Дисертації з теми "Magnetostrictive effects"

The goal of this work is to investigate the effect of electric and magnetic field on the optical resonances of electrostrictive and magnetorheological optical resonators. The optical resonances, also known as whispering gallery modes (WGM) or morphology dependent resonances (MDR) experience a shift in the transmission spectrum whenever the resonator changes its size and/or index of refraction. Their small size, the elimination of electrical cabling, and the high optical quality factor, Q, make them attractive for a large number of applications. In these studies, we investigate the magnetostrictive and the electrostrictive effect of fiber coupled photonic spherical resonators. The electrostrictive and the magnetostrictive effect are the elastic deformation of a solid when subject to an electric or magnetic field respectively. In these studies, three different configurations were investigated to tune the optical modes of the spherical optical resonator. In the first configuration, the resonator was fabricated by embedding magnetic micro particles in a polymeric matrix of PVC plastisol (commercial name super soft plastic, SSP). For these configurations we studied the WGM shift that was induced when the sphere was immersed in a static and a harmonic magnetic field. These results lead to the development of a magnetic flied sensor and a non-contact transduction mechanism for displacement measurements. The sphere showed a sensitivity to the magnetic field of 0.285 pm/mT and to the displacement of 0.402 pm/?m. These values lead to a resolution of 350 ?T and 248 nm respectively. The second configuration was a microsphere that was made of pure super soft plastic and was subject to a static and harmonic electric field. The results lead to the development of a non-contact displacement sensor whose sensitivity is 0.642 pm/?m and the resolution is 155 nm. Both studies also indicate for the first time that it is possible to couple light into a PVC compound and achieve high optical quality factor of the order of 106. The third configuration was a metglas film that was mechanically coupled to a PDMS microsphere. The results of these studies lead to the development of a magnetic field sensor with sensitivity and resolution of 0.6 pm/?T and 166 nT respectively. In conclusion, these studies lead to a fundamental understanding of the dynamical behavior of electrostrictive and magnetorheological optical resonators and its potential for sensing applications. In addition, these devices could be embedded into polymeric matrix for the development of materials with actuation and sensing capabilities.

Magnetostriction means that the dimensions of a material depend on its magnetization. The primary goal of this dissertation was to understand the effect of magnetostriction on the magnetic anisotropy of single crystal magnetostrictive thin films, where the epitaxial pinning of the material to a substrate could inhibit its conversion to new dimensions. In order to address this goal, several Fe-based binary alloys were deposited onto various substrates by molecular beam epitaxy. The samples were characterized by an array of techniques including electron diffraction, Rutherford backscattering, vibrating sample magnetometry, ferromagnetic resonance, and x-ray absorption spectroscopies. The attempted growths of crystalline magnetostrictive thin films resulted in successful depositions of Fe 1-xGa x and Fe 1-xZn x. Depositions onto MgO(001) substrates result in an in-plane cubic magnetic anisotropy, as expected from the cubic symmetry of the Fe-based thin films, and a strong out-of-plane uniaxial anisotropy that forces the magnetization to lie in the plane of the films. Depositions onto ZnSe/GaAs(001) substrates feature an additional in-plane uniaxial anisotropy. The magnitudes and signs of the in-plane anisotropies depend on the Ga content. Furthermore, the cubic anisotropy constant of Fe 1-xGa x samples deposited onto MgO substrates switches sign at a lower Ga concentration than is seen in bulk Fe 1-xGa x. The effect on the magnetic anisotropy of depositing a magnetostrictive material as an epitaxial thin film is influenced by the material's magnetostrictive properties and the substrate upon which it is deposited. In particular, pinning a magnetoelastic material to a substrate will modify its cubic anisotropy, and depositions on substrates compliant to an anisotropic strain relaxation may result in a strong in-plane uniaxial anisotropy. 'Co authored by Steven Albert, Tino Jaeger, Hongyan Li, Paul Rugheimer, Juergen A. Schaefer, Yves U. Idzerda, Elke Arenholz, Hongyan Li, Gerrit van der Laan, Damon A. Resnick, Christopher M. Kuster.'

A high frequency physical phase variable electric machine model was developed using FE analysis. The model was implemented in a machine drive environment with hardware-in-the-loop. The novelty of the proposed model is that it is derived based on the actual geometrical and other physical information of the motor, considering each individual turn in the winding. This is the first attempt to develop such a model to obtain high frequency machine parameters without resorting to expensive experimental procedures currently in use. The model was used in a dynamic simulation environment to predict inverter-motor interaction. This includes motor terminal overvoltage, current spikes, as well as switching effects. In addition, a complete drive model was developed for electromagnetic interference (EMI) analysis and evaluation. This consists of the lumped parameter models of different system components, such as cable, inverter, and motor. The lumped parameter models enable faster simulations. The results obtained were verified by experimental measurements and excellent agreements were obtained. A change in the winding arrangement and its influence on the motor high frequency behavior has also been investigated. This was shown to have a little effect on the parameter values and in the motor high frequency behavior for equal number of turns. An accurate prediction of overvoltage and EMI in the design stages of the drive system would reduce the time required for the design modifications as well as for the evaluation of EMC compliance issues. The model can be utilized in the design optimization and insulation selection for motors. Use of this procedure could prove economical, as it would help designers develop and test new motor designs for the evaluation of operational impacts in various motor drive applications.

Les capteurs magnéto-électrique (ME) sont une alternative prometteuse pour mesurer de faibles signaux magnétiques. Précédemment le choix était généralement de déposer des couches minces magnétostrictives sur un matériau piézoélectrique massif conduisant à des systèmes macroscopiques de taille milllimétrique. L’intégration de ces systèmes dans des MEMS (micro-electro-mechanical systems) requiertà la fois de résoudre les problèmes d’intégration de matériaux actif sur silicium, et de mesurer des petits signaux étant donné l’importante réduction de la réponse du système lorsqu’il est miniaturisé.Dans cette optique, le premier objectif de ce travail de thèse a été d’intégrer un matériau piézoélectrique sur un substrat de silicium tout en conservant une excellente qualité cristalline. Pb(Zr ₀ , ₅ ₂Ti ₀ ,₄₈)O₃ (PZT) a été retenu pour ces excellente propriétés piézoélectriques. L’intégration de la couche mince ce fait sur silicium qui est le substrat de prédilection pour la fabrication de microsystèmes avec les procédés microélectroniques standards. La qualité cristalline des matériaux actifs est directement corrélée aux couches d'adaptation utilisées pour obtenir une bonne qualité cristalline sur silicium. Pour cel l'intégration d'une tricouche composée de zircone stabilisée à l'yttrium (YSZ), d'oxyde de cérium (CeO₂) et de SrTiO₃ permet ensuitela croissance des pérovskites d'intérêt pour le dispositif. Le choix de l’électrode conductrice inférieure (SrRuO₃ ou La ₀ ,₆₆Sr₀₃₃MnO₃ dans le cas présent) permet de contrôler l’orientation de la maille de PZT.Une première étude des propriétés piézoélectriques de la couche mince de PZT sous la forme d’une poutre libre pour son intégration dans un système magnétoélectrique a été réalisé. La mesure de la déformation de la poutre induite par l'application d'une tension électrique permet d'extraire un coefficient d₃₁ de -53pmV⁻¹, valeur inférieure au matériau massif mais à l'état de l'art dans ce type de dispositif. Dans une seconde étape, l’utilisation de la poutre comme résonateur à été étudiée. L’étude dynamique du système a permis d'obtenir la fréquence de résonance et le facteur de qualité. Le déplacement de la fréquence caractéristique du système en fonction d'une contrainte induite par une tension DC a été investigué. Enfin, l'ajout d'une couche de matériau magnétostrictif (TbFeCo) sur la poutre a finalisé la structure du capteur. Le capteur ainsi obtenu a été caractérisé et une sensibilité d’une dizaine de micro Tesla a été obtenue
Magneto-electric (ME) sensors have been demonstrated as a promising alternative for the detection of weak magnetic signals with high sensitivity. To date, most applications focused on the use of bulk piezoelectric materials on which magnetostrictive thin films are deposited leading to millimeter-sized devices. The integration of such devices into micro-electro-mechanical systems (MEMS), bringing smaller size and lower power consumption, involves addressing several scientific issues ranging from the integration of active materials on silicon to the strong reduction in amplitude of generated signals related to the size reduction of the sensor.In this context, the first goal of this thesis work was to integrate high crystalline quality piezoelectric thin films on silicon.Pb(Zr ₓTi ₁ ₋₁)O₃ (PZT) with a morphotropic composition (x=0.52) having high electromechanical coupling factor was chosen. Silicon is a necessary template as it allows for the use of conventional clean room processes for the realization of the microsystem. The crystalline quality of the active films is directly linked to the buffer layers that promote the crystalline growth on silicon. For this purpose, Yttria-stabilized Zirconia (YSZ) was used in combination with CeO₂ and SrTiO₃ to allow further growth of epitaxial perovskites. The choice of the bottom electrode material (SrRuO₃ or La ₀ ,₆₆Sr₀₃₃MnO₃ in this work) further tunes the crystalline orientation of the PZT layer.To probe the potential of such PZT thin films for ME devices, the first step was to characterize the electromechanical properties of this material in a free standing cantilever structure. Under an applied electric field, the measured displacement of the epitaxial PZT-based cantilevers is characterized by a coefficient d₃₁ =-53pmV⁻¹ , a reduced value with respect to the bulk material but that can be enhanced by further optimizing the film growth. The second step consists in ascertaining the ability of the cantilever to be used as resonator. For that purpose, first characterizations of oscillators have been performed to extract the resonant frequencies and the associated quality factors. Then, the resonant frequency shift with DC bias-induced stress was measured. Finally, a magnetostrictive layer of TbFeCo was added on the PZT cantilevers to sense magnetic field based on the ME effect. The resulting resonant frequency shift with external applied magnetic field was characterized with a typical sensitivity of 10’s of µT

Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.

