Orientador: Marcelo Knobel / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-03T22:09:03Z (GMT). No. of bitstreams: 1 Piccin_Rafael_M.pdf: 28985729 bytes, checksum: afa3d6ef86c868ccfd0239d4dfa72438 (MD5) Previous issue date: 2004 / Resumo: Nesta dissertação de mestrado são apresentados resultados do efeito do campo dipolar magnético em conjuntos formados por fios e microfios ferromagnéticos amorfos de composição Fe77,5Sil2,5B15. Devido à estrutura de domínios peculiar destes fios magnéticos, eles podem ser, em princípio, aproximados a dipolos magnéticos, permitido a análise destes campos magnetostáticos nestas estruturas macroscópicas. Os estudos foram realizados em conjuntos de fios colocados lado a lado. As curvas de magnetização foram medidas variando-se o número de elementos. Além disso, no caso de dois fios, medidas em função da separação entre os dois elementos foram realizadas. Estas medidas magnéticas foram feitas empregando um magnetômetro desenvolvido durante o projeto e também utilizado um magnetômetro SQUID. Os resultados são explicados considerando o campo dipolar gerado por cada fio do conjunto, que altera os valores do campo de inversão da magnetização e faz com que platôs sejam observados nas curvas de magnetização. Os efeitos do campo dipolar também foram observados em medidas de magnetoimpedância, onde a presença do fio rico em Fe desloca as curvas de magnetoimpedância de um fio amorfo base Co, de modo análogo a um campo bias / Abstract: The results of the magnetic dipolar field in an array of amorphous ferromagnetic wires and microwires of composition Fe77,5Si12,5B15 are presented in this dissertation. Due to their peculiar domain structure, in principle, they can be approximated to magnetic dipoles, a11owing the analysis of the magnetostatic field among these macroscopic entities. The studies were carried out in arrays of wires placed side by side. The magnetic loops were measured changing the number of elements. Moreover, in the case of two parallel wires, measurements as function of the distance between the wires were performed. The magnetic measurements were carried out in an inductive magnetometer developed during this project and a1so using a SQUID magnetometer. The results are explained considering the dipolar field created by each wire in the array. The dipolar field changes the reversal field of the wires, a11owing the appearance of plateaus during the demagnetization. The effects of the dipolar field were also observed in magnetoimpedance measurements when a Fe-rich wire in placed near an amorphous Co-based wire, its presence dislocates the Co-based wire magnetoimpedance curves, ana1ogously as a bias field / Mestrado / Física / Mestre em Física

Made available in DSpace on 2014-06-12T18:06:41Z (GMT). No. of bitstreams: 2 arquivo945_1.pdf: 1972275 bytes, checksum: faf69ec63b01c315fa88949dbaf22651 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2009 / Sandoildo Freitas Tenório, Antônio; Domingues Coutinho Filho, Maurício. Phase transitions and thermodynamics of quasione- dimensional quantum rotor and spin systems. 2009. Tese (Doutorado). Programa de Pós-Graduação em Física, Universidade Federal de Pernambuco, Recife, 2009.

M.Sc. / Please refer to full text to view abstract.

