Tesi sul tema "Magnetic pressure"
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1
Renshaw, Matthew Peter. "Magnetic resonance studies at elevated temperature and pressure". Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709303.
2
Ghannadzadeh, Saman. "Investigating magnetism and superconductivity using high magnetic fields". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:4b78618e-89a3-424e-a673-59d363a2605d.
Abstract (sommario):
This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF2(H2O)2(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H2O(glycine)2]ClO4 is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO4 is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe1-xCoxAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.
3
Wensley, Joanne Ruth. "High pressure studies on magnetic metals near quantum critical points". Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609926.
4
Furukawa, Masaru. "Localized pressure-driven MHD instabilities in reversed-magnetic-shear tokamaks". Kyoto University, 2001. http://hdl.handle.net/2433/150492.
Abstract (sommario):
Kyoto University (京都大学)0048
新制・課程博士
博士(エネルギー科学)
甲第9045号
エネ博第32号
新制||エネ||9(附属図書館)
UT51-2001-F375
京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻
(主査)教授 若谷 誠宏, 教授 近藤 克己, 教授 佐野 史道
学位規則第4条第1項該当
5
Blaauw, Leopold V. "X-ray Scattering and Absorption and High Pressure Magnetisation Studies of 3d Transition Metal Magnetic Compounds and the Development of High Pressure Magnetic Compton Scattering". Thesis, University of Warwick, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.490685.
Abstract (sommario):
Magnetic Compton scattering was performed on the weak itinerant ferromagnet MnSi providing a direct measurement of the one dimensional spin-dependent electron momentum density of the sample. These results were then compared to band structure calculations performed by the Bristol positron group. The complementary technique of X-ray magnetic circular dichroism was utilised to measure the spin to orbital magnetic moment ratio of NiO nanoparticles. High pressure magnetisation studies were performed on the rare Earth perovskite EUO.5SSr0.42Mn03 and led to the construction of a new ambient and high pressure magnetic phase diagram. The technique of magnetic Compton scattering has been extended into the high pressure regime via the use of a diamond anvil cell capable ofpressures ofup to 20 GPa. The rare Earth perovskite EU0.5SSr0.42Mn03 exhibits a large magnetic moment in the ferromagnetic metal phase. The phase diagram proposed by previous authors was reanalysed in the light of new magnetometry data. A SQUID clamp cell was used to extend the study of the new phase diagram into the high pressure regime. Behaviour which is indicative of. the formation of ferromagnetic clusters within an antiferromagnetic matrix is observed at low temperatures and fields. The application of pressure radically suppresses antiferromagnetism with the sample and favours ferromagnetic ordering via the double exchange mechanism. A new high pressure phase diagram is proposed. The intermetallic compound MnSi exhibits weak ferromagnetism below a temperature of 29K under an applied field of 0.6T. Band structure calculations, when compared to experimentally obtained magnetic Compton profiles fail to adequately explain· the observed momentum densities for pz < 2 a.u. However, the features observed in the profiles at higher momentum are described well by the LMTO theory. The discrepancies at low momentum are the result of the incorrect weighting of the d-like bands. The spin moment was measured as 0.36 ± 0.005 J.lB f.u.-1 indicating a 10% orbital contribution to the total observed moment of 0.4 J.lB f.u.-I A study ofNiO nanoparticles revealed a critical surface to volume ratio which results in a large enhancement of the ferromagnetisation. X-ray absorption spectra indicate that this enhancement is due to the formation of a thicker surface layer of Ni2+ spin symmetry and that the contribution to the spectra from Ni2+ low spin and Ni3+ symmetries does not change as particle size is reduced. X-ray magnetic circular dichroism studies show that the spin and orbital moments are parallel and that as particle size is decreased the orbital to spin moment ratio is increased from the bulk value of 17% to 25%. The development of the equipment and experimental technique of high pressure magnetic Compton scattering utilising a diamond anvil cell is discussed in detail. It is shown that this technique can be successfully applied to measure the one dimensional electron momentum densities of samples situated within the anvil cell. The development and adaptation of the cell for several complementary laboratory techniques is also discussed.
6
McNally, Graham Michael. "High pressure synthesis and neutron diffraction studies of new magnetic manganites". Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/31185.