Magnetostriction of core laminations is one of the main sources of transformer acoustic noise. The magnetostriction of grain oriented silicon steel is extremely sensitive to applied compressive stress. A measurement system using piezoelectric accelerometers has been designed and built. This was optimized for magnetostriction measurements under stress within the range of 10 MPa to -10 MPa on large as-cut sheets. This system was used for characterization of wide range of grain-oriented grades. Laboratories around the world are using many different methods of measurement of the magnetostrictive properties of electrical steel. In response to this level of interest, an international round robin exercise on magnetostriction measurement has been carried out and eight different magnetostriction-measuring systems have been compared. Results show a reasonable correlation between the different methods. In this study the influence of factors such as the domain refinement process, curvature, and geometry on the magnetostriction of 3% grain oriented silicon steel were investigated. The study shows that both laser scribing and mechanical scribing have a similar effect on the sample’s domain structure and would cause an increase in magnetostriction. A proposed domain model was used successfully to estimate the effect of scribing on magnetostriction. Correlation between magnetostriction of 3% grain oriented silicon steel with transformer vibration was investigated. It was shown that increasing the clamping pressure to 4Nm can decrease the out of plane vibration in the joint regions due to the increase of friction and reduction of air gap which reduces the air gap flux and consequently the Maxwell forces. Also it has been shown that the primary source for the differences between the vibration of the cores under the same magnetic excitation and clamping pressure in the measured cores is due to the differences in the magnetostriction characteristics of the grades of electrical steels. Correlations between the magnetostriction harmonics and the vibration of the cores have been determined.

Die aus magnetisierbaren Partikeln und einer elastischen Matrix bestehenden magnetorheologischen Elastomere sind ein Verbundwerkstoff mit magnetisch steuerbaren Eigenschaften. In der vorliegenden Arbeit wird ein kontinuumsmechanisches Modell zur Beschreibung der relevanten physikalischen Phänomene bereitgestellt. Die Lösung zugehöriger Randwertaufgaben basiert auf der erweiterten Finiten Elemente Methode. Zur Verifikation und Validierung des Modells werden analytische Referenzlösungen zweidimensionaler Problemstellungen herangezogen. Die Homogenisierung des magnetomechanischen Feldproblems erfolgt mit kleinen Deformationen. Aus einer Volumenmittelung der lokal inhomogenen Feldverteilungen ergeben sich makroskopische Variablen. Auf Basis dieser Größen lassen sich Aussagen über das effektive Verhalten ableiten. Somit ist neben den rein magnetischen und mechanischen Materialeigenschaften das gekoppelte magnetomechanische Verhalten analysierbar. Darunter sind aktuatorische Spannungen, magnetostriktive Dehnungen und der magnetorheologische Effekt zu verstehen
Magnetorheological elastomers are composite materials consisting of magnetizable particles embedded in an elastic matrix. Their properties can be altered by an external magnetic field. In this work a continuum based formulation is applied to model relevant physical phenomena. Boundary value problems are solved by the extended Finite Element Method. For the purposes of verification and validation analytic solutions are provided. The homogenization of the magnetomechanical field problem is limited to small deformations. Macroscopic variables are obtained by volume averaging. In addition to macroscopic magnetic and mechanical properties the effective behavior is analyzed in terms of actuatoric stresses, magnetostrictive strains and the magnetorheological effect

The aim of this thesis work was to develop magnetoelectric (ME) composites consisting of piezoelectric and magnetostrictive components. The piezoelectric constituent was selected as a PZT ceramic modified by strontium, bismuth and manganese. The magnetostrictive phase was nickel ferrite (NF) ceramic doped by cobalt, copper and manganese. The properties of component phases were optimized in order to enhance the ME effect in the composite. In the first part of the thesis, effects of sintering temperature on the dielectric and piezoelectric properties of PZT and on the electrical and magnetic properties of NF ceramics were investigated in the temperature range covered from 1150 to 1250 °
C. The best piezoelectric properties in PZT were attained at 1250 °
C. At this sintering temperature, values of piezoelectric strain coefficient, dielectric constant, and electromechanical coupling coefficient were 434 pC/N, 1320 and 0.48, respectively. NF ceramics showed poor densification
80 %TD was attained at 1250 °
C. In order to obtain higher densities in ferrites, Bi2O3 was used as a sintering aid. Addition of Bi2O3 enhanced densification up to 97 %TD, and improved electrical and magnetic properties of ferrites. Highest DC-resistivity of 1.15*10^8 ohm-cm and highest magnetostriction of ~26 ppm were attained in NF ceramics doped with 1 wt% Bi2O3. In the second part of the thesis, ME composites were manufactured either as bulk composites or as laminated composites. The efficiency of different composite types was evaluated in terms of voltage output in response to the applied magnetic field. Higher outputs were observed in laminated composites.

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.

Ce travail se situe dans le contexte de l’utilisation des ondes de spin comme vecteur d’information. Il explore la possibilité d’exciter l’aimantation dans de fines couches ferromagnétiques grâce au couplage magnéto-élastique. Cela permettrait un adressage non-inductif, efficace, et distant des ondes de spin. Dans un premier volet, nous avons développé un dispositif expérimental générant des ondes acoustiques de surface (ODS) électriquement, verrouillées en phase à des impulsions de laser sonde. La dynamique d’aimantation est détectée grâce aux effets magnéto-optiques (Kerr et Voigt). Nous étudions le couplage magnétoélastique résonant dans une couche mince du semiconducteur magnétique (Ga,Mn)As. Afin d’atteindre la résonance, la fréquence des ondes de spin est ajustée à celle des ODS par un champ magnétique. Nous isolons les contributions photo-élastique et magnéto-optique du signal, pour quantifier l’amplitude de la précession d’aimantation. Nous montrons que la précession observée est exclusivement déclenchée par l’ODS. La variation en champ de son amplitude correspond très bien à celle calculée, et elle est maximum au champ pour laquelle l’absorption de l’ODS est maximale, démontrant clairement la résonance magnétoacoustique. L’influence de la fréquence et de la puissance de l’ODS, ainsi que de la température sur l’efficacité du couplage est également explorée. Dans un deuxième volet, nous avons excité des ODS par effet thermoélastique grâce à un faisceau laser focalisé, et cela sur des couches magnétiques métalliques cette fois-ci (Ni, FeGa, Co), déposées sur un substrat transparent (verre, sapphire). Des cartes spatio-temporelles du déplacement de la surface et du signal magnéto-optique ont été obtenues. Un décalage du spectre magnéto-optique vers les hautes fréquences semble indiquer une excitation des ondes de spin par les ODS
In the developing field of spin wave-based information technology, this work investigates the possibility to use surface acoustic waves (SAW) to excite spin-waves in ferromagnetic thin layers relying on the magnetoelastic coupling. This would provide a non-inductive, efficient, and remote addressing of spin waves. In the first project we develop an experimental setup to generate electrically excited SAWs phase-locked to probe laser pulses. The magnetization dynamics is detected by an optical bridge using magneto-optical effects (Kerr and Voigt). We investigate the resonant magneto-elastic coupling in a thin film of the ferromagnetic semiconductor (Ga,Mn)As. To reach resonant coupling, the spin-wave frequency is scanned across the SAW frequency using a magnetic field. We disentangle the photoelastic contribution from the magneto-optical one, from which we obtain the amplitude of magnetization precession. We show that it is driven solely by the acoustic wave. Its field dependence is shown to agree well with theoretical calculations. Its amplitude resonates at the same field as the resonant attenuation of the acoustic wave, clearly evidencing the magnetoacoustic resonance with high sensitivity. The influence of temperature, SAW frequency and power on the coupling efficiency are studied. In the second project we use SAWs thermoelastically excited by a tightly focused laser beam on ferromagnetic metals (Ni, FeGa, Co) on a transparent substrate (glass, sapphire). Spatio-temporal maps of the surface displacement and magneto-optical signal are obtained. A high-frequency shift of the frequency spectrum of the latter gives a hint for spin-wave excitation by SAWs

Le but de ce travail etait d'obtenir des couches minces presentant une grande magnetostriction en champ faible et une temperature d'ordre elevee pour la realisation de microactionneurs en vue d'applications a l'automobile et en medecine. Les etudes anterieures sur couches minces ont porte essentiellement sur des alliages amorphes de type terfenol et terfenol-d (tbdyfe#2). Nous avons montre qu'a l'etat amorphe, il etait bien preferable de remplacer le fer par du cobalt, les alliages proches de la composition a-r#0#. #3#3co#0#. #6#6 (r = terre rare) presentant une temperature d'ordre et une magnetostriction plus elevee. De fait, nous avons pu preparer, caracteriser puis optimiser toute une serie d'echantillons a-(tb,dy)#1#-#x(fe,co)#x : le materiau tb#0#. #3#2(fe#0#. #4#5co#0#. #5#5)#0#. #6#8 presente une forte magnetoelasticite (b##,#2 = - 63. 5 mpa, ##,#2 = 10#-#3), et une forte magnetostrictivite a champ faible. Nous avons obtenu des performances encore superieures en deposant des multicouches de type spring magnet, ou le champ de saturation de la couche amorphe tb-co est reduit en augmentant l'aimantation moyenne. Ceci est accompli par le couplage magnetique avec des couches de fe-co. De plus, le magnetisme des couches amorphes magnetostrictives (tb-co) est renforce par le champ d'echange de leurs couches voisines (fe-co) cristallisees. Nous avons, a l'aide de ces nouveaux materiaux, realise des microactionneurs magnetostrictifs compenses vis a vis des derives thermiques. Ces actionneurs sont usines dans le silicium par des techniques utilisees en micromecanique avant depot des couches actives, puis caracterises par interferometrie et deflectometrie. Ces dispositifs ne sont pas encore optimises, mais ont neanmoins prouve la faisabilite de tels microactionneurs.

