation of the information is to be possible. This section also indicates the operational limits of this technique and the relevance of these on the results. The next 3 chapters (5-7) contain the results which form the main body of this thesis. The fabrication procedure of the films and the physical and compositional characterisation of the samples is detailed in chapter 5. Chapter 6 contains the results of the bulk magnetic characterisation of these MET films The quantitative micromagnetic results obtained from each film, at two recording densities, form the basis of chapter 7. We also detail small angle scattering experiments which were necessary to establish the validity of the quantitative results. The final chapter, chapter 8, suggests ways in which we feel that much of the information which we obtained, about the micromagnetic structure of the recorded samples, could be improved now that the DPC system has recently been modified. We also comment on the relevance of new techniques now available and how these might add to our existing knowledge of the micromagnetic structure produced within ferromagnetic materials when subjected to localised magnetic fields

Directed Energy Deposition of Ni-Mn-Ga Magnetic Shape Memory Alloy

Processing of magnetic shape memory alloys – active materials that show strains of up to 12 % in a magnetic field and that are being targeted for application as actuators, sensors and energy harvesters – suffers from challenges including time intensive production and macrosegregation that leads to reduced yield. Furthermore, the brittle mechanical behavior of these materials largely eliminates the possibility of machining for a desired shape. This work explores directed energy deposition, an additive manufacturing or “3D printing” process, as an alternative processing route for Ni Mn Ga magnetic shape memory alloy. The magnetic properties, transformation behavior, and composition of the feedstock powder and deposits resulting from a laser metal deposition process are investigated against varied laser power. All samples are seen to possess favorable magnetic behavior and potentially favorable phase for magnetic field induced strain (MFIS) to take place. Additionally, the microstructure of the deposited samples is observed and its special features that may aid MFIS are discussed. Most notably, this thesis presents possible evidence of twin variants crossing deposition layers and of twin boundary motion in a magnetic field. Finally, a connection between lower laser power and reduced loss of Mn is considered

Spontaneous formation of spiral-like patterns with distinct periodic physical properties by confined electrodeposition of Co-In disks

Spatio-temporal patterns are ubiquitous in different areas of materials science and biological systems. However, typically the motifs in these types of systems present a random distribution with many possible different structures. Herein, we demonstrate that controlled spatio-temporal patterns, with reproducible spiral-like shapes, can be obtained by electrodeposition of Co-In alloys inside a confined circular geometry (i.e., in disks commensurate with the typical size of the spatio-temporal features). These patterns are mainly of compositional nature, i.e., with virtually no topographic features. Interestingly, the local changes in composition lead to a periodic modulation of the physical (electric, magnetic and mechanical) properties. Namely, the Co-rich areas show higher saturation magnetization and electrical conductivity and are mechanically harder than the In-rich ones. Thus, this work reveals that confined electrodeposition of this binary system constitutes an effective procedure to attain template-free magnetic, electric and mechanical surface patterning with specific and reproducible shapes

Bifunctional CoFe2O4/ZnO Core/Shell Nanoparticles for Magnetic Fluid Hyperthermia with Controlled Optical Response

Conjugation of optical and magnetic responses in a unique system at the nanoscale emerges as a powerful tool for several applications. Here, we fabricated bifunctional CoFe2O4-core/ZnO-shell nanoparticles with simultaneous photoluminescence in the visible range and ac magnetic losses suitable for hyperthermia. The structural characterization confirms that the system is formed by a ≈7 nm CoFe2O4 core encapsulated in a ≈1.5-nm-thick semiconducting ZnO shell. As expected from its high anisotropy, the magnetic losses in an ac magnetic field are dominated by the Brown relaxation mechanism. The ac magnetic response of the core/shell system can be accurately predicted by the linear response theory and differs from that one of bare CoFe2O4 nanoparticles as a consequence of changes in the viscous relaxation process due to the effect of the magnetostatic interactions. Concerning the optical properties, by comparing core/shell CoFe2O4/ZnO and single-phase ZnO nanoparticles, we found that the former exhibits a broader optical absorption and photoluminescence, both shifted to the visible range, indicating that the optical properties are closely associated with the shell-morphology of ZnO. Being focused on bifunctional nanoparticles with an optical response in the visible range and a tunable hyperthermia output, our results can help to address current open questions on magnetic fluid hyperthermia.Fil: Lavorato, Gabriel Carlos. Centro Brasileiro de Pesquisas Físicas; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Lima, Enio Junior. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Vasquez Mansilla, Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Troiani, Horacio Esteban. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Zysler, Roberto Daniel. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Winkler, Elin Lilian. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Characterisation of focused ion beam nanostructures by transmission electron microscopy