Hard disk drives (HDDs) have been the dominant secondary memory device in computing for over 50 years, while more recently magnetoresistive random access memory (MRAM) has emerged as a candidate for primary computing memory. Both HDDs and MRAM store information in the polarity of a magnetic layer, which is written and read by non relativistic mechanisms. There is now gathering interesting in using relativistic mechanisms whose origins lie with spin-orbit coupling (SOC) for MRAM writing because of potential benefits in terms of scalability, device design, and efficiency. This thesis investigates the fundamental physics of SOC phenomena that can write (spin-orbit torque (SOT), Neel order SOT) or read (anisotropic magnetoresistance (AMR), magnetic gating) the magnetic state by the application of electrical current. These phenomena are studied in ferromagnetic and antiferromagnetic semiconducting materials that offer a relevant electrical conductivity for integration into commercial electronic devices. Effective magnetic fields which parametrise the SOT phenomenon are measured in the diluted magnetic semiconductor (Ga,Mn)As using a technique based upon experimental planar Hall effect measurements and analytical fitting with a free energy equation for coherent magnetization rotation. It is found that effective magnetic fields which originate from Dresselhaus SOC increase in magnitude with increasing temperature, whereas those originating from Rashba SO have no significant temperature dependence within experimental uncertainty. The size of the measured effective fields per unit of current density, as well as the ratio of Dresselhaus to Rashba effective field magnitudes averaged over all temperatures are comparable to previous experimental measurements. Sb-based diluted magnetic semiconductors (Ga,Mn (As0.9,Sb0.1) and (Ga,Mn)Sb are characterised by magnetic and transport measurements. The Curie temperature (Tc) of (Ga,Mn)(As0.9,Sb0.1) increases from 28K to 55K upon sample annealing. The Tc of as-grown (Ga,Mn)Sb is found to be 34K, and in contrast to (Ga,Mn)(As0.9,Sb0.1) does not change upon annealing, indicating a lack of interstitial Mn in (Ga,Mn)Sb. Field rotation transport measurements for current along various crystalline directions reveal significant crystalline and non crystalline contributions to the AMR of both as-grown and annealed (Ga,Mn)(As0.9,Sb0.1). An anomalous temperature dependence of the AMR of the annealed (Ga,Mn (As0.9,Sb0.1) sample for current along the [110] crystalline direction is accounted for by considering the relative sizes of the individual AMR contributions as a function of temperature. Results are shown of an attempt to vary the current flow through a non-magnetic GaAs/AlGaAs 2D electron gas (2DEG) by changing the magnetization orientation of an electrically insulated Fe gate layer. Such magnetic gating of electrical current is based upon the principle that, as a result of SOC, the electrochemical potential of a ferromagnet is anisotropic with respect to its magnetization orientation. The magnetic gating experiment proved to be unsuccessful due to an AMR-like signal arising in field rotation measurements of 2DEG samples both with and without the gate layer. The origins of this AMR-like signal are unknown, and it cannot not be accounted for by fitting analysis.

This extended abstract describes the background to the 14 research papers that the author, as staff candidate, is submitting for the award of PhD by published works. The core part of this work refers to the development of volumetric biomarkers within the human digestive tract using magnetic resonance imaging (MRI) and their application to answer novel biomedical research questions. In particular the author’s work has focussed on applying these techniques within the human colon and the first two papers (which detail this work) were led and written by the author. This work was pioneering in its field, the first time that physiologically undisturbed colon volumes were measured in healthy human subjects and in patients suffering from irritable bowel syndrome (IBS) and provided novel insights into the post-prandial symptoms experienced. Subsequently the effect of an experimental stress on this post prandial response was evaluated in healthy subjects, also the first time such an effect had been measured. The third paper, also written by the author, describes her work on the first clinical application of similar volumetric techniques to assess the human nasal airways and their response to pharmacological intervention, in this case the efficacy of a nasal decongestant. This document seeks to set the gastro-intestinal papers within their scientific and physiological background and to show their original contribution to the current understanding of the physiological processes within the human gastro-intestinal tract. Between mouth and anus, a complex myriad of mechanical, chemical and biological procedures interact to liquefy and transport food; to break it down into increasingly simpler chemical forms; absorb nutrients and then eject what is no longer required. MRI provides a unique window into the functions and form of this environment at the macroscopic level; a non-invasive tool for detecting and measuring the structure and physical movements of the abdominal organs and their contents, monitoring fluid transport and providing insights into the biological processing therein. This can provide quantitative biomarkers to rigorously assess the normal undisturbed physiology in health and disease and the effect of pharmacological interventions. It is a hitherto relatively unexplored area and it is the development and application of such measurements that form the bulk of the author’s research contained within the presented publications.