The main challenge in the design of vibration energy harvesters is the optimization of energy outcome relative to the applied excitation to reach a higher efficiency in spite of the weakness of ambient energy sources. One promising principle of vibration converters is magnetoelectricity due to the outstanding properties of magnetostrictive and piezoelectric laminate composites, which provide interesting possibilities to harvest energy from low amplitude and low frequency vibration with relatively high energy outcome. For these devices, ensuring high deformations in the magnetostricive layers, improvement of the magnto-mechanical and the electro-mechanical couplings are highly required for the optimization of the energy outcome. This thesis primarily aims to develop a model based harvester design for magnetoelectric (ME) converters. Based on a comprehensive understanding of the complex energy flow in magnetoelectric transducers, several design parameters are investigated. For instance, magnetostriction in a Terfenol-D plate is investigated by means of atomic force microscopy under similar conditions as within magnetoelectric transducers. A novel measurement approach was successfully developed to detect the evolution of magnetic domains and measure deformations in a Terfenol-D plate in response to externally non-uniform applied magnetic fields. Furthermore, a finite element model is developed to predict the induced voltage in the ME transducer as a response to the magnet’s displacement, corrected based on atomic force microscopy measurements, and used for the design of the harvester. The presented three- dimensional model takes into consideration the nonlinear behaviour of the magnetostrictive and piezoelectric materials. Additionally, three novel converters having different magnetic circuits are designed and analysed analytically based on Lindstedt-Poincaré method. The effects of the structure parameters, such as the nonlinear magnetic forces, the magnetic field distribution and the resonance frequency are discussed, and the electric output performances of the three designed converters are evaluated. In order to improve both mechanical and electrical coupling between the piezoelectric and the magnetostrictive layers, a bonding technique at room temperature is proposed which uses conductive polymer nanocomposites. Two magnetoelectric transducers are fabricated based on this technique having 1 wt.% and 2 wt.% concentration of multiwalled carbon nanotubes in epoxy resin. Another magnetoelectric transducer is fabricated by a classical technique for comparison purposes. In order to validate the design, a series of demonstrators are designed and fabricated according to the simulation and optimization results. The proposed design is composed by a cantilever beam, a magnetic circuit with several magnet arrangements and a magnetoelectric transducer, which is formed by a piezoelectric PMNT plate bonded to two magnetostrictive Terfenol-D layers. In this design, external vibrations are converted to magnetic field changes acting on the magnetostrictive layers leading to deformations, which are transmitted directly to the piezoelectric layer. The converters are tested under harmonic excitations and real vibration profiles reproduced by an artificial vibration source. Different parameters were investigated experimentally including the magnetic forces between the transducer and the magnetic circuit and the used bonding technique. Tuning the resonance frequency of the ME converter is also addressed using a simple screw/nut system, which allows to control the relative position and therefore the magnetic forces between the magnetic circuit and the transducer. The magnetoelectric transducer bonded with 2 wt.% concentration of multiwalled carbon nanotubes shows better output performances than the two other ME transducers under similar excitations. A maximum power output of 2.42 mW is reached under 1 mm applied vibration at 40 Hz. This performance presents an improvement of minimum 20 % of the reached energy outcome by other magnetoelectric vibration converters using single ME transducer at comparable applied excitations.
Die größte Herausforderung bei der Konstruktion von Vibrations-Energiewandlern ist die Optimierung der gewonnenen Energie im Verhältnis zur angewandten Anregung, um trotz schwacher Umgebungsenergiequellen einen hohen Wirkungsgrad zu erreichen. Ein vielversprechendes Prinzip von Vibrationswandlern ist die Magnetoelektrizität aufgrund der hervorragenden Eigenschaften von magnetostriktiven und piezoelektrischen Verbundwerkstoffen, die interessante Möglichkeiten bieten, Energie aus niederfrequenten Schwingungen mit kleinen Amplituden zu gewinnen. Bei diesen Wandlern ist die Sicherstellung hoher Verformungen in den magnetostriktiven Schichten, die Verbesserung der magnetisch-mechanischen und der elektromechanischen Kopplungen für die Optimierung des Energieertrages sehr wichtig. Diese Arbeit zielt in erster Linie auf die Entwicklung eines modellbasierten Entwurfs für magnetoelektrische (ME) Wandler ab. Basierend auf einem umfassenden Verständnis des komplexen Energieflusses in magnetoelektrischen Wandlern werden mehrere Entwurfsparameter untersucht. So wird beispielsweise die Magnetostriktion in einer Terfenol-D-Platte mittels Rasterkraftmikroskopie unter ähnlichen Bedingungen untersucht wie in magnetoelektrischen Wandlern. Dabei wurde eine neuartige Messmethode erfolgreich entwickelt, um die Entwicklung von magnetischen Domänen zu erfassen und die Deformation in einer Terfenol-D-Platte als Reaktion auf extern ungleichmäßig angelegte Magnetfelder zu messen. Darüber hinaus wird ein Finite-Elemente-Modell entwickelt, um die induzierte Spannung im ME-Wandler als Reaktion auf die Verschiebung des Magneten vorherzusagen, welches auf der Grundlage von Atomkraftmikroskopie Messungen korrigiert und für den Entwurf des Energiewandlers verwendet wird. Das vorgestellte dreidimensionale Modell berücksichtigt das nichtlineare Verhalten der magnetostriktiven und piezoelektrischen Materialien. Zusätzlich werden drei neuartige Wandler mit unterschiedlichen Magnetkreisen nach dem Lindstedt-Poincaré Verfahren konzipiert und analytisch analysiert. Die Auswirkungen der Strukturparameter, wie die nichtlinearen Magnetkräfte, die Magnetfeldverteilung und die Resonanzfrequenz, werden diskutiert und die elektrischen Ausgangsleistungen der drei ausgelegten Wandler ausgewertet. Um die mechanische und elektrische Kopplung zwischen der piezoelektrischen und der magnetostriktiven Schicht zu verbessern, wird eine bei Raumtemperatur prozessierbare Verbindungstechnik vorgeschlagen, bei der leitfähige Nanokomposite verwendet werden. Zwei magnetoelektrische Wandler werden basierend auf dieser Technik mit einer Konzentration von 1 wt.% und 2 wt.% an mehrwandigen Kohlenstoff-Nanoröhren in Epoxidharz hergestellt. Ein weiterer magnetoelektrischer Wandler wurde zu Vergleichszwecken mit einer klassischen Technik hergestellt. Für die Validierung des Entwurfes wird eine Reihe von Demonstratoren entsprechend den Simulations- und Optimierungsergebnissen konstruiert und gefertigt. Der vorgeschlagene Entwurf besteht aus einem Trägerbalken, einem Magnetkreis mit mehreren Magnetanordnungen und einem magnetoelektrischen Wandler, der aus einer piezoelektrischen PMNT-Platte besteht, die mit zwei magnetostriktiven Terfenol-D-Schichten verbunden ist. Bei dieser Konstruktion werden externe Schwingungen in Magnetfeldänderungen umgewandelt, die auf die magnetostriktiven Schichten wirken und zu Verformungen führen, die direkt auf die piezoelektrische Schicht übertragen werden. Die Wandler werden unter harmonischen Anregungen und mit realen Schwingungsprofilen getestet, die von einer künstlichen Schwingungsquelle reproduziert werden. Verschiedene Parameter wurden experimentell untersucht, darunter die magnetischen Kräfte zwischen dem Wandler und dem Magnetkreis sowie die verwendete Verbindungstechnik. Die Abstimmung der Resonanzfrequenz des ME-Wandlers erfolgt ebenfalls über ein einfaches Schrauben-Mutter-System, das es ermöglicht, die relative Position und damit die magnetischen Kräfte zwischen Magnetkreis und Wandler zu steuern. Der magnetoelektrische Wandler, der mit einer Konzentration von 2 wt.% mehrwandiger Kohlenstoff-Nanoröhrchen verbunden ist, zeigt bessere Ausgangsleistungen als die beiden anderen ME-Wandler bei ähnlichen Anregungen. Eine maximale Ausgangsleistung von 2,42 mW wird bei 1 mm angelegter Vibration bei 40 Hz erreicht. Diese Leistung stellt eine Verbesserung von mindestens 20 % im Vergleich zu anderen magnetoelektrischen Schwingungsumrichtern dar, welche mit einem einzigen ME-Wandler bei vergleichbaren Anregungen getestet werden.

Amorphous and polycrystalline microwires cast from ferromagnetic Fe-based or Co-based alloys in glass envelope demonstrate unique magneto-anisotropic and high frequency impedance properties that make them very attractive for sensor applications. Magnetic anisotropies of different types result from the inverse magnetostriction effect (positive or negative) at the interface between the glass shell and the metal core, in the presence of the residual stresses induced during the Taylor-Ulitovski casting method. Therefore, the glass shell is not just isolation, but also is one of most important factors that defines the physical properties of microwires. In particular, magnetic anisotropy allows high frequency impedance to be tuned by external stimuli such as magnetic field, tensile stress, or temperature. In the project, these effects are explored for the creation of low density microwire inclusions that might introduce tuneable microwave properties to polymer composite materials. The project aims to study high frequency impedance effects in ferromagnetic wires in the presence of tensile stress, temperature, and magnetic field. The integration of microwave equipment with mechanical and thermal measurement facilities is a very challenging task. In the project, we develop new experimental techniques allowing comprehensive study of composite materials with electromagnetic functionalities. The wire surface impedance recovered from such measurements can then be used to model the microwave response from wire-filled composites in free space. The obtained results significantly expand the horizon of potential applications of ferromagnetic wires for structural health monitoring.