Abstract (sommario):
With the discovery of appreciable room temperature magnetoresistance (MR) in high Curie temperature (Tc) ferrimagnetic double perovskites such as Sr2FeMoO6, research surrounding other materials of this type has expanded. Most ferrimagnetic double perovskites of the formula A2BB'O6 have non-magnetic A-site cations, such as Sr2+, Ca2+ or Ba2+. Replacing non-magnetic cations with magnetic variants offers further possibilities to tune magnetic effects. This thesis focuses on the substitution of non-magnetic A-site cations with relatively small magnetic Mn2+ cations. This substitution is made possible through the use of high-pressure/temperature (P/T) synthesis, and the characterisation of structural and magnetic properties of new phases discovered through these syntheses. The first of these new phases to be reported herein is Mn2FeReO6, which can be described as the Mn analogue of the well-known ferrimagnetic double perovskite Ca2FeReO6. These materials are well ordered with Fe3+/Re5+ on B-sites and crystallise in a P21/n structure. Mn2FeReO6 shows a high Tc of 520 K due to ferrimagnetic Fe/Re magnetic order above RT, and a large saturated magnetisation of 5.0 μB, which peaks at 75 K. Interestingly, the A-site Mn2+ (3d5) magnetic order has the effect of causing a spin reorientation of the Fe/Re sublattice observed by neutron powder diffraction (NPD) at temperatures below ~75 K. This causes the MR to exhibit the expected negative intergrain tunnelling behaviour above the transition and colossal positive behaviour below. Also reported are a series of perovskite related structures with formulae CaxMn2- xFeReO6 (x = 0.5, 1.0, 1.5). Of particular note among these is CaMnFeReO6, which exhibits 1:1 A-site ordering of Ca/Mn and adopts the P42/n space group. This material belongs to a family of newly discovered 'double double' perovskites, in which Ca/Mn order in columns pointing along the c-axis and Mn has alternating tetrahedral and square planar coordination environments. MR in this material remains negative down to 20 K, potentially due to the presence of Ca disrupting magnetic interactions between Mn2+ cations and suppressing the spin transition. Alternating coordination environments in the double double perovskite structure type were exploited in the synthesis of Ca(Mn0.5Cu0.5)FeReO6. This material also crystallises in the P42/n structure and is well ordered on B-sites, as evidenced by X-ray powder diffraction. Neutron diffraction yields, in addition to columnar order, a slight preference for Cu to occupy the square planar sites and for Mn to occupy tetrahedral sites. This doping of square planar sites with Cu has the effect of enhancing magnetic properties compared to CaMnFeReO6, increasing the saturated magnetisation, raising the ferrimagnetic ordering temperature of the B-sites from 500 to 560 K, and also having a profound effect on the observed MR effects, as a switch in the sign of the MR is observed in this material through a magnetic transition. Finally, B-site substitution has been experimented with in the synthesis of CaMnMnReO6. This also possesses the combined A and B-site orders observed in CaMnFeReO6 and an unusual magnetic structure, with perpendicular A and B-site magnetism due to frustration, deviating greatly from the magnetic structures of materials with B-site Fe/Re. In summary, this thesis compiles the synthesis and analysis of a series of new double perovskites, double double perovskites and a new 'triple double' five-fold cation ordered structure with a general formula of AA'0.5A''0.5BB'O6. These materials show that new types of structural ordering can be used to increase the number of degrees of freedom available for tuning the interplay between many different magnetic cations in different coordination environments.
7
Sokolov, Dmitriy A. "Effect of pressure on transport and magnetic properties of layered manganites". Thesis, University of Edinburgh, 2002. http://hdl.handle.net/1842/14442.
8
Rochette, P., G. Fillion, R. Ballou, F. Brunet, B. Ouladdiaf e L. Hood. "High pressure magnetic transition in pyrrhotite and impact demagnetization on Mars". AMER GEOPHYSICAL UNION, 2003. http://hdl.handle.net/10150/623401.
Abstract (sommario):
Using neutron diffraction under pressure at room temperature, we observed that pyrrhotite undergoes a ferrimagnetic to paramagnetic transition at about 2.8 GPa. Complete demagnetization of remanence at the same pressure is confirmed in an independent experiment. Such a process provides a quantitative explanation of the magnetic structure of the Martian Southern Hemisphere assuming that pyrrhotite is the major magnetic mineral and that our static experiments can be extrapolated to dynamic pressure conditions. Indeed, the 3 GPa isobaric line during the two large impacts of Argyre and Hellas separates the magnetized and unmagnetized zones. We also infer a reinterpretation of Martian meteorites paleomagnetic signal.