No description available.

Increased signal to noise ratio in high field magnetic resonance imaging (MRI) allows the acquisition of high resolution images and the development of the quantitative techniques for measuring tissue properties. This detailed information can provide a better understanding of the structure and function of the healthy brain but, more importantly, it can also provide methods for explaining pathological processes in neurodegenerative diseases. The work described in this thesis investigated iron and myelin content in the brain using 7T MRI. Multiple sclerosis (MS) is one of the most common demyelinating diseases. White matter (WM) lesions detected in MS with conventional MRI techniques show poor correlation with the disease progression. In this work investigation of degeneration of the WM as well as cortical and deep grey matter (GM) in MS and clinically isolated syndrome (CIS) suggestive of MS was performed using high resolution quantitative MRI techniques. Iron plays an important role in the physiological processes of the healthy brain, but its excessive accumulation in the particular brain structures accompanies neurodegeneration in Parkinson’s disease (PD). Studies analysing anatomy and quantitative properties of these structures, in vivo and post mortem, comparing PD patients with healthy controls are presented in this thesis. Investigation related to iron is supported by the simulations aimed at understand the effects of tissue microstructure related to iron on the MR signal. The presented analyses provide a better understanding of the complex dependencies of different MR contrasts on myelin and iron content.

Anisotropic magnetic materials have been proposed over the past twenty years or so as candidates for high density storage, so-called Single-Molecule Magnets (SMMs). These may in future be used to store data at the level of an individual molecule. Separately, isotropic materials may be harnessed for their large magnetocaloric effect which enables them to be used as refrigerants. This can potentially replace the increasingly rare and therefore expensive 3He and 4He currently employed either separately or in 3He-4He dilution refrigerators. This thesis examines the use of lanthanide(III) ions for these applications, by detailing the synthesis, characterisation and performance of three new classes of zero-dimensional compounds, {LnIII2}, {LnIII2ZnII4} and {LnIII7}. These are assessed by several techniques including SQUID magnetometry, heat capacity measurements luminescence spectroscopy, Electron Paramagnetic Resonance and ab initio calculations. In doing so we extended the use of a tripodal ligand widely employed in 3d chemistry to 4f ions, and found only the second 3d-4f phosphonate SMM. Investigating several members of a known three-dimensional lanthanide(III) polymer, {LnIII}n, showed the isotropic gadolinium(III) analogue is one of the very best of all known magnetic refrigerants in the low-temperature regime.

Superconductivity and ferromagnetism are two antagonistic physical phenomena which their coexistence in a uniform material can be resolved only under extraordinary conditions. The reason for that is the phonon-mediated attraction energy between electrons which results in the formation of the so-called Cooper pairs, is usually smaller that the exchange (Zeeman) interaction between electrons which tend to align the electron spins. However, non-zero total momentum Cooper pairs can be accomplished even in the presence of an exchange field as surprisingly! predicted first by Fulde and Ferrel [1] and independently by Larkin and Ovchinikov [2] nearly 50 years ago. This coexistence has already been observed experimentally in both bulk samples [3, 4] and in thin films [5-7] which result from a different type of electron-pairing mechanism which electrons with spin pointing in the same direction team up to form Cooper pairs with one unit of spin, resulting in the so-called triplet superconductivity. Apart from this so-called ferromagnetsuperconductors which both superconducting and ferromagnetism order parameters are present in a uniform material, hybrid systems [8] are made form materials with different or even mutually exclusive properties. Therefore the overall property can be strongly affected by the interaction between constituent materials. The present work, concerns such a hybrid system where Nb, a superconducting metal having transition temperature below 9.5K, is placed in contact with a ferromagnetic metal, Gd with bulk Curie temperature of around 290 K in a form of a nanocomposite. The mutual immiscibility of these two elements gives us the opportunity to take advantage of both the superconduction and ferromagnetism properties of the constituents and further study the transport and magnetic behavior of the system and their effects on each other specially on the critical current of the superconductor which is expected to be modified by the proximity of the ferromagnetic metal.

