Tailoring Magnetic, Mechanical and Dielectric Properties of Magnetorheological Elastomers based on Natural Rubber and Iron by Magnetic Field Assisted Curing for Possible Applications

If magnetism is universal in nature, magnetic materials are ubiquitous. A life without magnetism is unthinkable and a day without the influence of a magnetic material is unimaginable. They find innumerable applications in the form of many passive and active devices namely, compass, electric motor, generator, microphone, loud speaker, maglev train, magnetic resonance imaging, data recording and reading, hadron collider etc. The list is endless. Such is the influence of magnetism and magnetic materials in ones day to day life. With the advent of nanoscience and nanotechnology, along with the emergence of new areas/fields such as spintronics, multiferroics and magnetic refrigeration, the importance of magnetism is ever increasing and attracting the attention of researchers worldwide. The search for a fluid which exhibits magnetism has been on for quite some time. However nature has not bestowed us with a magnetic fluid and hence it has been the dream of many researchers to synthesize a magnetic fluid which is thought to revolutionize many applications based on magnetism. The discovery of a magnetic fluid by Jacob Rabinow in the year 1952 paved the way for a new branch of Physics/Engineering which later became magnetic fluids. This gave birth to a new class of material called magnetorheological materials. Magnetorheological materials are considered superior to electrorheological materials in that magnetorheology is a contactless operation and often inexpensive.Most of the studies in the past on magnetorheological materials were based on magnetic fluids. Recently the focus has been on the solid state analogue of magnetic fluids which are called Magnetorheological Elastomers (MREs). The very word magnetorheological elastomer implies that the rheological properties of these materials can be altered by the influence of an external applied magnetic field and this process is reversible. If the application of an external magnetic field modifies the viscosity of a magnetic fluid, the effect of external magnetic stimuli on a magnetorheological elastomer is in the modification of its stiffness. They are reversible too. Magnetorheological materials exhibit variable stiffness and find applications in adaptive structures of aerospace, automotive civil and electrical engineering applications. The major advantage of MRE is that the particles are not able to settle with time and hence there is no need of a vessel to hold it. The possibility of hazardous waste leakage is no more with a solid MRE. Moreover, the particles in a solid MRE will not affect the performance and durability of the equipment. Usually MR solids work only in the pre yield region while MR fluids, typically work in the post yield state. The application of an external magnetic field modifies the stiffness constant, shear modulus and loss modulus which are complex quantities. In viscoelastic materials a part of the input energy is stored and released during each cycle and a part is dissipated as heat. The storage modulus G′ represents the capacity of the material to store energy of deformation, which contribute to material stiffness. The loss modulusG′′ represents the ability of the material to dissipate the energy of deformation. Such materials can find applications in the form of adaptive vibration absorbers (ATVAs), stiffness tunable mounts and variable impedance surfaces. MREs are an important material for automobile giants and became the focus of this research for eventual automatic vibration control, sound isolation, brakes, clutches and suspension systems

Trasporte eléctrico y magnetismo en sistemas electrodo - aislante - electrodo: hacia el desarrollo de dispositivos del tipo juntura tunel.