9
Prescimone, Alessandro. "Effect of high-pressure on molecular magnetism". Thesis, University of Edinburgh, 2010. http://hdl.handle.net/1842/4619.
Abstract (sommario):
The effect of pressure on a number of magnetically interesting compounds such as single-molecule magnets and dimeric copper and manganese molecules has been investigated to probe the validity of ambient magneto-structural correlations. The first chapter is an introduction to the equipment and methodologies that have been adopted to carry out the experimental high-pressure work. The second chapter reports the first combined high-pressure single crystal X-ray diffraction and high pressure magnetism study of four single-molecule magnets (SMMs). At 1.5 GPa the structures [Mn6O2(Et-sao)6(O2CPh(Me)2)2(EtOH)6] (1) – an SMM with a record effective anisotropy barrier of ~86 K – and [Mn6O2(Etsao) 6(O2C-naphth)2(EtOH)4(H2O)2] (2) both undergo significant structural distortions of their metallic skeletons which has a direct effect upon the observed magnetic response. Up to 1.5 GPa pressure the effect is to flatten the Mn-N-O-Mn torsion angles weakening the magnetic exchange between the metal centres. In both compounds one pairwise interaction switches from ferro- to antiferromagnetic, with the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with respect to the plane of the three metal centres. High pressure dc χMT plots display a gradual decrease in the low temperature peak value and slope, simulations showing a decrease in |J| with increasing pressure with a second antiferromagnetic J value required to simulate the data. The “ground states” change from S = 12 to S = 11 for 1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in |D|, while out-of-phase (χM //) ac data show a large decrease in the effective energy barrier for magnetisation reversal. The third SMM is the complex [Mn3(Hcht)2(bpy)4](ClO4)3·Et2O·2MeCN (3·Et2O·2MeCN) that at 0.16 GPa loses all associated solvent in the crystal lattice, becoming 3. At higher pressures structural distortions occur changing the distances between the metal centres and the bridging oxygen atoms making |J| between the manganese ions weaker. No significant variations are observed in the JT axis of the only MnIII present in the structure. Highpressure dc χMT plots display a gradual decrease in the low temperature peak value and slope. Simulations show a decrease in J with increasing pressure although the ground state is preserved. Magnetisation data do not show any change in |D|. The fourth SMM, [(tacn)6Fe8O2(OH)12](ClO4)3.9Br4.1⋅6H2O, (4) is the largest inorganic compound ever studied at high-pressure. Up to 2.0 GPa the conformation of the complex remains largely unaffected, with the counter ions and water molecules moving around to accommodate a compression of the unit cell volume. High pressure magnetic susceptibility data collected up to 0.93 GPa confirm minimal changes in the intra-molecular exchange interactions. The third chapter focuses on three hydroxo-bridged CuII dimers: [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (5), [Cu2(OH)2(tben)2](ClO4)2 (6) and [Cu2(OH)2(bpy)2](BF4)2 (7) have been structurally determined up to 2.5, 0.9 and 4.7 GPa, respectively. 6 and 7 have never been reported before. Pressure imposes important distortions in the structures of all three complexes, particularly on the bond distances and angles between the metal centres and the bridging hydroxo groups. 5 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in different magnetic behaviours of the complexes as seen in dc susceptibility measurements up to ~0.9 GPa: J becomes less antiferromagnetic in 5 and 6 and more ferromagnetic in 7. The fourth chapter shows the compression of two oxo-bridged MnII/MnIII mixed valence dimers: [Mn2O2(bpy)4](ClO4)3⋅3CH3CN, (8) has been squeezed up to 2.0 GPa whilst [Mn2O2(bpy)4](PF6)3⋅2CH3CN⋅1H2O, (9) could be measured crystallographically up to 4.55 GPa. 9 has never been reported before, while 8 has been reported in a different crystallographic space group. The application of pressure imposes significant alterations in the structures of both complexes. In particular, in 8 the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond distances, 9 shows essentially analogous behaviour: the Mn-Mn distance and nearly all the Mn-N bonds shrink significantly. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 8 and 0.84 GPa for 9, but neither display any significant differences with respect to their ambient data.