The theoretical part of this thesis deals with principles and characteristics of the surface acoustic wave filter. It explained the principle of magnetostriction and piezoelectric effect, which uses a filter. In the practical part in the program Comsol Multiphysics are made three simple models of SAW filter. Some modes are founded and are compared to previously known results. In the next phase of project is further studied using the parametric analysis. In the last phase of project is applied global optimization PSO at admittance characteristic from simple 2D structure. The results are compared with the commercially produced devices.

Doutoramento em Engenharia Física
Este trabalho expõe um estudo teórico e experimental das propriedades anisotrópicas magnetoelétricas (ME) em diferentes compósitos contendo monocristais piezoelétricos (PE), maioritariamente sem chumbo na sua composição, com vista a diversas aplicações multifuncionais. Uma descrição linear do efeito ME em termos de campos elétricos, magnéticos e elásticos e constantes materiais é apresentada. Um modelo fenomenológico quasi-estático é usado para ilustrar a relação entre as constantes materiais, sua anisotropia e os coeficientes MEs transversais de tensão e carga. Subsequentemente, este modelo é empregue para estimar o máximo coeficiente ME direto de tensão expectável numa série de compósitos tri-camadas de Metglas/Piezocristal/Metglas em função da orientação do cristal PE. Demonstra-se assim como os efeitos MEs são fortemente dependentes da orientação cristalina, o que suporta a possibilidade de se gerarem coeficientes MEs de tensão elevados em compósitos contendo monocristais PEs sem chumbo como o niobato de lítio (LiNbO3; LNO), tantalato de lítio (LiTaO3), ortofosfato de gálio (GaPO4; GPO), quartzo (SiO2), langatato (La3Ga5.5Ta0.5O14) e langasite (La3Ga5SiO14) através da otimização da orientação cristalina. Uma técnica experimental dinâmica de lock-in para a medição da impedância e efeito ME direto é exposta. O formalismo descritivo desta técnica, assim como um arranjo experimental desenvolvido para o efeito são apresentados. O esquema e características deste, assim como diferentes formas de reduzir o ruído e a indesejável indução mútua são exploradas. Um estudo comparativo do efeito ME direto em compósitos tri-camadas de Metglas e monocristais de LNO e PMN-PT conectados de forma simples é exposto. Embora o PMN-PT possua piezocoeficientes de carga muito superiores aos do LNO, o coeficiente ME direto de tensão demonstrou-se comparável entre ambos os compósitos devido a uma muito menor permitividade dielétrica do LNO. Cálculos teóricos indicam aínda que as propriedades MEs poderão ser significativamente melhoradas (até 500 V/(cm.Oe)) através da otimização do ângulo de corte do LNO, espessura relativa entre camadas ferroelétrica/ferromagnética e uma melhor colagem entre o Metglas e o LNO. Vantagens da utilização do material ferroelétrico LNO em compósitos MEs são discutidas. Num estudo subsequente, as propriedades dinâmicas anisotrópicas de impedância e MEs em compósitos tri-camadas de Metglas e monocristais PEs sem chumbo de LNO e GPO são exploradas. Medições foram realizadas em função do corte de cristal, magnitude e orientação do campo magnético de polarização e frequência do campo de modulação. Coeficientes MEs altamente intensos em certos modos de ressonância são explorados, e a sua relação com as propriedades materiais dos cristais e geometria dos compósitos é investigada. Um coeficiente ME de até 249 V/(cm.Oe) foi aqui observado num compósito com um cristal de LNO com corte 41ºY a 323.1 kHz. Mostramos assim que compósitos multicamadas contendo cristais sem chumbo de LNO e GPO podem exibir efeitos MEs anisotrópicos relativamente elevados. Demonstramos também que o controlo da orientação dos cristais PEs pode em princípio ser usado na obtenção de propriedades MEs anisotrópicas desejáveis para qualquer aplicação. Características únicas como elevada estabilidade química, piezoeletricidade linear e robusteza térmica abrem verdadeiras perspetivas para a utilização de compósitos baseados no LNO e GPO em diversas aplicações. Eventualmente, compósitos bi-camadas contendo lâminas PEs com bidomínios de LNO com corte 127ºY foram estudados tanto teoricamente como experimentalmente. Estas lâminas de LNO possuem uma estrutura de bidomínios com vetores de polarização espontânea opostos ao longo da direção da sua espessura (i.e. uma estrutura de macrodomínios ferroelétricos “head-to-head” ou “tail-to-tail”) Medições de impedância, efeito ME e densidade de ruido magnético equivalente foram realizadas nos compósitos operando sob condições quasi-estáticas e de ressonância. Coeficientes MEs de até 578 V/(cm.Oe) foram obtidos a ca. 30 kHz sob ressonâncias de dobramento usando cristais PEs com 0.5 mm de espessura. Medições de densidade de ruído magnético equivalente demosntraram valores de até 153 pT/Hz1/2 a 1 kHz (modo quasi-estático) e 524 fT/Hz1/2 sob condições de ressonância. É de esperar que uma otimização adicional das técnicas de fabrico, geometria dos compósitos e circuitos de detenção possa permitir reduzir estes valores até pelo menos 10 pT/Hz1/2 e 250 fT/Hz1/2, respetivamente, e a frequência de ressonância em pelo menos duas ordens de grandeza. Estes sistemas poderão assim no futuro ser usados em sensores vetoriais de campo magnético simples e sensíveis, passivos e estáveis e operáveis a elevadas temperaturas.
This work presents a theoretical and experimental study of the anisotropic magnetoelectric (ME) properties of differently structured composites featuring piezoelectric (PE) single-crystals, mainly lead-free, for diverse multifunctional applications. A linear description of the ME effects in terms of electric, magnetic and elastic fields and material constants is offered. An averaging quasi-static phenomenological model is used to illustrate the relation between the material constants, their anisotropy and the transversal direct ME voltage and charge coefficients. Subsequently, the aforementioned model is employed in the calculation of the maximum expected direct ME voltage coefficient for a series of tri-layered Metglas/Piezocrystal/Metglas composites as a function of the PE crystal orientation. The ME effects are shown to be strongly dependent on the crystal orientation, which supports the possibility of inducing large ME voltage coefficients in composites comprising lead-free PE single crystals such as lithium niobate (LiNbO3; LNO), lithium tantalate LiTaO3, gallium phosphate (GaPO4; GPO), quartz (SiO2), langatate (La3Ga5.5Ta0.5O14) and langasite (La3Ga5SiO14) through the optimization of the crystal orientation. An experimental dynamic lock-in technique for the measurement of the impedance and direct ME effect is presented. The formalism describing this technique and an implemented custom-made setup are introduced. The scheme and characteristics of the latter as well as ways to reduce the noise and the undesirable mutual induction are explored. A comparative study of the direct ME effect in simply bonded tri-layered laminates of Metglas and LNO and PMN-PT crystals is exposed. Though PMN-PT has much larger charge piezocoefficients than LNO, the direct magnetoelectric voltage coefficient is found to be comparable in both trilayers due to the much lower dielectric permittivity of LNO. Calculations show that the ME properties can be significantly improved (up to 500 V/(cm·Oe)) via an optimization of the cut angle of LNO, relative thickness ratio of the ferroelectric/ferromagnetic layers and a better bonding between Metglas and LNO. Advantages of using the LNO ferroelectric in ME composites are discussed. In a subsequent study, the dynamic impedance and ME anisotropic properties of tri-layered composites of Metglas and single-crystalline lead-free PE of LNO and GPO are explored. Measurements have been performed as a function of the crystal-cut, magnitude and orientation of the magnetic bias field and frequency of the modulated field. Greatly enhanced ME coefficients in certain resonance modes are explored, and their relation to the material properties of the crystals and the geometry of the composites is investigated. The largest ME coefficient of up to 249 V/(cm·Oe) was observed for a composite with a 41ºY-cut LNO crystal at 323.1 kHz. We thus show that multilayers comprising lead-free LNO and GPO crystals can exhibit relatively large anisotropic ME effects. We also demonstrate that the control of the PE crystal’s orientation can in principle be used to obtain almost any desired quasi-static and resonant anisotropic ME properties for any given application. Such unique features as chemical stability, linear piezoelectricity and thermal robustness open up a real perspective of using lead-free LNO and GPO based ME tri-layers in various applications. Eventually, bi-layered composites comprising PE bidomain plates of 127ºY-cut LNO were studied both theoretically and experimentally. The LNO plates possessed an engineered bidomain structure with opposite spontaneous polarization vectors along the thickness direction (i.e. a “head-to-head” or “tail-to-tail” ferroelectric macrodomain structure). Impedance, ME effect and equivalent magnetic noise density measurements have been performed on the composites operating under quasi-static and resonant conditions. ME coefficients of up to 578 V/(cm·Oe) were obtained at ca. 30 kHz at the bending resonance using 0.5 mm thick piezoelectric crystals. Equivalent magnetic noise density measurements yielded values down to 153 pT/Hz1/2 at 1 kHz (quasi-static mode) and 524 fT/Hz1/2 under resonant conditions. A further optimization of the fabrication techniques, laminate geometry and detection circuit is expected to allow reducing these values down to at least 10 pT/Hz1/2 and 250 fT/Hz1/2, respectively, and the resonance frequency by at least two orders of magnitude. Such systems may in future thus find use in simple and sensitive, passive and stable, low-frequency and high-temperature vector magnetic field sensors.

Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen
The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured

Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen.
The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured.

