Ce travail de thèse est consacré à l’étude du couplage d’échange dans des nano-structures de type ferromagnétique-multiferroïque, avec un intérêt particulier dans la compréhension du renversement en température et angulaire de l’aimantation.Au niveau théorique, un modèle numérique de renversement en température de l’aimantation dans des nanoparticules de type cœur-coquille sera présenté. Le code source du programme, implémenté au cours de cette thèse, a été rendu disponible pour la communauté scientifique sous licence libre. Il permet notamment d’introduire une dispersion en taille des particules, et démontrera le rôle clé de la distribution en taille et de la température dans la réponse magnétique des nanoparticules.Au niveau expérimental, une bicouche composée d’un ferromagnétique Ni81Fe19, et d’un multiferroïque magnétoélectrique à température ambianteBiFeO3, est étudiée. Ces couches sont déposées par pulvérisation cathodique radiofréquence, selon différentes épaisseurs de BiFeO3. Leur structure ainsi que leur morphologie sont caractérisées par diffraction des rayons X, microscopie à force atomique et microscopie électronique à transmission, révélant en particulier la polycristallinité de BiFeO3. Le renversement de l’aimantation est analysé par magnétométrie vectorielle à échantillon vibrant, fournissant des mesures angulaires à température ambiante et à 77 K, à l’aide d’un cryostat à immersion développé au cours de cette thèse ; ainsi que par magnétométrie SQUID, avec l’application de deux protocoles spécifiques de refroidissement en température (entre 10 K et 380 K). Les résultats montrent un comportement similaire à ceux obtenus sur des bicouches épitaxiées. Une propriété intrinsèque du BiFeO3 sera proposée comme étant un mécanisme possible conduisant au comportement en température obtenu, à savoir le cantage des spins de BiFeO3 conduisant à une contribution biquadratique du couplage d’échange. Finalement, un phénomène nouveau dans les matériaux couplés par échange sera mis en évidence à température ambiante, c’est-à-dire un traînage angulaire des axes d’anisotropie. / This dissertation presents a study of the exchange coupling in ferromagnetic-multiferroic nanostructures, with specific interest in understanding the thermal and angular reversal of the magnetization.A theorical numerical model of the thermal magnetization reversal in core-shell nanoparticles is presented. The program source code, implemented during this thesis, is freely avaibale to the scientific community under an open-source license. This model, developed during this thesis, allows diameter size dispersion, and demonstrates the key role of the size distribution and temperature in the magnetic response of nanoparticles.The experimentally studied bilayer is composed of a ferromagnetic material, Ni81Fe19, and a room temperature magnetoelectric multiferroic, BiFeO3. Different thicknesses in BiFeO3 were deposited. The structure and morphology of the bilayers were studied using X-ray diffraction, atomic force microscopy and transmission electron microscopy, revealing in particular the BiFeO3 polycristallinity. The magnetization reversal was probed by vectorial vibrating magnetometry, at room temperature and 77 K, using a self-developped immersive cryostat. The SQUID magnetometry allowed the measurement of two specific cooling protocols between 10 K and 380 K. The results of these two different protocols are similar to the ones obtained for measurements previously reported on expitaxial BiFeO3. An intrinsic property of BiFeO3 is proposed as being the driving mechanism for the thermal dependent magnetization reversal: the canting of the BiFeO3 spins leading to a biquadratic contribution to the exchange coupling. Finally, a new phenomenon in exchange coupled materials is shown at room temperature, which corresponds to an angular training of the anisotropy axes.

No description available.

Field and laboratory experiments were conducted to elucidate various factors influencing the temperature-dependent larval eclosion of winter moth, Operophtera brumata L, in New England. We found no difference in duration of the embryonic stage of eggs reared from larvae collected in Massachusetts (MA) and on Vancouver Island, British Columbia (BC), where winter temperatures are rarely below freezing. The number of growing degree days (GDD) required for larval eclosion declined with the number of days chilled in the laboratory and number of days below freezing in the field, confirming the findings of previous studies. Thus, eggs hatched with fewer GDD, when the spring came later than usual. Date of oviposition had no effect on date of hatch. Eggs laid by naturally occurring (feral) females hatched sooner with lower GDD than eggs from laboratory-reared females from MA and BC held on the same trees over the winter. South-facing eggs on the stems of trees hatched on average 1.6 days sooner than north-facing eggs. Growing degree days calculated from bi-hourly measures of temperature were 15% greater than GDD estimates based on the average of daily maximum and minimum temperatures, as used by many GDD estimates made for online sources. Over two years, the mean GDD in ⁰C for hatch of feral eggs based on bihourly temperature measurements, a 1 Jan start date and a 3.9⁰C developmental threshold was 176.53 ± 6.35SE