10
Lenfeldt, Niklas. "The search for reversibility of idiopathic normal pressure hydrocephalus : aspects on intracranial pressure measurements and advanced MRI techniques in combination with CSF volume alteration /". Umeå : Univ, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-1422.
11
Maude, D. K. "Investigation of advanced semiconductor materials using high pressure and high magnetic fields". Thesis, University of Nottingham, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381086.
12
Foster, Timothy John. "High pressure and high magnetic field studies of low dimensional semiconductor structures". Thesis, University of Nottingham, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315090.
13
Weissner, Stefan. "Numerical and experimental investigation of the load/unload behavior of subambient pressure hard disk drive sliders /". Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2001. http://wwwlib.umi.com/cr/ucsd/fullcit?p3027050.
14
Meier, Thomas. "High Sensitivity Nuclear Magnetic Resonance at Extreme Pressures". Doctoral thesis, Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-203643.
Abstract (sommario):
Moderne Hochdruckforschung entwickelt sich rasant zu einer der vielfältigstens und überraschensten Disziplinen der Festkörperphysik. Unter Benutzung von Diamantstempelzellen können Drücke erreicht werden, die den Bedingungen im Inneren unserer Erde ähneln.
Eine Anwendung von Kernmagnetischen Resonanzexperimenten (NMR) in Diamantstempelzellen galt jedoch fur lange Zeit als unmöglich. In der vorliegenden Arbeit wird ein neuartiger Ansatz weiterentwickelt, der Radiofrequenz-(RF)-Mikrospulen benutzt, die direkt zwischen den Diamantstempeln platziert werden, und somit zu einer
signififikanten Sensitivitatssteigerung führen.
Es ist gelungen, Hochdruckzellen zu entwickeln, die fur die speziellen Anforderungen der NMR zugeschnitten sind. Des Weiteren konnte eine nicht metallische, nicht magnetische Dichtung entwickelt werden, die zudem zu einer signififikanten Stabilisierung des Probenvolumens führt. Eine breit angelegte Analyse der Leistungsfähigkeit dieser
neuartigen NMR-Hochdruckprobenköpfe zeigt deren Leistungsfähigkeit mit sehr hohen Empfifindlichkeiten sowie einer exzellenten RF Anregung und Zeitauflösung.
Drei Anwendungsbeispiele, die das Potenzial dieses Ansatzes in dieser Arbeit
unterstreichen, werden vorgestellt. Bei Drücken von bis zu 4 GPa werden die elektronischen und dynamischen Eigenschaften von elementarem Gallium untersucht. Unter höheren Drücken ist es gelungen, einen druckinduzierten Isolator-Metall-Übergang in dem ternaren Chalkogenid AgInTe2 zu beobachten. Schlussendlich ist es gelungen, die strukturellen und elektronischen Eigenschaften von Rubin bei Drücken von bis zu 30.5
GPa zu untersuchen, was einer Verdreifachung des bisher zugänglichen experimentellen Druckbereiches entspricht und die NMR fur moderne Hochdruckanwendungen möglich macht.
15
Bloom, Scott Harris. "Superconducting and normal compounds : some high field/high pressure effects /". Thesis, Connect to Dissertations & Theses @ Tufts University, 1989.
Abstract (sommario):
Thesis (Ph.D.)--Tufts University, 1989.Submitted to the Dept. of Physics. Includes bibliographical references (leaves 192-204). Access restricted to members of the Tufts University community. Also available via the World Wide Web;
16
DaSilva, Jack Guy. "Pressure dependent magnetic investigation of tetracyanoethylene-based molecule-based magnets and their analogues". Thesis, The University of Utah, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3593070.