This thesis focuses on the role of interfacial strain in heterostructures to modify the magnetism of thin ferromagnetic films due to the inverse magnetostrictive effect, defined as the change of magnetization produced in ferromagnetic materials by an external stress. Thus, the magnetic control can be obtained without applying an external field by using heterostructures composed of a non-magnetic layer characterized by a temperature-driven structural phase transition coupled to a ferromagnetic layer. In such heterostructures, the magnetization of the ferromagnetic layer is modified through changes in the stress field at the interface when the structural phase transition in the non-magnetic layer (actuator) is carried out. In this work, we used NiTi shape memory alloy as the actuator to modify the magnetic behavior of ferromagnetic films through the magneto-elastic coupling in novel NiTi/Ni and NiTi/Co heterostructures. NiTi, when near its equiatomic composition, is a shape memory alloy that undergoes a reversible structural phase transition with temperature, providing stress on the ferromagnetic film. We chose this alloy because NiTi exhibits a large recovery stress with transition temperatures above room temperature for Ti-rich NiTi films, which is of interest for technological applications of the heterostructures. Since the right microstructure of NiTi is important to observe structural phase transition and it defines the characteristic of the transition, an extensive review on previous research on NiTi is detailed in this thesis. Thus, to ensure large stress during the NiTi structural transition with temperature, the NiTi alloy must be near its equiatomic composition with a thickness above 800 nm. Both characteristics were confirmed by Rutherford Backscattering analyses. The crystal structure and its transition with temperature were studied by X-ray diffraction measurements. In-plane magnetization and hysteresis measurements with temperature, performed on a superconducting quantum interference device (SQUID) magnetometer, prove the magneto-elastic coupling that was observed as an enhancement in the magnetic moment of the ferromagnetic layer. Such enhancement becomes the feature of magneto-elastic coupling in these novel NiTi/ferromagnetic heterostructures.
Esta tese estuda o papel da tensão interfacial em filmes heterogêneos na modificação do magnetismo de camadas ferromagnéticas finas por meio do efeito magnetoestritivo inverso, definido como a mudança de magnetização produzida em materiais ferromagnéticos por um estresse externo. Tecnologicamente, isto visa ter um grau de controle magnético do material sem a aplicação de um campo externo, usando heteroestruturas compostas por uma camada não magnética caracterizada por uma transição de fase estrutural acionada pela temperatura, acoplada a uma camada ferromagnética. Em tais heteroestruturas, a magnetização da camada ferromagnética é modificada através de alterações no campo de tensão na interface quando a transição de fase estrutural na camada não magnética (atuador) é realizada. Assim, utilizamos a liga com memória de forma NiTi como atuador, para modificar o comportamento magnético de filmes ferromagnéticos através do acoplamento magnetoelástico em novas heteroestruturas de NiTi/Ni e NiTi/Co. O NiTi, quando próximo à sua composição equiatômica, é uma liga com memória de forma que sofre uma transição de fase estrutural reversível com a temperatura, proporcionando tensão no filme ferromagnético. Escolhemos esta liga porque o NiTi apresenta uma grande tensão de recuperação com temperaturas de transição acima da temperatura ambiente, para filmes de NiTi ricos em Ti, o que é de interesse para aplicações tecnológicas das heteroestruturas. A microestrutura do NiTi é fundamental para favorecer a transição de fase estrutural e definir as suas características. Assim, uma extensa revisão de pesquisas anteriores sobre NiTi é detalhada nesta tese. Para garantir um grande estresse durante a transição estrutural do NiTi com a temperatura, o filme de NiTi deve estar próximo de sua composição equiatômica e ter espessura acima de 800 nm. Ambas as características foram confirmadas pelas análises de espectroscopia de retroespalhamento Rutherford. A estrutura cristalina e sua transição com a temperatura foram estudadas por medidas de difração de raios X. Medidas de magnetização e histerese em função da temperatura, com campo aplicado no plano dos filmes, realizadas em um magnetômetro SQUID, comprovaram a existência do acoplamento magnetoelástico, o qual se manifestou através de variações no momento magnético da camada ferromagnética. Essas mudanças de magnetização, observadas principalmente na heteroestrutura com Ni, torna-se a característica principal do acoplamento magnetoelástico nesses novos materiais.

Le compose tbfe#2 presente des proprietes de magnetostriction geante a temperature ambiante. Nous avons mene une etude des conditions de croissance, des proprietes structurales et des proprietes magnetiques des couches minces epitaxiees de tbfe#2(111). Les couches elaborees par depot laser pulse (pld) ont des epaisseurs de 60-240 a. Nous avons determine les effets de la temperature de depot sur la morphologie des couche, observee par microscopie afm. Nous montrons qu'un depot en gradient de temperature (type b) permet d'obtenir des couches plus continues qu'un depot a temperature constante elevee (type a). L'effet de la couche tampon et de la terre rare sur la morphologie ont ete etudies. Les relations d'epitaxie ont ete determinees et nous montrons qu'elles dependent de l'etat de surface de la couche tampon. Une etude detaillee des cliches rheed est presentee. Nous avons mesure des deformations de maille dans les echantillons. Les couches a presentent une dilatation dans le plan de la couche et une contraction suivant l'axe de croissance 111 alors que les couches b sont dilatees dans le plan de meme que sur l'axe 111. Nous avons etudie les proprietes magnetiques des echantillons de type a et b par vsm, squid et effets magneto-optique (kerr et faraday). Les echantillons a ont une anisotropie planaire qui n'apparait pas sur les couches b. Nous proposons un modele de calcul d'anisotropie base sur le couplage magnetoelastique reliant les deformations mesurees a l'anisotropie. Ce modele permet d'obtenir un accord qualitatif avec les observations experimentales. Enfin nous proposons un processus d'elaboration de nanostructures auto-assemblees de fe. Dans ce travail preliminaire la structure des nanostructures obtenues a ete analysee par rheed, diffusion centrale des rx, diffraction x et afm. Nous presentons les premiers resultats de l'etude magnetique macroscopique (mesures vsm) et locale, mesures par microsquid et dichroisme magnetique circulaire des rx.

Ce memoire decrit des etudes de dilatation thermique et de magnetostriction effectuees au moyen d'un dilatometre capacitif sur des monocristaux de composes a fermions lourds, entre 0. 2 et 300 k et sous des champs magnetiques allant jusqu'a 11 t. Dans une premiere partie, sont decrits des resultats sur l'alliage ce#0#. #9#8#5y#0#. #0#1#5ru#2si#2, qui completent une etude precedente du compose ceru#2si#2 pur et de l'alliage ce#0#. #9#5la#0#. #0#5ru#2si#2. Leur energie caracteristique obeit a une loi d'echelle. Aucun des de ces trois composes ne presente d'ordre magnetique a longue distance, mais des correlations antiferromagnetiques a courte portee, lesquelles disparaissent lors d'une transition induite sous champ, qualifiee de pseudometamagnetique. Les effets d'alliage sur la dilatation volumique et sur la largeur de cette transition sont discutes, et une explication basee sur un effet kondo en presence des correlations est presentee. Le compose cecu#6 montre des effets magnetovolumiques tres anisotropes. A tres basse temperature, une faible transition pseudometamagnetique a pu etre detectee. Le comportement sous champ est discute en terme d'effet kondo a un site. La formation d'un gap semiconducteur dans cenisn n'a pas de forte influence sur les proprietes magnetovolumiques. Par contre, nos experiences confirment l'existence d'un pseudogap de spin tres anisotrope. Les modeles theoriques qui tentent d'expliquer ce comportement nouveau dans les semiconducteurs magnetiques sont discutes

Dehnungssensoren werden zur Überwachung von sicherheitsrelevanten Bauteilen, besonders in Bauteilen aus faserverstärkten Polymermatrixverbundwerkstoffen eingesetzt. Durch deren Integration in das Bauteilinnere werden sie vor schädigenden mechanischen sowie korrosiven Einwirkungen geschützt. Dies gewährleistet eine zuverlässige sowie dauerhafte Funktion. Verschiedene Ansätze zur Weiterentwicklung integrierbarer Dehnungssensoren werden international untersucht. Die Verringerung des Sensordurchmessers auf Abmaße im Bereich des Durchmessers von Verstärkungsfasern ist dabei ein bedeutendes Entwicklungsziel. Insbesondere bei der Integration in Bauteile aus faserverstärkten Kunststoffen sorgen zum Durchmesser von Fasern vergleichbare Sensordurchmesser für eine optimale Sensoranbindung. Die Bildung von Harznestern sowie schwächender Unstetigkeiten kann mittels dünner Sensoren verhindert werden. Dies gewährleistet eine artefaktefreie Dehnungsmessung. Drei verschiedene Ansätze für neuartige Dehnungssensoren mit kleinem Querschnitt wurden in dieser Arbeit untersucht
Strain sensors are used for structural health monitoring issues, certainly in parts with high safety requirements made of fibre-reinforced plastic composites. The integration of these sensors inside the parts protects them against any mechanical and corrosive impact. The sensor functionality can be enhanced by integration. There is a lot of international research effort to further develop integratable strain sensors. Different approaches are currently pursued. This thesis presents the results of investigations on three different approaches for novel strain sensors. The main goal of these investigations was to minimise the sensor diameter down to the diameter of reinforcing fibres. The small diameter allows for an optimum and artefact free integration of the sensors. The formation of resin nests and notches to the material structure can be prevented by integrating sensor with a smaller diameter. The strain measurement and monitoring is enhanced and more reliable then