The temperature dependence of the refractive index and the density of aqueous binary mixtures of water and ethyl alcohol (C₂H₅OH) were measured by using a modified Michelson interferometer and a narrow glass capillary tube over the temperature range of 278≤T≤353 K for solutions of 100, 75,65, 50, 25, 10 and 0 volume percent ethyl alcohol. The temperature was cycled over both increasing and decreasing directions to explore hysteresis in the cycling. The data are discussed and compared with the Lorentz-Lorenz (LL) formula. A more accurate formula which fits the experimental data better than the LL relation was derived. An attempt was made to determine the nature of the solvent-solute interaction through any changes that were found in the refractive index for He-Ne laser light and IR diode signals and to analyze the refractive index and density results to test the accuracy of the available mixing rules in predicting the refractive index values and the density of binary systems. Conductivity measurements (d. c.) over the temperature range 278≤T≤353 K of aqueous solutions of NaCl at various concentrations were made and used to establish transport properties of ions in solution. The dynamical properties of the electrolytes were used to establish the nature of hydrogen bonding in aqueous binary mixture systems. Rate equations for ion formation and recombination were used to establish the temperature ranges in which hydrogen bonding dominated in forming polymeric species. From experimental data on the binary mixtures with water, a better understanding of water in its different functions and aggregation is possible. The water molecule itself and its response to the environment are understood when suitable studies are made of the forces in the system. In this work, some qualitative aspects of the interactions and dynamics of the water molecule have been investigated. Classical molecular dynamics simulations were tried to explain some of the thermodynamical properties of the water molecule.

This study aimed to investigate methods for determining the temperature dependence of the refractive index and absorption of IR-transparent materials through literary studies and experimental tests. Results from the experimental trials were hard to obtain due to the inherent difficulty of measuring optical properties and yielded only temperature trends of transmittance and reflectance. Despite this, the results could be used for speculation regarding the temperature dependency of the refractive index and the absorption which provides insights into the optical properties of a material. There are several suggestions to improve measurements using this method for future work to be able to determine precise values of the properties. Two additional methods have been reviewed with a literary study, the minimum deviation prism method and the improved Swanepoel thin film method. Both methods are regarded as promising candidates for determining refractive index and its temperature dependence with good accuracy. However, the improved Swanepoel method stands out as the more promising candidate. The determination of absorption and its temperature dependence is established to be inherently hard to determine with the experimental method and the two researched methods, thus suggestions for measuring the absorption in future work are given.

The spawning time differs between sympatric ecotypes of lake-dwelling whitefish (Coregonus lavaretus) and this temporal divergence can be linked to the type of spawning habitat used (i.e., streams, littoral or profundal). It has been hypothesized that the divergence in spawning time between spawning habitats is due to differences in incubation temperature regimes, and a need to synchronize hatching with the spring zooplankton bloom. An alternative hypothesis is that the hatching time of eggs is controlled by cues such as rising temperatures or increasing day length, and the divergence in spawning time occurs due to later maturation in colder habitats. To test these hypotheses, whitefish eggs were sampled on different spawning grounds and the median hatching time was estimated using logistic regression. Temperature logger data and information based on interviews and earlier studies were used to study accumulation of thermal energy during incubation in the form of degree-days. According to this thesis, the hypothesis that hatching time is dependent on rising temperatures and day length is supported, due to high variation in the amount of accumulated thermal energy, and little variation in hatching time. The egg sampling was not successful on profundal spawning grounds, nor on littoral spawning grounds of old populations, which made the results of this study weaker. Further studies are needed to fully confirm either of these hypotheses regarding spawning time of mature whitefish and hatching time of eggs. Sampling methods need to be advanced in order to enable egg sampling on all types of spawning grounds.

Scope and contents: The static and dynamic electrical characteristics of silicon n-channel MOS FETs are studied down to cryogenic temperatures. Particular emphasis is directed towards the effect of interface states on the temperature dependence of both the pinch-off voltage and 1/f noise. / No abstract included. / Thesis / Master of Engineering (MEngr)

No description available.

No description available.

No description available.