Abstract (sommario):
Several metal-tetracyanoethylene (TCNE) compounds, including the [bis(pentamethylcyclopentadienyl)iron(III)][tetracyanoethylene], [FeCp*2][TCNE], family of molecule-based magnets and two cyanide based MBMs were investigated by pressure dependent DC magnetic measurements. The 0-D electron transfer salts: [FeCp*2][TCNE], ferromagnetic [FeCp*2][TCNQ] (TCNQ = 7,7,8,8-tetracyanoquinodimethane), metamagnetic [FeCp*2][TCNQ], [FeCp*2][HCBD] (HCBD = hexacyanobutadiene), and [FeCp*2][DDQ] (DDQ = 2,3-dichloro-5,6-dicyano-p-benzoquinone) exhibited an array of magnetic behavior both at ambient and applied pressure. [FeCp*2][TCNE] and [FeCp*2][HCBD] exhibited weak ferromagnetism above 4.2 and 3.1 kbar, respectively. The ferromagnetic polymorph of [FeCp*2][TCNQ] displayed linear increase to the critical temperature, Tc, and the bifurcation temperature, Tb, reaching 5.01 and 5.46 K, respectively at 10.3 kbar. The coercive field, Hcr, displayed exponential-like increase, reaching 550 Oe at 10.3 kbar. The metamagnetic polymorph of [FeCp*2][TCNQ] displayed linear increase of the Tc at low applied pressure, reaching 2.90 at 2.9 kbar, then transitioned to a paramagnetic state at further applied pressure. [FeCp*2][HCBD] transitioned from a paramagnetic state at ambient pressure to a weak ferromagnetic state at 3.1 kbar with a Tc, H cr, and Hc of 2.46 K, 25 Oe, and 2,200 Oe, respectively. The Tc and H c then increased linearly with further applied pressure to 4.80 K and 10,000 Oe, while the Hcr increased exponentially to 795 Oe, at 11.4 kbar. [FeCp*2][DDQ] exhibited paramagnetic behavior at ambient and applied pressures up to 9.2 kbar. The structurally related 2-D MnII(TCNE)I(H2O) and 3-D MnII(TCNE) 3/2(I3)1/2, showed significant increases to the Tc, Tb, and Hcr with applied pressure. A high- and low-pressure regions were observed for MnII(TCNE)I(H2O). 2-D [Ru2(O2CBu t)4][M(CN)6] · H2O (M = Fe, Cr) displayed suppression of hysteretic properties at high applied pressure and irreversibility of the suppression. A Mean Field (MF) analysis of three structurally related non-cubic Prussian blue analogues (PBA) was performed to assess the intensity of their coupling modes. These values were framed by the reinvestigation of several known cubic PBAs and comparing the coupling intensities, as well as evaluating the MF theory in the context of these structures as several had been evaluated by other means.
17
Giriat, Gaetan. "Instrumentation development for magnetic and structural studies under extremes of pressure and temperature". Thesis, University of Edinburgh, 2012. http://hdl.handle.net/1842/7931.
Abstract (sommario):
The study of the magnetic and structural properties of matter under extreme conditions is a fast developing field. With the emergence of new techniques and innovative instruments for measuring physical properties, the need for compatible pressure generating devices is constantly growing. The work described in this thesis is focused on development, construction and testing of several high pressure (HP) cells of novel design. One of the cells is intended for single crystal X-ray diffraction (SXD) studies at low temperature (LT) and the other three HP devices are designed for a Magnetic Property Measurement System (MPMS), two of which are suitable for dc susceptibility studies and the other one is aimed at high frequency ac susceptibility measurements. HP crystallographic studies are routinely carried out in diamond anvil cells (DAC) at room temperature while ambient pressure SXD studies are often conducted at LT to reduce atomic vibrations and obtain more precise structural data as well as to study LT phases. Combining HP with LT gives access to a whole new area on the phase diagrams but due to the size of the existing DACs this is generally achieved by cooling down the cells inside a cryostat and it is mainly possible at synchrotrons where dedicated facilities exist. A miniature DAC which can be used with commercially available laboratory cry-flow cooling systems and achieves pressures in excess of 10 GPa has been developed. The design of the pressure cell is based on the turnbuckle principle and therefore it was called TX-DAC. Its dimensions have been minimised using Finite Element Analysis (FEA) and the final version of the cell weighs only 2.4 g. The cell is built around a pair of 600 μm culet Boehler-Almax anvils which have large conical openings for the diffracted beam. The TX-DAC is made of beryllium copper (BeCu) alloy which has good thermal conductivity and allows quick thermal equilibration of the cell. The MPMS from Quantum Design is the most popular instrument for studies of magnetic properties of materials. It is designed to measure ac and dc magnetic susceptibility of sample with detectable signals as low as 10-8 emu. The MPMS has a sample chamber bore of 9 mm in