Nous avons conçu et réalisé un microscope associant l'imagerie du champ magnétique à l'échelle micrométrique avec l'imagerie topographique en champ proche, destiné à l'étude de la supraconductivité et du magnétisme à basse température. L'intégration du microscope à micro-SQUID dans un réfrigérateur à dilution offre un domaine d'étude encore inexploré en imagerie magnétique. Le champ magnétique est mesuré par un micro-SQUID qui est une boucle de 1 µm de diamètre fabriquée par lithographie électronique et comportant deux jonctions Josephson. L'imagerie topographique est implémentée à travers la microscopie de force. Un résonateur mécanique à quartz en forme de diapason constitue le capteur de force. Il maintient la pointe de Si contenant le micro-SQUID à quelques dizaines de nanomètres de la surface de l'échantillon et permet de connaître la topographie. En mode d'imagerie, le microscope atteint une sensibilité magnétique de 10-3 F0/Hz1/2 et une résolution spatiale magnétique inférieure à 2 µm. A l'aide du microscope, nous avons observé la dynamique des vortex dans un film d'aluminium contenant un réseau de trous. Nous avons pu mettre en évidence le rôle de la taille des trous, le dépiégeage des vortex quand la température approche Tc , la supraconductivité de surface et la relaxation de la distribution des vortex au cours du temps.

La manipulation de la matière au niveau nanométrique a ouvert depuis une quinzaine d'années de nouveaux champs fondamentaux et applicatifs pour les scientifiques et les industriels. Dans ce nouveau paradigme, la nanoélectronique quantique se propose de fonder une nouvelle électronique basée sur les phénomènes quantiques de la matière et plus particulièrement sur la nature quantique des électrons. Ce projet de thèse s'articule autour d'un système électronique quantique hybride supraconducteur/nanotube de carbone (CNT) dénommé nano-SQUID. Ce dispositif présente une boucle supraconductrice contenant deux jonctions CNT en parallèle. Il couple de façon unique les propriétés d'un interféromètre supraconducteur SQUID avec celles de jonctions Josephson à boîte quantique moléculaire. A travers des expériences de transport réalisées, à des températures de quelques dizaines de milli-Kelvins, dans un cryostat à dilution inversé, nous avons étudié les interactions électroniques entre une boîte quantique nanotube et des électrodes supraconductrices. Nous nous sommes particulièrement focalisés sur l'influence de l'état de spin du nanotube sur le courant supraconducteur, qui peut, dans certaines conditions, conduire à la réalisation d'un jonction-. Par un contrôle électrostatique des paramètres microscopiques du dispositif nous avons ainsi pu définir un diagramme de phase expérimental des transitions 0- d'une jonction Josephson à boîte quantique. La dernière partie de cette thèse a porté sur l'utilisation du nano-SQUID comme magnétomètre. En effet, en couplant un aimant moléculaire au CNT composant le SQUID, il a été montré théoriquement qu'il est possible de détecter le retournement d'aimantation d'un spin unique. Nous avons ainsi couplé au nano-SQUID l'aimant moléculaire Double Decker Holmium et réalisé les premières mesures de détections magnétiques aux résultats prometteurs.