La presente tesis es un estudio dedicado a la optimización y desarrollo de sistemas del tipo juntura túnel. La metodología utilizada para la realización de la tesis consistió, en primer lugar, en la optimización de las componentes independientes de la juntura túnel: electrodo y barrera aislante. Posteriormente se optimizaron los procesos de fabricación para el desarrollo y caracterización de dispositivos del tipo juntura túnel en su forma final. En la primera parte de la tesis se analizan detalladamente los resultados obtenidos de la caracterización eléctrica y topografica de barreras aislantes en sistemas electrodo - barrera. Los sistemas bicapas estudiados, GdBa_2Cu_3_7/SrTiO_3, Nb/Ba_0,05Sr_0,95TiO_3 y YBa_2Cu_3O_7/SrTiO_3, fueron caracterizados utilizando un microscopio de fuerza atómica en modo conductor. Se propuso un modelo fenomenológico basado en los resultados experimentales, que permitió la obtención de parámetros críticos para el desarrollo de dispositivos del tipo juntura túnel con nuevas funcionalidades. La información obtenida de la caracterización de los sistemas bicapas (homogeneidad de crecimiento, baja densidad de defectos y de pinholes) indican un muy buen control de los parámetros de crecimiento de las barreras. Por otro lado, se obtuvo un buen comportamiento aislante para espesores mayores a 2 nm sin la presencia de pinholes en la barrera. La similitud en la estequiometría de las barreras (SrTiO_3) permitió comparar los distintos sistemas estudiados en términos de conductividad eléctrica. Se verificó que el modelo fenomenológico permite comparar la conductividad eléctrica de los sistemas mediante uno de los parámetros definidos en el modelo fenomenológico (obtenido de los ajustes lineales de las curvas I(V)). De los 3 sistemas estudiados, las bicapas GdBa_2Cu_3O_7/SrTiO_3 presentaron un mayor valor de longitud de atenuación de los portadores de carga a través de la barrera y una muy baja densidad de defectos superficiales. Las bicapas YBa_2Cu_3O_7/SrTiO_3 y Nb/Ba_0,05Sr_0,95TiO_3 permitieron validar el modelo fenomenológico propuesto para el análisis de la respuesta corriente - voltaje obtenida con el microscopio de fuerza atómica en modo conductor. La segunda parte de la tesis abarca conceptos de magnetismo y microfabricación para el desarrollo de junturas túnel magnéticas. Durante la caracterización de las películas ferromagnéticas individuales de Co_90Fe_10 (CoFe) se logró aumentar valor del campo coercitivo de films de 10 nm de espesor al incrementar la temperatura de depósito. Esto se debe a un aumento del tamaño de grano de los films. El aumento de la temperatura del sustrato durante el crecimiento influye en la morfología y las propiedades magnéticas de los films de CoFe favoreciendo la formación de granos y la pérdida del eje preferencial de magnetización. Estos resultados permitieron la fabricación de sistemas Co_90Fe_10/M_gO/Co_90Fe_10 con distintas orientaciones relativas accesibles con campo magnético para el estudio del acople magnético entre los films de CoFe. La caracterización eléctrica de estos sistemas, particularmente la respuesta corriente - voltaje obtenida con el microscopio de fuerza atómica en modo conductor, indicó que las propiedades de transporte eléctrico de las junturas presentan un alto grado de reproducibilidad. Se analizó además la inuencia del sustrato utilizado en la corriente túnel que atraviesa la barrera aislante. Por otro lado, se discuten los fenómenos relacionados a la optimización de las propiedades magnéticas de electrodos ferromagnéticos para la fabricación de junturas túnel Co_90Fe_10/MgO/Co_90Fe_10 y Co_90Fe_10/MgO /Fe_20Ni_80. En particular, se estudió el acople magnético entre capas ferromagnéticas y la inuencia del sustrato utilizado para el crecimiento de las tricapas. La optimización de los electrodos magnéticos involucró el análisis de la inuencia de la presencia de un aislante entre dos capas magnéticas en el acople de los electrodos. Se logró el desacople de films de 10 nm de Co_90Fe_10 y Fe_20Ni_80 separados por un espaciador de MgO de 2 nm. Finalmente se detallan los pasos para la fabricación de una red de junturas túnel magnéticas y su caracterización eléctrica a bajas temperaturas. El sistema estudiado fue la tricapa Co_90Fe_10 (10 nm)/M_gO (8 nm)/ Fe_20Ni_80 (10 nm) crecido sobre un sustrato de M_gO. La caracterización eléctrica confirmó la buena calidad de la junturas fabricadas. Las junturas obtenidas presentaron un comportamiento altamente resistivo (~ MΩ). Las mediciones de la corriente túnel en función de la temperatura permitieron descartar la presencia de pinholes en la barrera. El transporte de los portadores de carga es por efecto túnel a través de la barrera aislante. Las curvas de conductancia diferencial permitieron calcular el valor medio de la altura de la barrera de potencial (φ = 3.1 eV) a partir del modelo de Brinkman. Los resultados obtenidos en cada uno de los capítulos se complementan y son relevantes para la optimización de junturas túnel, debido a que brindan información crítica para su correcto funcionamiento. En la presente tesis se lograron obtener los primeros avances para la fabricación de arreglos de junturas túnel que permitan el desarrollo de dispositivos.

