This thesis describes Nuclear Orientation and radiofrequency magnetic susceptibility experiments on the enhanced nuclear antiferromagnet holmium vanadate (HoVO₄) from 1 K down to about 1 mK and in applied fields of up to 2.4 tesla. Adiabatic demagnetisation of single crystals of HoVO₄ allowed a mean nuclear spin temperature of about 1 mK to be reached. R.f. magnetic susceptibility measurements at approximately 2.4 mK and in an applied field of about 1.5 x 10^-2 T identified the spin 'flop' phase of the nuclear anti-ferromagnet HoVO₄. The Néel temperature T_N was determined to be 4.0 (2) mK which is in good agreement with the value of 4.8 mK calculated for the dipole-dipole interaction of the enhanced magnetic moments at the Ho site. Gamma-ray anisotropy measurements of ^166mHoVO₄ showed that in low applied fields (B ≮ 0.5 x 10^-2 T) and temperatures of about 1 mK the antiferromagnetic domains were uniformly distributed in the a-a' plane of the tetragonal zircon crystal structure. This was interpreted in terms of pinning of the nuclei due to crystalline imperfections. The spin 'flop' phase was identified in an applied field of 1.2 x 10^-2 T which is in good agreement with the susceptibility measurements. Demagnetisation experiments down to the relatively high residual fields of 4.2 x 10</sup>-2</sup> T and 7.4 x 10</sup>-2</sup> T and temperatures of about 2 mK showed that the effective field at the nucleus was much smaller than the applied field thus indicating that there was some antiferromagnetic ordering even at these relatively high fields. The magnetic dipole moment of ^166mHo was estimated to be 3.60 (6) μ_N from gamma-ray anisotropy measurements of some gamma transitions of known NO parameters. This value is in good agreement with the value of 3.69μ_N calculated for the Nilsson configuration [6337./2]_n [5237/2]_p for the extra-core neutron and proton respectively. Mixing ratio (δ) measurements for gamma-ray transitions between the (K^π = 2⁺) γ-vibrational and (K^π = 0⁺) ground state rotational bands of the daughter nucleus ¹⁶⁶Er showed that Ml transition probabilities are predominantly via ΔK = 1 band mixing which is in good agreement with systematics for deformed nuclei in this mass region. Nuclear refrigeration experiments on a powdered compressed HoVO₄ - copper pill allowed a lattice temperature of 3.9 (3) mK to be reached. Thermal contact measurements on a similar pill showed that the effective area of contact between the salt particles and copper was only about 30 cm² which was a small fraction of the total surface area available. Preliminary work on a HoVO₄ - gold pill enabled a ⁶⁰Co ̲C̲o thermometer to be cooled to 6.0 (l) mK.

Powder neutron diffraction and magnetic susceptibility measurements of the antiferromagnetic phases of Mn_xNi_1-xO,Mn_xCo_1-xO, (Mn_xFe_1-x)_zO and (Co_xFe_1-x)_zO reveal that the magnetic moments of unlike ions are always effectively collinear, despite the presence of competing anisotropies. The magnetic moments of Mn_xNi_1-xO (x = 0.24,0.48 and 0.77) at 5K are confined to (111) planes by dipole-dipole forces, and the small trigonal exchangestrictions are the products of opposed antiferromagnetic Mn²⁺-Mn²⁺ and ferromagnetic Ni²⁺-Mn²⁺ nearest neighbour interactions. In Mn_xCo_1-xO (x = 0.05, 0.10, 0.25,0.36) at 5K, the orbital degener- acy of Co²⁺ is removed by both Jahn-Teller (J.T) and spin-orbit coupling (S.O) mechanisms, leading to orthorhombic or triclinic symmetries and partially quenched Co²⁺ moments. Monoclinic symmetry is observed for 0.51<x<0.67, and Jahn-Teller stabilisations and spin-only Co²⁺ moments are evident. The cobalt concentration is too small to support a cooperative J.T stabilisation in Mn_0.83Co_0.17O, where a magnetostriction (c/a < 1) is observed (in agreement with previous AFMR results). Previous results for Co_xNi_1-xO, and those for Mn_xCo_1-xO, (Mn_xFe_1-x)_zO (x = 0.05, 0.1, 0.12, 0.23, 0.36,0.56,0.66,0.89) and (Co_xFe_1-x)_zO (x = 0.04,0.12,0.50,0.63,0.81) at 5K indicate that the anisotropy order for the iron group monoxides is CoO>MnO»Fe_zO≈NiO. The weak trigonal anisotropy of Fe²⁺ correlates with the near-cubic symmetries of (Co_xFe_1-x)_z( (x = 0.04,0.12) and (Mn_xFe_1-x)_zO (0.1<x<0.66), and a tetragonal (c/a>1) magnetostriction of Fe²⁺ is observed in (Co_xFe_1-x)_zO with x > 0.5. Measurements of vacancy-ferric interstitial ratios for (Mn_xFe_1-x)_zO and (Co_xFe_1-x)_zO suggest that non-stoichiometry is accommodated by 6:2 or 8:3 defect clusters in the former, and by larger units in the latter. The observed magnetic moments of defective samples are normally larger than those calculated according to a previous model for Fe_zO, and require the postulation of partial antiferromagnetic order around the clusters. The room temperature Mossbauer effect parameters of (Mn_xFe_1-x)_zO (0<x<0.975,0.910<z<1.0) indicate that while Fe²⁺ and Mn²⁺ ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.