<p>The magnetic properties of transition metal compounds have been studied using SQUID-magnetometry, magnetic force microscopy and Lorentz transmission electron microscopy. New magnetic materials have been found and their magnetic properties have been determined. How the magnetic properties of a material can be changed through e.g. chemical substitution of magnetic and nonmagnetic atoms and shape and size effects have also been studied. Three different sets of samples have been investigated: three new Mn-compounds, two substitution series of layered magnetic structures and ferromagnetic micronsized thin film elements.</p><p>The three Mn-compounds, Mn₃IrSi, IrMnSi and Mn₈Pd₁₅Si₇, show different magnetic ordering. Mn₃IrSi orders 'antiferromagnetically' at 210 K. IrMnSi forms a double cycloidal spin spiral below 460 K. Mn₈Pd₁₅Si₇ only shows short-range magnetic ordering.</p><p>Substituting Se with S in TlCo₂Se_2-xS_x changes the magnetic order from a spin spiral to a colinear ferromagnet for a composition of <i>x</i>=1.75. An intermediate region exists where the compound is neither a pure ferromagnet, nor purely a spin spiral, as evidenced by the magnetization versus field measurements for the <i>x</i>=1.3 and 1.5 samples. This is also seen in the temperature dependent susceptibility measurements. For the TlCu_2-xFe_xSe₂ compounds it was found that the ordering temperature and saturation magnetic moment per Fe-atom changed with composition <i>x</i>.</p><p>Ferromagnetic micronsized thin film elements in permalloy, Fe₂₀Ni₈₀, and epitaxial Fe/Co multilayers were studied. For the Fe/Co multilayer thin film elements it was found that it is possible to change the magnetization reversal process, by aligning the easy shape anisotropy axis with either the easy or the hard magnetocrystalline anisotropy axis. In the permalloy elements the effect of inter-elemental distance was found to determine the interval of fields where multidomain states were stable, so that for shorter inter-elemental distances multidomain states were stable for a shorter interval of fields. The domain structure of permalloy elements in rotating magnetic fields was also studied. Higher applied fields led to a broader interval of angles in which saturated states were stable.</p>

The goal of this study was to characterize the nutritional contribution of microalgae to white-legged shrimp and optimize fish meal (FM) and fish oil (FO) inclusion levels in their diets in the presence of microalgae. Phytoplankton composition was first determined in a typical Peruvian intensive commercial shrimp farm and in a semi-closed greenhouse-covered reservoir. A predominance of 76.3% cyanobacteria was observed for most of 9 months in all shrimp ponds. However, with the fertilization program in a reservoir tank, 60.7% diatoms and 22.8% cyanobacteria predominated. Thus, with the imposed fertilization regimen, the microalgae composition was manipulated to be different than that in commercial shrimp ponds. The microalgae composition was then evaluated along with different dietary levels of FM and squid meal (SM) in a feeding trial to evaluate the potential of phytoplankton to reduce FM and SM levels in shrimp feeds. Six diets were formulated to contain either 5, 10 or 20% SM combined with either 6.5 or 12% FM. Dietary effects on growth and survival were compared in a "clear-water system" (CWS) and a "green-water system" (GWS). Results suggest that 6.5% FM and 5% SM can be used as a cost-effective combination in feeds for shrimp. The effects of different dietary levels of FO and soybean lecithin (LT) on shrimp growth in CWS and GWS were evaluated in another feeding trial to determine if dietary phospholipids and phytoplankton increase the availability of essential fatty acids (EFAs) to shrimp. Six diets were formulated to contain 1, 2 or 3% FO combined with either 1 or 4% LT. Shrimp fed diets containing 1% LT and 1% FO in both systems had significantly lower weight gain and higher feed conversion ratio. Cephalothorax lipids and phospholipids were higher in shrimp fed diets containing 4% LT. Inclusion of 4% LT increased the availability of EFAs, and could contribute to reduce the FO in shrimp diets. The contribution of phytoplankton to shrimp weight gain, varied from 38.8 to 60.6%. This study demonstrated that cost-effective diets could be formulated with reduced inclusion levels of FM and FO considering the contribution of microalgae to the nutrition of shrimp.