Thermodynamics and Heat Transport of doped Spin-Ice Materials

In this thesis the low-temperature magnetism of the spin-ice systems Dy2Ti2O7 and Ho2Ti2O7 is investigated. In general, a clear experimental evidence for a sizable magnetic contribution kappa_{mag} to the low-temperature, zero-field heat transport of both spin-ice materials is observed. This kappa_{mag} can be attributed to the magnetic monopole excitations, which are highly mobile in zero field and are suppressed by a rather small external field resulting in a drop of kappa(H). Towards higher magnetic fields, significant field dependencies of the phononic heat conductivities kappa_{ph}(H) of Ho2Ti2O7 and Dy2Ti2O7 are found, which are, however, of opposite signs, as it is also found for the highly dilute reference materials (Ho0.5Y0.5)2Ti2O7 and (Dy0.5Y0.5)2Ti2O7. The dominant effect in the Ho-based materials is the scattering of phonons by spin flips which appears to be significantly stronger than in the Dy-based materials. Here, the thermal conductivity is suppressed due to enhanced lattice distortions observed in the magnetostriction. Furthermore, the thermal conductivity of Dy2Ti2O7 has been investigated concerning strong hysteresis effects and slow-relaxation processes towards equilibrium states in the low-temperature and low-field regime. The thermal conductivity in the hysteretic regions slowly relaxes towards larger values suggesting that there is an additional suppression of the heat transport by disorder in the non-equilibrium states. The equilibration can even be governed by the heat current for particular configurations. A special focus was put on the dilution series Dy2Ti2O7x. From specific heat measurements, it was found that the ultra-slow thermal equilibration in pure spin ice Dy2Ti2O7 is rapidly suppressed upon dilution with non-magnetic yttrium and vanishes completely for x>=0.2 down to the lowest accessible temperatures. In general, the low-temperature entropy of (Dy1-xYx)2Ti2O7, considerably decreases with increasing x, whereas its temperature-dependence drastically increases. Thus, it could be clarified that there is no experimental evidence for a finite zero-temperature entropy in (Dy1-xYx)2Ti2O7 above x>=0.2, in clear contrast to the finite residual entropy S_{P}(x) expected from a generalized Pauling approximation. A similar discrepancy is also present between S_{P}(x) and the low-temperature entropy obtained by Monte Carlo simulations, which reproduce the experimental data from 25 K down to 0.7 K, whereas the data at 0.4 K are overestimated. A straightforward description of the field-dependence kappa(H) of the dilution series with qualitative models justifies the extraction of kappa_{mag}. It was observed that kappa_{mag} systematically scales with the degree of dilution and its low-field decrease is related to the monopole excitation energy. The diffusion coefficient D_{mag} for the monopole excitations was calculated by means of c_{mag} and kappa_{mag}. It exhibits a broad maximum around 1.6 K and is suppressed for T<=0.5 K, indicating a non-degenerate ground state in the long-time limit, and in the high-temperature range for T>=4 K where spin-ice physics is eliminated. A mean-free path of 0.3 mum is obtained for Dy2Ti2O7 at about 1 K within the kinetic gas theory.

The storage and dissipation of magnetic energy in the quiet sun corona determined from SDO/HMI magnetograms

In recent years, higher cadence, higher resolution observations have revealed the quiet-Sun photosphere to be complex and rapidly evolving. Since magnetic fields anchored in the photosphere extend up into the solar corona, it is expected that the small-scale coronal magnetic field exhibits similar complexity. For the first time, the quiet-Sun coronal magnetic field is continuously evolved through a series of non-potential, quasi-static equilibria, deduced from magnetograms observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, where the photospheric boundary condition which drives the coronal evolution exactly reproduces the observed magnetograms. The build-up, storage, and dissipation of magnetic energy within the simulations is studied. We find that the free magnetic energy built up and stored within the field is sufficient to explain small-scale, impulsive events such as nanoflares. On comparing with coronal images of the same region, the energy storage and dissipation visually reproduces many of the observed features. The results indicate that the complex small-scale magnetic evolution of a large number of magnetic features is a key element in explaining the nature of the solar corona.

Trasporte eléctrico y magnetismo en sistemas electrodo - aislante - electrodo: hacia el desarrollo de dispositivos del tipo juntura tunel.