Actuellement, le marché des implants offre milliers de dispositifs différents pour diagnostiquer, traiter et suivre le patient. Dans la dernière décennie, d'importants travaux de recherche ont permis de rendre les dispositifs implantés plus durables et moins invasifs. La source d'énergie la plus utilisée reste néanmoins la pile électrochimique à usage unique. Pour un implant à long terme, il est nécessaire de remplacer la pile déchargée, ce qui nécessite une intervention chirurgicale coûteuse et invasive. Pour résoudre ces problèmes, deux approches ont été récemment proposées, la première consiste à utiliser l'énergie biomécanique du corps, et la deuxième consiste à transmettre de l'énergie depuis l'extérieur du corps humain. Dans le premier cas, par exemple, il est possible d'alimenter la nouvelle génération de pacemaker avec un MEMS piézoélectrique en utilisant l’énergie mécanique fournie par les battements cardiaques. Dans le deuxième cas, la télé-alimentation par induction entre deux bobines (l'une dans l'implant, l'autre à l'extérieur du corps) est une technologie de plus en plus répandue dans le secteur biomédical. Une différence importante entre ces deux techniques concerne les niveaux de puissance atteints : quelques µW pour la récupération d'énergie, et de quelques mW au W pour la télé-alimentation. Durant cette thèse, une nouvelle technologie de télé-alimentation a été développée. Le système considéré est constitué de deux blocs principaux, l'émetteur (situé à l’extérieur du corps) et le récepteur (situé dans l'implant). L'émetteur est constitué d’une bobine, sur laquelle il y a peu de contraintes en termes de taille et de position sur le corps. Le récepteur est un transducteur magnétoélectrique (ME). L’utilisation de ce système ME est notamment motivée par la possibilité de s’affranchir -au moins en partie- des contraintes d'alignement du système classique de bobine-bobine, alignement difficile à assurer dans le cas des dispositifs implantables. Le transducteur ME considéré est un composite constitué de matériaux piézoélectriques et magnétostrictifs. L’élément magnétostrictif a la propriété de se déformer mécaniquement, lorsqu'il est exposé à un champ magnétique. L’élément piézoélectrique se polarise et crée un champ électrique sous l’effet d’une contrainte mécanique. Ainsi, lorsque la bobine émettrice génère un champ magnétique, à la résonance, elle crée une déformation élastique du matériau magnétostrictif. Cette déformation mécanique est transmise à la couche piézoélectrique, dans laquelle apparaît un champ électrique, générant ainsi une tension électrique. Grâce à cette conversion magnéto-mécano-électrique, l'implant médical peut être alimenté ou rechargé. Pendant cette thèse, des transducteurs de différentes dimensions ont été réalisés et testés : 10mm et 16mm de diamètre, et de 2 à 5 mm d’épaisseur. Parmi de nombreux résultats de caractérisation, un résultat très intéressant montre qu’à diamètre égal les transducteurs les plus minces avec une fraction volumique de matériau magnétostrictif plus importante permettent d’atteindre des puissances plus importantes qu’avec des échantillons plus épais et dont la fraction volumique de matériau magnétostrictif plus faible. Ce résultat ouvre d’intéressantes perspectives pour la miniaturisation de transducteurs ME tout en conservant une efficacité satisfaisante dans le transfert d'énergie. Concernant les tests in vitro, les résultats obtenus sont très prometteurs, montrant des puissances suffisantes pour recharger un implant jusqu'à 20 mm de distance de la bobine émettrice, dans l'orientation la plus défavorable. Un tel résultat est impossible à obtenir dans le cas d’un système bobine-bobine dans la position la plus défavorable (bobines perpendiculaires).En conclusion, les résultats obtenus présentent des perspectives très intéressantes pour la télé-alimentation, en termes de dispositifs miniaturisés et d'adaptabilité à la localisation des dispositifs médicaux implantés
Nowadays, the market of implantable medical devices is very large and heterogeneous: of the order of 4,000 different device types tracked by the FDA in 2018 that can diagnose, monitor, and treat patients. Over the past decades, significant research has made to develop implanted systems more durable and less invasive. Despite dramatic progress in all directions, energy autonomy remains the Achilles heel of active implants. The most employed energy source remains today the single-use battery. For long-lasting implants such as pacemakers, the replacement of depleted batteries is necessary and requires a costly and invasive surgical procedure. To overcome these issues, different techniques have been investigated. The first approach consists in using the biomechanical energy available inside of the body, and the second is to transmit energy from outside of the human body. In the first case, it is possible, for example, to power the new generations of pacemakers using MEMS energy harvesting devices supplied by the heartbeat. In the second case, the wireless power transmission by induction between two coils (one in the implant, the other outside the body) is an increasingly widespread technology in the biomedical sector and in everyday life. The main difference between these two approaches is their power range: typically, a few microwatts for biomechanical energy harvesters, and milliwatts to watts for wireless power transmission. As part of this thesis, a new wireless power transmission technology has been developed. The considered system consists of two main blocks: the transmitter (out-body) and the receiver (located in the implant). The transmitter is a coil with no major constraints in terms of size and on-body position. The receiver is a magnetoelectric (ME) transducer. Investigating the use of a ME receiver, instead of the classical coil receiver, was in particular motivated by the willingness to reduce the alignment constraints of the classical coil-coil system, which is difficult to manage for implantable medical devices. The considered ME transducer is a composite made of piezoelectric and magnetostrictive layers. A magnetostrictive material has the property of mechanically deforming its structure, when exposed to a magnetic field. A piezoelectric material can be polarized and create an electric field under mechanical stress (direct effect). Therefore, the magnetic field, generated by the transmitter coil, induces an elastic deformation of the magnetostrictive material. This mechanical deformation is transmitted to the piezoelectric material, in which an electric field appears, generating an electric voltage across its electrodes. Thanks to the magnetic-mechanical-electrical energy conversion, achieved by the ME transducer, the medical implant can be wirelessly supplied or recharged. In this thesis, receivers with different sizes have been tested: 10 mm or 16 mm in diameter, and thickness comprised between 2 mm and 5 mm. Amongst numerous interesting experimental results, it was observed that thinner ME transducers with higher magnetostrictive volume ratio could generate higher electrical power than thicker samples with smaller magnetostrictive volume ratio. This result opens good prospects for the possibility of miniaturizing the ME transducer without losing efficiency in wireless power transfer. Concerning the in-vitro and phantom tests, the ME transducer exhibited very promising performances, converting enough power to recharge an implant up to 20 mm away from the transmitter coil, in the most unfavourable orientation. Such results are impossible to get using a coil-coil energy transmission system is the most unfavourable position (receiver perpendicular to the transmitter).In conclusion, the obtained results present very promising prospects for wireless energy transmission, in terms of miniaturised devices and adaptability to the localization of the implanted medical devices

Conselho Nacional de Desenvolvimento Científico e Tecnológico
Hybrid nanostructures which integrate two or more technologically interesting physical properties are fundamental for developing new generations of electronic devices. Exhibiting at least two coupled ferroic orders, multiferroics are an outstanding class of multifunctional materials. Compounds which present coupling between ferromagnetism and ferroelectricity are specially interesting. Although natural multiferroics are rarely found, the possibility of obtaining strain-mediated magnetoelectric coupling in composite structures, by integrating magnetostrictive and piezoelectric layers, paves the way to control electric properties by applying magnetic field or to the electric control of magnetism. Nevertheless, most scientific efforts have been on monophasic compounds or bulk composites. Considering the incorporation of magnetoelectric nanostructures in devices, expanding the scope of the magnetoelectric effect and targetting it to different kinds of applications is needed. Besides new characterization techniques, seeking new alternative materials to the lead-based piezoelectrics or oxide-based magnetostrictives is necessary. Recently, a few works using semiconductors such as ZnO and AlN, or amorphous magnetic alloys such as those based on Co, Fe and Ni, have been reported. In spite of not presenting remarkable piezoelectric and magnetostrictive effects, the features of such materials are promising for high frequency applications, for instance. Considering these issues, four independent surveys are presented. Firstly, the origin of the coupling, latest advances and current scenario of the field are reviewed. Then magnetostriction measurements in thin films are addressed by employing a direct technique based on the cantilever-capacitance method. The goals are to study magnetoelastic properties of some materials whose magnetostriction are not found very often in literature, and to check the reliability of this technique for investigating thin films. In this sense, measurements of some amorphous magnetic alloys mainly based on Co, Fe and Ni are performed. Most samples presents larger magnetoelastic response for magnetic field applied along the magnetization easy axis, as opposed to the theoretically expected. Two investigations on aluminum nitride thin films are reported. Firstly, the growth of AlN films onto several different substrates and buffer layers is studied. Films grown onto glass and polyimide show excellent structural properties for eletromechanical systems and flexible electronics applications. Samples with low residual stress on silicon substrates, suitable for incorporating in existing technologies, are obtained. Secondly, bilayers composed by AlN and ferromagnetic films are investigated. In addition to the structural and morphological properties of the AlN films which are checked, the magnetic characterization of the structures also contributes to design multilayers for exploring the magnetoelectric effect. Finally, problems involving electric fields in scanning probe microscopies are adressed. Surface images of AlN piezoelectric films are systematically acquired. Among other major observations, the possibility of getting reliable piezoresponse images of strongly polarized areas as well as of visualizing ferroelastic domains, is demonstrated. Furthermore, a new microscopy for investigating a sample s ferro and piezoelectric properties is proposed, exploring the direct piezoelectric effect. By utilizing acoustic excitation and electrical detection, the potency of this technique is illustrated with measurements on quartz and AlN surfaces.
Nanoestruturas híbridas, integrando duas ou mais propriedades físicas de grande interesse tecnológico, são fundamentais para o desenvolvimento de novas gerações de dispositivos eletrônicos. Uma classe interessante de materiais multifuncionais são os multiferróicos, que exibem pelo menos duas ordens ferróicas acopladas. Dentre eles, os que apresentam acoplamento entre ferromagnetismo e ferroeletricidade despertam interesse especial. Apesar de serem raros de ocorrer naturalmente, a possibilidade de gerar efeito magnetoelétrico em estruturas compósitas, intermediado pela deformação elástica entre camadas magnetostrictivas e piezoelétricas, abre caminho para que seja possível controlar propriedades elétricas aplicando-se campo magnético, ou propriedades magnéticas aplicando-se campo elétrico. Todavia, a maior parte das pesquisas atuais ainda envolve compostos monofásicos ou compósitos em forma massiva. Tendo em vista a incorporação de nanoestruturas magnetoelétricas em dispositivos, é fundamental ampliar a abrangência do efeito magnetoelétrico e direcioná-lo para diferentes tipos de aplicações. Para isto, além de novas técnicas de caracterização, é necessário buscar-se materiais alternativos aos tradicionais piezoelétricos baseados em chumbo e magnetostrictivos baseados em óxidos. Recentemente tem-se encontrado trabalhos pontuais onde são utilizados piezoelétricos semicondutores como ZnO e AlN, e ligas magnéticas amorfas como as baseadas em Co, Fe e Ni. Mesmo sem apresentar efeitos piezoelétrico e magnetostrictivo com magnitudes notáveis, as características destes materiais são promissoras para aplicações envolvendo altas frequências, por exemplo. Neste necessário, são apresentados quatro estudos independentes entre si. Primeiramente, é realizada uma revisão sobre a origem do acoplamento, os últimos avanços e o panorama atual das pesquisas na área. Em seguida, através de uma técnica direta baseada no método do cantiléver-capacitância, aborda-se o problema das medidas de magnetostricção em amostras na forma de filmes finos. Os objetivos são estudar as propriedades magnetoelásticas em alguns materiais que não são frequentemente abordados pela literatura, e avaliar a potencialidade da técnica para a análise de filmes finos. Para isto, são realizadas medidas principalmente em ligas ferromagnéticas amorfas baseadas em Co, Fe e Ni. Para a maioria das amostras analisadas, a resposta magnetoelástica é maior quando o campo magnético é aplicado na direção do eixo de fácil magnetização, de forma contrária à esperada teoricamente. São apresentadas duas investigações envolvendo filmes finos de nitreto de alumínio. Primeiro é estudado o crescimento de filmes de AlN sobre vários substratos e camadas semente. Filmes crescidos sobre vidro e poliimida apresentam excelentes propriedades estruturais para aplicações em sistemas eletromecânicos e eletrônica flexível. Amostras obtidas com baixos valores de tensão residual, sobre substratos de silício, são interessantes para incorporação em tecnologias existentes. Segundo, são investigadas bicamadas de AlN com filmes ferromagnéticos. Além das propriedades estruturais e morfológicas dos filmes de AlN, a análise das características magnéticas das estruturas contribui para o design de multicamadas que exploram o efeito magnetoelétrico. Finalmente, são abordados problemas em medidas de microscopias de varredura por sonda envolvendo campos elétricos. Imagens da superfície de filmes piezoelétricos de AlN foram coletadas sistematicamente. Entre outras observações importantes, demonstra-se que é possível adquirir imagens confiáveis de piezo-resposta em regiões fortemente polarizadas, e visualizar a formação de domínios ferroelásticos. Também é proposta uma nova técnica de microscopia, para investigar as propriedades ferro e piezoelétricas de uma amostra, explorando o efeito piezoelétrico direto. Utilizando excitação acústica e detecção elétrica, o potencial da nova técnica é demonstrado com imagens de superfícies cristalinas de quartzo e AlN.