This thesis will present research that studies the role of disorder, geometric frustration and the long range dipolar interaction on the collective behaviour of several insulating, rare earth magnets. Experiments were performed at low temperatures to measure the specific heat and magnetic susceptibility of several materials. Susceptibility was measured with a SQUID magnetometer that has been designed and constructed primarily for the study of slow dynamics in glassy systems. Specifically, this thesis will discuss three distinct topics. The first is the series of materials LiHo(x)Y(1-x)F(4), which are manifestations of the dilute, dipolar coupled Ising model. The low-x portion of the phase diagram has become a rather contentious issue in recent years with both theoretical and experimental groups disagreeing on the existence of a spin glass freezing transition and one experimental group arguing for the existence of an exotic "antiglass'' or spin liquid state resulting from quantum entanglement at x=0.045. We present specific heat and dynamical susceptibility measurements on four stoichiometries in this series: x = 0.018, 0.045, 0.080 and 0.012. No evidence of an unusual antiglass state is observed. Instead, our results show evidence, at all dilution levels studied, of a spin glass freezing transition. Interpretation of experimental data is found to be complicated by the anomalously slow dynamics in these materials. The relaxation time scales are found to increase as the concentration of Ho(3+) ions is reduced, an effect which can be attributed to single-ion physics and the importance of the nuclear hyperfine coupling in this system. A second set of materials studied here is a series of several Gd garnet materials, the most famous of which is Gd(3)Ga(5)O(12) (GGG), a material previously argued to be a disorder-free spin glass. Our specific heat experiments reproduce previous experiments on GGG and show that the homologous Gd garnets Gd(3)Te(2)Li(3)O(12) and Ga(3)Al(5)O(12) do not share the same glassy physics but exhibit sharp ordering features. By experimenting with the introduction of random site dilution, it is concluded that a 1-2% off-stoichiometry inherent in GGG is likely a special kind of disorder that is particularly effective in inducing random frustration and the formation of a spin glass. Finally, specific heat measurements on the pyrochlore antiferromagnet Gd(2)Sn(2)O(7) (GSO) are presented. While GSO has generally been found to be a well behaved and well understood model magnet, with long range order developing at around 1 K, like many other geometrically frustrated magnets, it has been discovered to possess persistent spin dynamics down to very low temperatures as measured by μSR and Mössbauer spectroscopy. Measurement of the low temperature limit of the specific heat when compared with linear spin-wave theory, however, presents a consistent picture of gapped magnon excitations that freeze out at low temperatures and make the existence of the proposed dynamic ground state unlikely.

Fabrication of nanoscale Josephson junctions and Superconducting Quantum Interference Devices (SQUID) is very promising but challenging topic in the superconducting electronics and device technology. In order to achieve best sensitivity of SQUIDs and to reproduce them easily with a straightforward method, new fabrication techniques for realization of nanoSQUIDs needs to be investigated. This study concentrates on investigation of new fabrication methodology for manufacturing nanoSQUIDs with High Temperature Bi-Crystal Grain Boundary Josephson Junctions fabricated onto SrTiO3 bi-crystal substrates using YBa2Cu3O7-δ (YBCO) thin-films. In this process nanoscale patterning of YBCO was realized by using electron beam patterning and physical dry etching of YBCO thin films on STO substrates. YBCO thin films were deposited using RF magnetron sputtering technique in the mixture of Ar and O2 gases and followed by annealing at high temperatures in O2 atmosphere. Structural characterization of YBCO thin films was done by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Superconducting properties of thin films was characterized by AC magnetic susceptibility measurements. Nanoscale structures on YBCO thin films were fabricated by one E-Beam Lithography (EBL) step followed by Reactive Ion Etching (RIE) and physical dry etching. First SiO2 thin film were deposited on YBCO by RF magnetron sputtering and it was patterned by EBL using Polystyrene (PS) as resist material and RIE. Then SiO2 was used as an etch mask for physical dry etching of YBCO and nanoscale structures on YBCO were formed.

This thesis is based on Gordon-Schaefer model, and assesses Argentine shortfin squid¡¦s stock by using the data of Southwest Atlantic from FAO between 1983 and 2009. First, estimate the equilibrium level of the open-access fishery and dynamic optimization fishery and compare to each other. Then estimate annual Argentine shortfin squid¡¦s stock size, comparing the stock size with the equilibrium level of the two fishery models. The result is that Argentine shortfin squid¡¦s stock size has no crisis of extinction now in Southwest Atlantic. In addition, simulate Argentine shortfin squid¡¦s stock size under management and no management status in the future. The result is that it will make the Argentine shortfin squid sustainable development under dynamic optimization fishery, and this fishery model will be a good management. Finally, this thesis based on the catch of Southwest Atlantic Argentine shortfin squid, which we figure out the fluctuation of catch by literatures, and do the sensitivity analysis.