La presente tesis es un estudio dedicado a la optimización y desarrollo de sistemas del tipo juntura túnel. La metodología utilizada para la realización de la tesis consistió, en primer lugar, en la optimización de las componentes independientes de la juntura túnel: electrodo y barrera aislante. Posteriormente se optimizaron los procesos de fabricación para el desarrollo y caracterización de dispositivos del tipo juntura túnel en su forma final. En la primera parte de la tesis se analizan detalladamente los resultados obtenidos de la caracterización eléctrica y topografica de barreras aislantes en sistemas electrodo - barrera. Los sistemas bicapas estudiados, GdBa_2Cu_3_7/SrTiO_3, Nb/Ba_0,05Sr_0,95TiO_3 y YBa_2Cu_3O_7/SrTiO_3, fueron caracterizados utilizando un microscopio de fuerza atómica en modo conductor. Se propuso un modelo fenomenológico basado en los resultados experimentales, que permitió la obtención de parámetros críticos para el desarrollo de dispositivos del tipo juntura túnel con nuevas funcionalidades. La información obtenida de la caracterización de los sistemas bicapas (homogeneidad de crecimiento, baja densidad de defectos y de pinholes) indican un muy buen control de los parámetros de crecimiento de las barreras. Por otro lado, se obtuvo un buen comportamiento aislante para espesores mayores a 2 nm sin la presencia de pinholes en la barrera. La similitud en la estequiometría de las barreras (SrTiO_3) permitió comparar los distintos sistemas estudiados en términos de conductividad eléctrica. Se verificó que el modelo fenomenológico permite comparar la conductividad eléctrica de los sistemas mediante uno de los parámetros definidos en el modelo fenomenológico (obtenido de los ajustes lineales de las curvas I(V)). De los 3 sistemas estudiados, las bicapas GdBa_2Cu_3O_7/SrTiO_3 presentaron un mayor valor de longitud de atenuación de los portadores de carga a través de la barrera y una muy baja densidad de defectos superficiales. Las bicapas YBa_2Cu_3O_7/SrTiO_3 y Nb/Ba_0,05Sr_0,95TiO_3 permitieron validar el modelo fenomenológico propuesto para el análisis de la respuesta corriente - voltaje obtenida con el microscopio de fuerza atómica en modo conductor. La segunda parte de la tesis abarca conceptos de magnetismo y microfabricación para el desarrollo de junturas túnel magnéticas. Durante la caracterización de las películas ferromagnéticas individuales de Co_90Fe_10 (CoFe) se logró aumentar valor del campo coercitivo de films de 10 nm de espesor al incrementar la temperatura de depósito. Esto se debe a un aumento del tamaño de grano de los films. El aumento de la temperatura del sustrato durante el crecimiento influye en la morfología y las propiedades magnéticas de los films de CoFe favoreciendo la formación de granos y la pérdida del eje preferencial de magnetización. Estos resultados permitieron la fabricación de sistemas Co_90Fe_10/M_gO/Co_90Fe_10 con distintas orientaciones relativas accesibles con campo magnético para el estudio del acople magnético entre los films de CoFe. La caracterización eléctrica de estos sistemas, particularmente la respuesta corriente - voltaje obtenida con el microscopio de fuerza atómica en modo conductor, indicó que las propiedades de transporte eléctrico de las junturas presentan un alto grado de reproducibilidad. Se analizó además la inuencia del sustrato utilizado en la corriente túnel que atraviesa la barrera aislante. Por otro lado, se discuten los fenómenos relacionados a la optimización de las propiedades magnéticas de electrodos ferromagnéticos para la fabricación de junturas túnel Co_90Fe_10/MgO/Co_90Fe_10 y Co_90Fe_10/MgO /Fe_20Ni_80. En particular, se estudió el acople magnético entre capas ferromagnéticas y la inuencia del sustrato utilizado para el crecimiento de las tricapas. La optimización de los electrodos magnéticos involucró el análisis de la inuencia de la presencia de un aislante entre dos capas magnéticas en el acople de los electrodos. Se logró el desacople de films de 10 nm de Co_90Fe_10 y Fe_20Ni_80 separados por un espaciador de MgO de 2 nm. Finalmente se detallan los pasos para la fabricación de una red de junturas túnel magnéticas y su caracterización eléctrica a bajas temperaturas. El sistema estudiado fue la tricapa Co_90Fe_10 (10 nm)/M_gO (8 nm)/ Fe_20Ni_80 (10 nm) crecido sobre un sustrato de M_gO. La caracterización eléctrica confirmó la buena calidad de la junturas fabricadas. Las junturas obtenidas presentaron un comportamiento altamente resistivo (~ MΩ). Las mediciones de la corriente túnel en función de la temperatura permitieron descartar la presencia de pinholes en la barrera. El transporte de los portadores de carga es por efecto túnel a través de la barrera aislante. Las curvas de conductancia diferencial permitieron calcular el valor medio de la altura de la barrera de potencial (φ = 3.1 eV) a partir del modelo de Brinkman. Los resultados obtenidos en cada uno de los capítulos se complementan y son relevantes para la optimización de junturas túnel, debido a que brindan información crítica para su correcto funcionamiento. En la presente tesis se lograron obtener los primeros avances para la fabricación de arreglos de junturas túnel que permitan el desarrollo de dispositivos.