This thesis is an experimental study of structural and magnetic properties, and hyperfine interactions in crystalline alloys Cu2GdIn, CrNiP, CrNiAs, EuCu2Si2, Cr2FeSe4 and icosahedral quasicrystals Al60Cr19.9Fe0.1Ge 20, Ag50In36Gd14, Zn77Fe 7Sc16. The Heusler alloy Cu2GdIn is shown to crystallize in the L21 crystal structure (space group Fm3¯ m) with a lattice constant of 6.6643(3) A. It is an antiferromagnet with the Neel temperature of 9.6(1) K, the effective magnetic moment of 7.98(4) muB per Gd atom, and the paramagnetic Curie temperature of --41.2(9) K. Its Debye temperature is 229(5) K. The alloys CrNiP and CrNiAs crystallize, respectively, in the Co 2P-type structure (space group Pnma) and the Fe 2P-type structure (space group P6¯2 m), with the lattice parameters a = 5.7965(1) A, b = 3.5337(1) A, c = 6.8123(2) A, and a = 6.1128(2) A, c = 3.6585(1) A. CrNiP is shown to be a three-dimensional Heisenberg ferromagnetic with the Curie temperature of 142.9(6) K, whereas CrNiAs is demonstrated to be a mean-field ferromagnet with the Curie temperature of 171.9(1) K. A long standing controversy concerning the Ni magnetic moment in these alloys is solved unequivocally: Ni atoms carry a magnetic moment of 0.14 muB in CrNiP and 0.15(3) mu B in CrNiAs. The Debye temperatures of CrNiP and CrNiAs are, respectively, 261(3) K and 221(1) K. EuCu2Si2 is one of the first Eu-based alloys in which intermediate valence behavior of Eu was observed. It is demonstrated here that there are no valence fluctuations of Eu in this alloy: Eu atoms are divalent in the temperature range 2-300 K. The Debye temperature of EuCu 2Si2 is shown to be 236(4) K. Cr2FeSe4 is found to have antiferromagnetism with weak ferrimagnetism due to the two different distortions of the octahedral Cr3+ and the octahedral Fe2+. A negative magnetization is found in a magnetic field of 5 De and below due to the coercive-field magnetization reversal. The magnetization reversal disappears in a magnetic field larger than 65 Oe. The icosahedral quasicrystals Al60Cr19.9Fe 0.1Ge20, Ag50In36Gd14, and Zn77Fe7Sc16 are shown to have a primitive six-dimensional Bravais lattice with a six-dimensional lattice constant of 6.558(2) A, 7.805(2) A, and 8.087(1) A, respectively. Al 60Cr19.9Fe0.1Ge20 is a paramagnet with the effective magnetic moment of 0.312(3) muB per Cr/Fe atom. Its Debye temperature is 463(15) K. The temperature dependence of the electrical conductivity of Al60Cr19.9Fe0.1Ge20 is well accounted for quantitatively by theories of quantum interference effects. Ag50In36Gd14 and Zn77Fe 7Sc16 are shown to be spin glasses with the spin freezing temperature of 4.25(5) K and 7.75(2) K, respectively. The hyperfine magnetic fields at 155Gd and 57Fe nuclei in g50 In36Gd14 and Zn77Fe7Sc 16 are shown to set in at temperatures larger than the corresponding freezing temperatures. The frequency dependence of the freezing temperature in Zn77Fe7Sc16 is shown to be equally well accounted for by the Vogel-Fulcher law and the power law. Analysis of the aging effects observed in Zn77Fe7Sc16 leads to a major finding that the nature of the spin-glass state in this quasicrystal is fundamentally different from that of a canonical spin glass. The Debye temperatures of Ag50In36Gd14 and Zn 77Fe7Sc16 are, respectively, 199(2) K and 443(8) K.