Ion irradiation is an effective tool for the modifcation and control of the properties of magnetic thin films. Basic magnetic properties such as coercivity and local anisotropy direction can be altered in NiFe (Permalloy) films, whilst for Co/Pd multilayers, ion irradiation results in a transition from perpendicular to in-plane magnetisation. This ability to tailor magnetic properties in a controlled manner can be used as a tool for nanoscale patterning. Results are presented from investigations into the effect of Ga+ ion dose on the magnetic and structural properties of permalloy thin film systems. Systems consisting of a permalloy layer of either 10nm or 20nm, and one or more non-magnetic layers of Al or Au were deposited by thermal evaporation and irradiated in a focused ion beam (FIB) with a 30kV Ga ion source. The presence of the non-magnetic layers allows irradiation induced mixing with the magnetic layer, effectively creating alloyed regions with different properties to the rest of the film. At low ion doses, no signifcant effect on either the magnetic or structural properties were observed. Bright field TEM images of the irradiated regions revealed that increasing the dose to 1x10^15 ions/cm^2 and above caused an increase in mean grain size from ~5nm to ~30nm. The Fresnel mode of Lorentz microscopy revealed that a reduction in the mean moment was also observed at these doses but no clear changes in coercivity or magnetisation reversal behaviour were observed until the systems were rendered non-magnetic. This occurred at 1x10^16 and 3x10^16 ions/cm^2 for systems with 10nm NiFe and 20nm NiFe respectively. Milling of the samples was evident at these high doses, meaning that it was not possible to magnetically pattern these systems without occasioning a change of 2nm and 6nm respectively in the thickness of the samples. Based on the above, structures were created to control the location of magnetic domain walls (DW). Lines were written by FIB in simple elements with dimensions <1micron, the aim being to create a higher density of DW than could be realised in equivalent homogeneous elements. Structures containing high DW densities are attractive for measuring domain wall magnetoresistive effects and have potential application in DW-based storage or logic devices. One geometry of interest is an element with `zigzag' edges. Results are be presented in chapter 4 showing how these can support either quasi-uniform magnetisation or multi-domain structures separated by DW with spacing <100nm. In chapter 5 irradiation of magnetic structures was again carried out, but this time in magnetic wires to create defect or pinning sites. Domain wall traps fabricated by ion irradiation were characterised, and irradiation line defects introduced along the wire. The lines were patterned at 90± and 45± to the length of the wire, and successfully pinned the domain walls at predefned locations. A 90 degree line irradiated at a dose of 1x10^15 ions/cm^2 was not able to provide a strong enough pinning site for a domain wall. However, when the angle of the line was changed to ±45 degrees it was possible to reproducibly pin domain walls at these sites. A relationship between the orientation of the irradiated line and the chirality of the domain wall that pinned at the site was observed. The effcts of irradiation on Co/Pd multilayers with perpendicular magnetic anisotropy was investigated in chapter 6. Irradiation causes magnetic systems with perpendicular magnetisation to undergo a transition from out-of-plane magnetisation to in-plane. A grid pattern was devised so that magnetic states with both in-plane and out-of-plane magnetisation could be observed. A combination of differential phase contrast microscopy and simulations of integrated magnetic induction were used to determine the orientation of magnetisation within the lines

Fabrication of ballistic suspended graphene with local-gating

Herein we discuss the fabrication of ballistic suspended graphene nanostructures supplemented with local gating. Using in-situ current annealing, we show that exceptional high mobilities can be obtained in these devices. A detailed description is given of the fabrication of bottom and different top-gate structures, which enable the realization of complex graphene structures. We have studied the basic building block, the p-n junction in detail, where a striking oscillating pattern was observed, which can be traced back to Fabry-Perot oscillations that are localized in the electronic cavities formed by the local gates. Finally we show some examples how the method can be extended to incorporate multi-terminal junctions or shaped graphene. The structures discussed here enable the access to electron-optics experiments in ballistic graphene

In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe2_2 van der Waals ferromagnet

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have N\'eel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe$_2$ compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases

Pulsed Laser Deposition of Thin Film Heterostructures

Thin films of Strontium Ruthenate have been grown on Strontium Titanate and Lanthanum Aluminate (100) substrates by pulsed laser deposition. X-ray diffraction results show that the films grown on the Strontium Titanate are amorphous and polycrystalline on the Lanthanum Aluminate. Resistances versus temperature measurements show that the films exhibit semiconducting characteristics. In addition to the growth of Strontium Ruthenate thin films, multilayer heterostructures of Terfenol-D thin films on polycrystalline Lead Titanate thin films were grown by pulsed laser deposition. By using a novel experimental technique called magnetic field assisted piezoelectric force microscopy it is possible to investigate the magnetoelectric coupling between the electrostrictive Lead Titanate and magnetostrictive Terfenol-D thin film. Upon examination of the produced thin films the phase and amplitude components of the piezoelectric signal experience changes in response to an applied in-plane magnetic field. These changes provide experimental evidence of a magnetoelectric coupling between the Terfenol-D and Lead Titanate layers