碩士
國立清華大學
材料科學(工程)研究所
84
Tb0.3Dy0.7Fe2(Terfenol-D)這個材料具有巨大磁致伸縮值已是眾所周知 ，本研究將添加碳、鉻、釩(C，Cr，V)三種元素來探討添加元素對磁伸縮 值及其他性質的影響。這三種元素中碳屬於插入型添加元素，鉻、釩則屬 於取代型的添加元素。三種元素添加合金[Tb 0.3Dy0.7Fe2Cx、Tb0.3 Dy0.7(Fe1-xCrx)2、Tb0.3Dy0.7(Fe1-xVx)2]皆是用arc-melting 熔配成 棒狀，然後將試片封於石英管中，在900℃加熱一禮拜做均質化。探討添 加元素對合金的磁性性質、結構 (XRD、金相)以及磁致伸縮的影響。隨著 碳的添加，其飽和磁化量漸漸降低，由 85.9 emu/g 降至 33.689emu/g (x=0.2)。而其居里溫度，由378℃降至x＝0.05時之374℃，在x=0.1時出 現第二個居里溫度(340℃) 表示有第二相形成，由 EPMA結果得知為含碳 的(Tb,Dy)Fe3相。磁致伸縮值 (l2T)則隨著碳添加量至x=0.1時有略微增 加，由1064 ppm升至1111 ppm，但是在x＞ 0.1時，第二相形成的量增加 導致磁致伸縮值急遽下降。對於鉻元素添加方面，飽和磁化量隨x值下降 ，至x=0.3時為 66.808emu/g 。其居里溫度值由378℃降到341℃ (x=0.3) ，磁致伸縮值(l2T)由1064 ppm降581ppm (x=0.3)。至於釩元素熔K加，可 增加Terfenol-D的強度，改善它的脆性性質，添加量到x=0.1時磁致伸縮 值尚可維持在900ppm以上。飽和磁化量略微下降，在x=0.1時為83.06 emu/g，居里溫度則下降5℃為373℃。

博士
國立中正大學
機械工程學系暨研究所
101
Based on the magnetoelastic effect, this study develops a non-contact strain sensor consisting of a magnetoelastic film by using low-priced and highly-sensitive Metglas 2826MB amorphous ribbons as the strain measure material. Furthermore, a planar spiral coil combined in a Colpitts oscillator circuit is used to excite the magnetoelastic film. Under a tensile load, a mechanical deformation of the amorphous ribbons occurs and alters its permeability and conductivity. This results in a variation of impedance on the film coupled with the planar spiral coil as well as in the corresponding voltage output to be measured. In addition, the study of the non-contact torque sensor in the giant magneto-impedance (GMI) effect base on amorphous micro wire has been conducted. The GMI strain sensor for torque measurement also can be applied to the rotation axis. The torque sensor materials used CoFeSiB amorphous micro wire, and used two circular hollow planar coil couplings for strain measurement and verification. The experimental results show that the measurement sensitivity can reach a sensitivity 7.24 mV/με of the non-contact strain sensor, a non-linearity of 4.4%, and a hysteresis of 3.6%. Moreover, measurement sensitivity can reach 11.7 mV/με of the torque sensor in GMI wire prototype. Instead of using a conventional coil wound around a bar in most of the other related works, the coil used for this designed sensor is of planar form which offers the advantages of a simplified manufacturing process, uniformity in coil characteristics, compactness, and low manufacturing cost. The circuit architecture for the proposed sensor is rather simple and low-priced compared with that of the widely-used frequency-domain system, requiring a function generator to generate a fixed-frequency steady state signal to excite the coil, and the received sensor response is sent to a lock-in amplifier to be amplified and measured. The advantages are a simplified manufacturing process, uniformity in coil characteristics, compactness, uncomplicated circuit architecture, and low manufacturing cost. Moreover, a promising potential for further improvement makes the designed sensor suitable for future practical applications.

碩士
國立交通大學
材料科學與工程學系所
104
In this study, we expect to develop an novel flexible, ferromagnetic and transparent thin film with large magnetostrictive effect composed of heteroepitaxial CoFe2O4. The study is mainly separated into two parts. One is transparent epitaxial CoFe2O4-SrTiO3 bulk heterojunction system, and the other one is flexible thin film with large magnetostrictive effect composed of heteroepitaxy CoFe2O4. Samples were fabricated through pulsed laser deposition method in this study. The epitaxial relationships, CFO[100]||STO[100], CFO[010]||STO[010] and CTO[001]||STO[001], between CFO and STO are confirmed by x-ray diffraction analysis. Atomic force microscopy and transmission electron microscopy images show CFO and STO entangling together, and this kind of system is called bulk heterojunction system. Superconductive quantum interference device is conducted to measure magnetic properties. The magnetization and magnetic anisotropy are dominated by the strain, which can be tuned by the composition ratio of CFO and STO. The transmission spectrum and x-ray magnetic circular dichroism further prove that it doesn’t need to sacrifice too much magnetization to achieve high transmission in the bulk heterojunction system. In another part of study, layer-by-layer growing mode in CFO film is discovered by reflection high energy electron diffraction. The epitaxial relationships, CFO[111]||Mica[001], CFO[11 ̅0]||Mica[010] and CFO[(11) ̅2]||Mica[100], between CFO and Mica are confirmed by x-ray diffraction analysis. CFO/Mica bimorph has solid magnetism to resist the deformation from SQUID and magnetic force microscopy analysis. Digital holographic microscopy shows that we can deform CFO/Mica bimorph by appling external magnetic field, and also determine high magnetostrictive behavior in this system. Then we combine these two parts of study. CFO and STO nanocomposite system can successfully fabricated on muscovite substrate. Although the magnetic behavior is similar to the second part, this system can’t exhibit excellent transmission. We expect to fabricate a novel transparent flexible magnetic film via the nanocomsite system.

Ph.D.
Spin—density—wave (SDW) effects are investigated in four dilute Cr—Ir alloy single crystals. The Ir concentrations in these crystals were chosen to cover all four magnetic phases existing on the magnetic phase diagram of the Cr—Ir system. Thermal expansion, electrical resistivity, elastic constants and ultrasonic attenuation are studied as functions of temperature and alloy concentration. The elastic constants are also studied as a function of applied hydrostatic pressure. The SDW effects in the Cr—Ir system are compared with those in other Cr alloys. The full temperature—concentration and temperature—pressure magnetic phase diagrams of the Cr—Ir system are determined. Existing theories are used to discuss the observations. The following major observations are made: Magnetoelastic interactions in Cr—Ir alloys are very large, resulting in well defined magnetic anomalies in the elastic constants and thermal expansion at all magnetic phase transition temperatures. Elastic constant measurements as a function of temperature appear to be a very sensitive tool to determine the magnetic phase transition temperatures of the Cr—Ir alloy system. Below TN of Cr—Ir alloys, where TN is the transition temperature from the incommensurate transverse spin—density-wave (TISDW) magnetic phase to the paramagnetic phase, elastic constant and thermal expansion measurements show the existence of hysteresis effects, which are probably due to a redistribution of antiferromagnetic domains. These hysteresis effects are the first evidence of such effects in Cr alloys. Spin fluctuation effects are shown to exist to temperatures well above the Neel temperatures of the Cr—Ir alloys. Analyses of the data for electrical resistivity measurements of Cr—Ir alloys show that the fraction of the electron and hole Fermi surface sheets that nests is roughly the same in the ISDW and CSDW (commensurate spin—density—wave) phases, making the resistivity anomaly near the ISDW—CSDW transition temperature very small or non—existing. Measurements of elastic constants as a function of applied pressure at different constant temperatures are shown to be a very powerful tool for an investigation of the interaction of the SDW with the acoustic phonons in dilute Cr—Ir alloys. It is found that the SDW in Cr—Ir alloys couples mainly with the longitudinal—mode acoustic phonons. Coupling to the shear—mode phonons is relatively small. Empirical correspondence is found between the temperature—concentration and temperature— pressure magnetic phase diagrams of the dilute Cr—Ir alloy system by using a linear scaling between pressure and concentration. Existing thermodynamic models fit the experimental results for the elastic constants and magnetovolume of the Cr—Ir alloys well. The main features of the temperature—concentration and temperature—pressure magnetic phase diagrams of the Cr—Ir alloy system are resonably well predicted by existing microscopic theories.