Thesis (Ph. D.)--McMaster University (Canada), 1990. / Source: Dissertation Abstracts International, Volume: 53-01, Section: B, page: 0310. Supervisor: J.E. Greedan.

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. Naresh Dalal, Florida State University, College of Arts and Sciences, Dept. of Chemistry and Biochemistry. Title and description from dissertation home page (Jan. 18, 2005). Includes bibliographical references.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2004. / Bibliography: leaves 188-210.

Nanomateriais têm sido amplamente estudados, como resultado de suas propriedades físicas e químicas diferenciadas, que oferecem um grande número de possibilidades para aplicações em biomedicina, principalmente na terapia de câncer e no desenvolvimento de estratégias de diagnóstico não invasivo. O óxido de ferro superparamagnético (SPION) é o principal material estudado como agente de contraste para imagem por ressonância magnética, devido à sua capacidade de reduzir o tempo de relaxação transversal (T2) em diferentes tecidos e sua menor toxicidade que os complexos de Gd3+ e Mn2+ usados atualmente. Entretanto, o acumulo de SPIONs pode ser facilmente confundido com sinais referentes à calcificação, depósito de metais pesados e sangramentos, e a alta susceptibilidade magnética do material promove distorções na imagem. Assim, alguns aspectos são desejáveis em material para que este tenha potencial para substituir o SPION, tais como forma nanoparticulada, para fácil modificação de superfície e possibilidade de funcionalização com agentes biosseletivos, e contraste positivo em T1. As nanopartículas (NPs) antiferromagnéticas de MnO atendem a todos os requisitos necessários para substituir o óxido de ferro. As NPs de MnO foram sintetizadas a partir da decomposição térmica do acetilacetonato de manganês(II) em uma variação do método poliol modificado, resultando na formação de NPs com tamanho médio de 21 ± 3,9 nm. Foi realizada a substituição de ligantes de superfície para que se substituísse o ácido oleico adsorvido sobre o material por 3-aminopropiltrimetoxisilano (APTMS) e foi determinada a concentração de grupamentos amino sobre a superfície das NPs. Posteriormente, obteve-se uma estrutura do tipo \"core/shell\" dispersível em meio aquoso e biocompatível pela reação dos grupos amino livres com o carboxilato da carboximetil dextrana (CMDex). O potencial de superfície e a estabilidade coloidal das NPs funcionalizadas foram caracterizados por mobilidade eletroforética e por espalhamento de luz dinâmico em água deionizada e em condições que mimetizavam o sangue. As NPs apresentaram toxicidade em células cancerosas de carcinoma cervical humano (HeLa). Entretanto, não foi observada toxicidade significativa na linhagem de células não cancerosas NCTC clone L929. Tanto as NPs como sintetizadas quanto as recobertas com CMDex apresentaram controle de tamanho e forma, apresentando distribuição de tamanho compatível com o esperado para as aplicações em biomedicina. / Nanomaterials have been widely studied as a result of their interesting physical and chemical properties, which offer a large number of possibilities for applications in biomedicine mainly in cancer therapy and the development of strategies for non-invasive diagnosis. The superparamagnetic iron oxide nanoparticles (SPION) is the main studied material as contrast agent for magnetic resonance imaging (MRI) due to its ability to reduce the transverse relaxation time (T2) in different tissues and lower toxicity than Gd3+ and Mn2+ complexes currently used. However, this SPIONs accumulation can be confused with signals from calcification, bleeding or metal deposits, and the high magnetic susceptibility distorts the background image because its ferromagnetic behavior. Some aspects are desirable to replace SPIONs, such as nanoparticulate form for simple surface modification and labeling with targeting agents, and positive longitudinal T1 relaxation time contrast ability. The antiferromagnetic MnO NPs attend all these requirements and overcome the drawback of using SPION. In our study, MnO NPs were synthesized by the thermal decomposition of Mn(II) acetylacetonate by a variation of the modified polyol process resulting in spherical nanoparticles with average size of 21 ± 3,9 nm. The ligand-exchange step was used to replace the oleic acid adsorbed on the as-synthesized NPs surface by 3-aminopropyltriethoxysilane (APTMS) and the total free amine groups on the NPs surface was determined. After that, a biocompatible and water-dispersible core/shell structure was obtained by coating with carboxymethyl dextran (CMDex) using the free amine-terminal group from APTMS and the carboxylate groups present in the CMDex molecules conjungation. Surface potential and colloidal stability of these functionalized NPs were evaluated by electrophoretic mobility and dynamic light scattering techniques in both water and artificial blood by using the Simulated Body Fluid (SBF) medium. While the water-dispersible NPs have shown toxicity in the human cell line derived from cervical cancer (HeLa), they have not shown significantly cytotoxicity in the healthy fibroblast cells (cell line L929). Both the as-synthesized and coated NPs present controlled size and shape and the final NPs size distribution and magnetic properties are compatible with the expected for biomedical applications.