This dissertation focuses on three separate studies. First, magnetization of the Mn12- acetate was studied by low temperature hysteresis loops and DC magnetization data on magnetically aligned Mn12-acetate micro-crystals. Secondly, Mn12-acetate thin films were fabricated and characterized by AFM and STM. Finally, magnetization of the film material was also studied. Enhanced alignment of Mn12-acetate micro-crystals as compared to prior studies was verified by observation of several sharp steps in low temperature hysteresis loops. It was found that ~ 0.5 T is sufficient to orient the micro-crystals in an organic solvent to a degree comparable to a single crystal. The degree of the alignment was controlled by varying the magnetic field at room temperature and during the cooling process. Subsequently, low temperature hysteresis loops and DC magnetizations were measured for each prepared orientation state of a sample. The high temperature magnetic anisotropy responsible for the alignment could not be measured, possibly due to its small magnitude. Mn12-acetate was deposited onto Si/SiO2 by a solution evaporation method. Atomic force microscopy studies revealed that 2 nm thick films of molecular level smoothness were formed. Mn12-acetate was also deposited onto a Highly Ordered Pyrolytic Graphite (HOPG) surface for scanning tunneling microscopy (STM) studies. A self-assembled triangular lattice was observed in the Mn12-acetate thin films by STM at room temperature under ambient conditions. These STM images show typical center to center intermolecular separations of about 6.3 nm and height corrugation of less than 0.5 nm. Magnetization measurements were not successful in Mn12-acetate thin films due to the small amount of material in the film and the large background signal from the substrate. Therefore, a sample for the magnetization measurements, called “film material”, was made by evaporating a dilute solution of Mn12-acetate powder in acetonitrile. Significant changes in magnetic properties of the film material were observed from magnetization measurements. The blocking temperature of the film material was found to increase to TB > 10 K at low magnetic fields.

The molecular magnet [FeII(£G)FeII(£N)(ox)2(Phen)2]n, whose chemical formula is C28H16Fe2N4O8 for unity, has been studied by magnetization measurements, neutron diffraction, and field-dependent specific heat. From the magnetization measurements, the quasi-ferrimagnetic behavior at T>Tm region can be well described with alternating Land&#x00E9; factors within 1D Ising chain model. However, in T<Tm region, the construction of long-range magnetic ordering due to the increase of interchain interaction was investigated, which is consistent with the anomaly shown in the low temperature specific heat measurement. Furthermore, an intrinsic antiferromagnetic configuration is deduced from analyzing Bragg pattern of neutron scattering. In specific heat measurement, a £f-type anomaly indicating the long-range magnetic ordering was observed. In addition, the magnetic entropy due to this anomaly is much smaller than expected value indicating the spin fluctuated as short-range ordering at T>Tm.

Abstract Methyl Viologen is a kind of organic molecular magnet of current interest which comprises with organic radicals (unpaired electrons). The molecular formula of the sample is (C12H14N2)2+X2-, where ¡§X2-¡¨ denotes a non-magnetic anion (CdI42-, I3-I- etc.). We perform the magnetization (using SQUID magnetometer) and electron paramagnetic resonance (EPR) measurements. Two absorption peaks are observed in EPR spectrum indicating that there are two radical forms in our samples. One of the radicals exists in every sample having the g1-factor value 2.004. The other radical which has the g2-factor 2.001 exists only in certain samples. Interestingly, we find that these samples exhibit ferromagnetism at room temperature. We use Lorentzian profile to quantitatively analyze the EPR spectrum of the samples and derive the ratio (A2/A1) of two different absorption peaks, which represents the amount of g2 radical. The remanent magnetization and saturation magnetization are found to increase as the ratio value increases, indicating that the strength of ferromagnetism is strongly correlated with g2 radical.