M.Sc.
Spin-density-wave (SDW) effects are investigated in a Cr + 1.9 at.% Fe alloy single crystal, where the Fe concentration lies just below the triple point found in the temperatureconcentration magnetic phase diagram of the Cr-Fe alloy system. The crystal is expected to undergo a commensurate (C) SDW to an incommensurate (I) SDW phase transition at a temperature Tc, and an ISDW-P (paramagnetic) phase transition at the Neel temperature, TN. The magnetoelastic properties and the anharmonic behaviour of this crystal were studied with the aid of velocity of sound measurements as function of temperature and pressure. Electrical transport measurements were carried out using the standard fourprobe method. In order to determine the various phases present in the crystal a preliminary neutron-diffraction study was also done. Fe belongs to the group-8 magnetic transition metals, possessing localized magnetic moments. The SDW effects in the Cr + 1.9 at.% Fe crystal are therefore compared with that of Cr-Ru and Cr-Ir alloys, as Ru and Ir also belong to the group-8 transition metals, however these impurities are nonmagnetic. The following observations were made: The longitudinal mode elastic constants and the bulk modulus show a prominent change in the slope at Tc, and a sharp, deep minimum at TN. For the c' shear propagation mode peaks were seen at Tcl as well as TN and the c4 4 propagation mode showed no anomalies at either phase transition temperatures. The longitudinal ultrasonic wave velocities for the cL propagation mode were measured as a function of temperature at different constant pressures. TN obtained from these measurements varies linearly with increasing pressure. High-pressure ultrasonic wave velocity measurements were taken at various constant temperatures in the range of 230 K to 350 K for the C L, c44 and c' propagation modes of the Cr + 1.9 at.% Fe alloy single crystal. This was used to determine the pressure derivatives of the second order adiabatic elastic constants (acu /ap). The pressure derivatives of the second order adiabatic elastic constant are shown to be a very powerful tool for investigating the interaction of the SDW with the acoustic phonons in the Cr-Fe crystal. II The long-wavelength acoustic-mode Gitmeisen parameters, calculated from (acu/ap), showed that the SDW in the Cr + 1.9 at.% Fe alloy single crystal couples mainly with the longitudinal acoustic phonons. Coupling to the shear modes is relatively small. The mean acoustic-mode GrOneisen parameter shows a small maximum between Tc, and TN. It increases on heating through TN, reaching a large maximum value above TN, and then decreases with further increase in the temperature. The electrical resistivity was measured_in the temperature region of 4 Kt() 900 K in order to obtain the nonmagnetic component of the resistivity at all temperatures. Only the Neel phase transition was observed in these measurements with no resistivity anomalies taking place at -Va. The experimental results on the resisitivity were analyzed according the model of Chui et al.. The magnetic component of the electrical resistivity was calculated from the model with and without the inclusion of the effects of resonant impurity scattering of the conduction electrons by the local impurity states lying in the SDW energy gap. The magnetic contributions were found to be appreciable above TN, even up to temperatures as high as 1.5TN. The neutron-diffraction experiments show that the Cr + 1.9 at.% Fe crystal remains in the ISDW phase at all temperatures below TN. This is an unexpected result as a CSDW-ISDW phase transition is expected at To, the temperature of the observed anomaly in elastic constant and thermal expansion measurements on the crystal

Giant magnetostrictive RFe2 type (R represents rare earths) intermetallics form an important class of magnetic materials keeping in view of their potential applications as sensors and/ or actuators. In this thesis, one such mixed rare earth compound (Tb,Dy)Fe2 has been chosen for investigations. Being a technologically important material system, several investigations concerning physical and magnetic properties of the material and effect of processing parameters on magnetic properties have been reported in the available literature. However, existing literature does not provide a clear insight into some important aspects such as phase equilibria, evolution of texture and microstructure of directionally solidified Tb-Dy-Fe alloys. Therefore, the present work was undertaken to bring out tangible process-structure-property correlations with an emphasis to clarify the grey areas in the available literature. The investigation on the nature of ternary phase equilibria of Tb-Dy-Fe was taken up with an aim to understand the effect of Tb/Dy ratio on phase equilibria and magnetic properties of TbxDy1-xFe1.95 (x=0-1) alloys. Microstructural and micro-chemical analysis along with study of lattice parameter has been used to predict the nature of phase equilibria and the deviation from the assumed pseudo-binary behaviour. Further, from the microstructural investigations and study of lattice parameter and Curie temperature, a schematic sketch of a section of the ternary diagram, where (Tb,Dy) / Fe =1.95, was formulated and presented. Directional solidification technique is the most widely adopted method for processing the (Tb,Dy)Fe2, to impart grain orientation for practical applications. Therefore, it was aimed in the present study to understand the evolution of texture and microstructure in directionally solidified Tb0.3Dy0.7Fe1.95 alloy by modified Bridgman and zone melting techniques. The alloy was directionally solidified by modified Bridgman technique with a series of growth rates 5 - 100 cm/h, at a constant temperature gradient of 150oC/ cm. Microstructural investigation revealed formation of island banding at lower growth rate and peritectic coupled growth at higher growth rates. The texture study indicated a transition of growth texture from <113> to <110> and finally to <112> with increase of growth rate. A mechanism based on atomic attachment kinetics is proposed to explain the orientation selection with growth rate. The texture and microstructure have been correlated with magnetostriction and static strain co-efficient (dλ/dH) of the Bridgman solidified alloys. The solidification morphology observed in Bridgman solidified samples was found to be mostly plane front. Therefore, in order to understand the microstructure and texture evolution in cellular/ dendritic regime, directional solidification of Tb0.3Dy0.7Fe1.95 was attempted by zone melting technique with a lesser temperature gradient of 100oC/cm. A detailed texture study indicated a transition in preferred growth direction from <110> to <112> with increase of growth rate. In this case of cellular/ dendritic growth regime, a mechanism based on atomic attachment kinetics has been proposed and the preferred morphologies of the solid-liquid interface for <110> and <112> growth have been modelled. The modelled interfaces have been correlated to the shape of cell/ dendrite cross-section observed for the growth rates adopted in this study. Apart from the investigation carried out on the (Tb,Dy)Fe2 alloys, attempts have been made to understand the role of microstructure, especially the ordered phases on the magnetostriction of an emerging magnetostrictive material Fe-Ga. A series of alloy compositions of Fe-x at % Ga (x=17, 20, 23 and 25) were prepared and subjected to different thermal treatments and characterized for microstructural features and magnetostriction. Microstructure investigation of slow cooled, quenched and quenched + aged alloys reveals formation of ordered DO3 phase from disordered A2 phase by first order transformation in 17 and 20 at% Ga alloys, whereas for 23 and 25 at% alloys, the transformation takes place by continuous ordering. It could be observed that large magnetostriction arises owing to the presence of disordered A2 phase or ordered DO3 phase alone. The magnetostriction however decreases substantially when these two phases are co-existing.

Dehnungssensoren werden zur Überwachung von sicherheitsrelevanten Bauteilen, besonders in Bauteilen aus faserverstärkten Polymermatrixverbundwerkstoffen eingesetzt. Durch deren Integration in das Bauteilinnere werden sie vor schädigenden mechanischen sowie korrosiven Einwirkungen geschützt. Dies gewährleistet eine zuverlässige sowie dauerhafte Funktion. Verschiedene Ansätze zur Weiterentwicklung integrierbarer Dehnungssensoren werden international untersucht. Die Verringerung des Sensordurchmessers auf Abmaße im Bereich des Durchmessers von Verstärkungsfasern ist dabei ein bedeutendes Entwicklungsziel. Insbesondere bei der Integration in Bauteile aus faserverstärkten Kunststoffen sorgen zum Durchmesser von Fasern vergleichbare Sensordurchmesser für eine optimale Sensoranbindung. Die Bildung von Harznestern sowie schwächender Unstetigkeiten kann mittels dünner Sensoren verhindert werden. Dies gewährleistet eine artefaktefreie Dehnungsmessung. Drei verschiedene Ansätze für neuartige Dehnungssensoren mit kleinem Querschnitt wurden in dieser Arbeit untersucht.
Strain sensors are used for structural health monitoring issues, certainly in parts with high safety requirements made of fibre-reinforced plastic composites. The integration of these sensors inside the parts protects them against any mechanical and corrosive impact. The sensor functionality can be enhanced by integration. There is a lot of international research effort to further develop integratable strain sensors. Different approaches are currently pursued. This thesis presents the results of investigations on three different approaches for novel strain sensors. The main goal of these investigations was to minimise the sensor diameter down to the diameter of reinforcing fibres. The small diameter allows for an optimum and artefact free integration of the sensors. The formation of resin nests and notches to the material structure can be prevented by integrating sensor with a smaller diameter. The strain measurement and monitoring is enhanced and more reliable then.