Orientador: Pascoal José Giglio Pagliuso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-23T00:52:55Z (GMT). No. of bitstreams: 1 Rosa_PriscilaFerrariSilveira_D.pdf: 147107080 bytes, checksum: e6ecb488eaabaaa03a84f0aa02f1bec0 (MD5) Previous issue date: 2013 / Resumo: O entendimento microscópico da intrincada relação entre magnetismo e supercondutividade não-convencional e atualmente um dos grandes problemas em aberto em física da matéria condensada. Em especial, compostos com estrutura cristalina tetragonal parecem favorecer a emergência de tal fenômeno. Os compostos intermetálicos tetragonais BaFe2As2 e EuFe2As2 cristalizam na estrutura tetragonal ThCr2Si2 (I 4/mmm) com camadas de FeAs separadas por átomos de bario/europio. Ambos os compostos apresentam uma distorção estrutural (tetragonal para ortorrômbica) acompanhada for uma transição de fase magnética do tipo onda de densidade de spin (ou SDW, na sigla em inglês) em TSDW = 140 K e TSDW = 190 K, respectivamente. E notável que esta fase magnética pode ser suprimida em direção ao estado supercondutor através de substituição quica ou pressão aplicada. Neste trabalho, apresentamos o estudo sistemático das propriedades do composto intermetálico tetragonal BaFe2As2 em função de tr^es parâmetros: substituição de Eu no sitio cristalográfico do Ba; substituição de metais de transição TM = Mn, Co, Ni, Cu e Ru no sitio cristalográfico do Fe e/ou pressão hidrostática. Para tal proposito, primeiramente sintetizamos amostras de alta qualidade através do método alternativo de fluxo metálico de In. A caracterização macroscópica dos compostos foi realizada através das técnicas experimentais de medidas de susceptibilidade magnética, calor especifico, resistividade elétrica em pressão ambiente e sob pressão hidrostática, além da caraterização estrutural através da difração de pó de raios-X. No que diz respeito a caracterização microscópica, a investigação experimental foi realizada através da técnica de ressonância de spin eletrônico (RSE) utilizando como provas os ons paramagnéticos de Eu2+ e Mn2+/Cu2+, além do estudo de absorção de raios-X (EXAFS) tanto na borda K do As quanto na borda do Fe. Dessa forma, foi possível investigar a dinâmica de spins no plano e fora do plano de FeAs e sua relação com as distorções locais do material. Nossos resultados evidenciam que a diminuição da distância Fe-As esta intimamente ligada a supressão da fase SDW e a localização das bandas 3d do Fe no plano Fe-As. O aumento do caráter orbital planar xy/x2 - y2 na superfície de Fermi parece ser um ingrediente propício para a emergência da supercondutividade nessa classe de compostos / Abstract: The microscopic understanding of the intricate interplay between magnetism and unconventional superconductivity is currently one of the great open questions in condensed matter physics. In particular, compounds with a tetragonal crystal structure seems to be favorable to the emergence of such phenomena. The intermetallic compounds BaFe2As2 and EuFe2As2 crystallize in the tetragonal ThCr2Si2-type structure (I 4/mmm) with FeAs sheets separated by barium/europium layers. Both compounds exhibit a structural distortion accompanied by a magnetic spin-density wave (SDW) phase transition at TSDW = 140 K and TSDW = 190 K, respectively. Remarkably, this SDW phase can be tuned toward a superconducting state by substitution and applied pressure. In this thesis, we will present a systematic study of the intermetallic tetragonal compound BaFe2As2 as a function of three parameters: Eu substitution in the Ba crystallographic site, transition metal (TM) substitution (TM = Mn, Co, Ni, Cu, and Ru) in the Fe site, and/or applied hydrostatic pressure. For this purpose, we have grown high-quality single crystals by the alternative In-ux method. The macroscopic characterization has been made by measurements of magnetic susceptibility, specic heat and electrical resistivity at ambient pressure and under hydrostatic pressure. Concerning the microscopic investigation, the experimental approach consists in using electron spin resonance (ESR) technique employing paramagnetic ions of Eu2+ and Mn2+/Cu2+ as probes in the Ba and FeAs planes, respectively and X-ray absorption spectroscopy (XANES and EXAFS) in both As and Fe K edges. In this manner, it was possible to study the site specific spin dynamics and its relation with local distortions in the material. Our results evidentiate that the decrease in the Fe-As distance is intimately related to the SDW phase suppression and to a localization of the Fe 3d bands in the FeAs plane. This increase in the planar xy/x2 - y2 orbital character at the Fermi surface appears to be a propitious ingredient to the emergence of superconductivity in this class of materials / Doutorado / Física / Doutora em Ciências

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