Coupling of magnetic, strain and electric polarization fields in the structure of multiferroic material

The comprehensive study on the coupling of magnetism, electrical polarization and the crystalline lattice with the off-stoichiometric effects in self-doped multiferroic hexagonal h-LuMnxO3±δ (0.92≤x≤1.12) ceramic oxides was carried out for the PhD work. There is a complex coupling of the three ferroic degrees. The cancelation of the magnetic moments of ions in the antiferromagnetic order, electric polarization with specific vortex/antivortex topology and lattice properties have pushed researchers to find out ways to disclose the underlying physics and chemistry of magneto-electric and magneto-elastic couplings of h-RMnO3 multiferroic materials. In this research work, self-doping of Lu-sites or Mn-sites of h-LuMnxO3±δ ceramics prepared via solid state route was done to pave a way for deeper understanding of the antiferromagnetic transition, the weak ferromagnetism often reported in the same crystalline lattices and the ferroelectric properties coupled to the imposed lattice changes. Accordingly to the aim of the PhD thesis, the objectives set for the sintering study in the first chapter on experimental results were two. First, study of sintering off-stoichiometric samples within conditions reported in the bibliography and also extracted from the phase diagrams of the LuMnxO3±δ, with a multiple firings ending with a last high temperature step at 1300ºC for 24 hours. Second, explore longer annealing times of up to 240 hours at the fixed temperature of 1300 ºC in a search for improving the properties of the solid solution under study. All series of LuMnxO3±δ ceramics for each annealing time were characterized to tentatively build a framework enabling comparison of measured properties with results of others available in literature. XRD and Rietveld refinement of data give the evolution the lattice parameters as a function to x. Shrinkage of the lattice parameters with increasing x values was observed, the stability limit of the solid solution being determined by analysis of lattice parameters. The evolution of grain size and presence of secondary phases have been investigated by means of TEM, SEM, EDS and EBSD techniques. The dependencies of grain growth and regression of secondary phases on composition x and time were further characterized. Magnetic susceptibility of samples and magnetic irreversibility were extensively examined in the present work. The dependency of magnetic susceptibility, Neel ordering transition and important magnetic parameters are determined and compared to observation in other multiferroics in the following chapter of the thesis. As a tool of high sensitivity to detect minor traces of the secondary phase hausmannite, magnetic measurements are suggested for cross-checking of phase diagrams. Difficulty of previous studies on interpreting the magnetic anomaly below 43 K in h-RMnO3 oxides was discussed and assigned to the Mn3O4 phase, with supported of the electron microscopy. Magneto-electric coupling where AFM ordering is coupled to dielectric polarization is investigated as a function of x and of sintering condition via frequency and temperature dependent complex dielectric constant measurements in the final chapter of the thesis. Within the limits of solid solubility, the crystalline lattice of off-stoichiometric ceramics was shown to preserve the magneto-electric coupling at TN. It represents the first research work on magneto-electric coupling modified by vacancy doping to author’s knowledge. Studied lattices would reveal distortions at the atomic scale imposed by local changes of x dependent on sintering conditions which were widely inspected by using TEM/STEM methods, complemented with EDS and EELS spectroscopy all together to provide comprehensive information on cross coupling of distortions, inhomogeneity and electronic structure assembled and discussed in a specific chapter. Internal interfaces inside crystalline grains were examined. Qualitative explanations of the measured magnetic and ferroelectric properties were established in relation to observed nanoscale features of h-LuMnxO3±δ ceramics. Ferroelectric domains and topological defects are displayed both in TEM and AFM/PFM images, the later technique being used to look at size, distribution and switching of ferroelectric domains influenced by vacancy doping at the micron scale bridging to complementary TEM studies on the atomic structure of ferroelectric domains. In support to experimental study, DFT simulations using Wien2K code have been carried out in order to interpret the results of EELS spectra of O K-edge and to obtain information on the cation hybridization to oxygen ions. The L3,2 edges of Mn is used to access the oxidation state of the Mn ions inside crystalline grains. In addition, rehybridization driven ferroelectricity is also evaluated by comparing the partial density of states of the orbitals of all ions of the samples, also the polarization was calculated and correlated to the off-stoichiometric effect.

Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale

The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology.

Badanie sprzężeń magnetosprężystych w multiferroikach heterostrukturalnych

Wydział Fizyki

Modern advances in glass-coated microwires: A significant distinction as a soft magnet

174 p.