diameter and this puts a constraint on the dimensions of the pressure cells. However, several types of clamp piston-cylinder cells and DACs have been designed for the MPMS. The former are used for measurements at pressure up to 2 GPa and the later can be used for studies at higher pressure. Taking advantage of the turnbuckle principle, a DAC (TM-DAC) and a piston-cylinder cell (TM-PCC) for dc magnetic studies were built. They allow HP measurements to be performed at the full sensitivity of MPMS. Both pressure cells are made of BeCu and their small dimensions combined with symmetrical design is the key to an ideal background signal correction. The TM-DAC is 7 mm long and 7 mm in diameter, it weighs 1.5 g and with 800 μm culet anvils it can generate a sample pressure of 10 GPa. Inherently the sample volume is limited to approximately 10-3 mm3 and the signal corresponding to this volume of some weakly magnetic material remains below the sensitivity of the MPMS. This constraint led us to the development of the TM-PCC – a piston-cylinder variant of the turnbuckle design. With a 4 mm3 sample volume it allows the study of weakly magnetic samples in the range 0-1.9 GPa. The TM-PCC uses two zirconia pistons of 2.5 mm in diameter; it is 10 mm long, 7 mm in diameter and weights 2.7 g. Conventional metallic pressure cells perform well in dc mode however in ac susceptibility measurements, the Eddy currents set in the cells’ body lead to a screening effect which can significantly obscure the signal from the sample. This problem was solved by designing a composite piston-cylinder cell made with Zylon fibre and epoxy resin. The sample is located in the middle of the cell in the 2.5 mm bore and the pressure is transmitted through zirconia pistons. Keeping the metallic parts away from the sample resolves any interference issue. The composite cell performs well in a pressure range of 0-1 GPa. The performance of the pressure cells developed within this project is illustrated by studies of various systems at high pressure.
18
Alikacem, Mohamed. "Investigation of low dimensional semiconductor structures using high pressure and high magnetic fields". Thesis, University of Nottingham, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315113.
19
Daniil, Dimitra. "Rock Magnetic Studies of Magnetite and Quartzite: Results at Ambient Conditions and From Diamond Anvil Pressure Experiments (~2 GPa)". Thesis, Uppsala universitet, Geofysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-437026.
Abstract (sommario):
Numerous techniques are used to study the magnetic history and properties of rocks, which provides important data on the Earth’s interior. In this study we conducted a series of experiments on two different sample sets, comprising pure magnetite (Set1) and a mixture of magnetite-hemoilmenite-bearing quartzite (Set2), in order to gather information about their magnetic susceptibility and remanent magnetization. A miniature diamond anvil cell (mDAC) constructed from hardened non-magnetic alloy (40HNU) was used for high pressure experiments. With the help of high sensitivity susceptibility bridge (MFK1-FA Kappabridge, AGICO), we measured the susceptibility of three samples and of the mDAC. Consequently, we loaded one of the samples in the mDAC and acquired susceptibility measurements during the compression and decompression of the system. For remanence measurements we used a 2G Enterprises SQUID magnetometer. We measured the natural remanent magnetization (NRM) and performed alternating field (AF) demagnetization on ten samples and on the mDAC. Additionally, isothermal and anhysteretic remanent magnetization (IRM and ARM) acquisition were performed on nine and one samples, respectively and demagnetization on three and one samples, respectively. Results showed that Set1 samples consist of both SD and MD magnetite grains. Regarding Set2 samples, the main magnetic carriers are SD and MD magnetite or titanium bearing magnetite grains, while there are indications that hematite and ferrimagnetic titanohematite might be present as well. The presence of eddy currents, due to the conducting materials of the mDAC, resulted in masking the signal of the sample making it difficult to draw any safe conclusions regarding how the susceptibility of the sample changes with pressure. All in all, there was a satisfactory response of the instruments working with very small amounts of material and an adequate signal stability of both the samples and the mDAC. Our study provided us with an insight into high pressure experiments and showed possible directions for future studies.Många metoder används för att studera magnetiska egenskaper i berg, vilket ger viktig information om Jordens inre. I denna studie genomförde vi en rad experiment på två olika uppsättningar prov, innefattande ren magnetit (Set1) och en blandning av magnetit-hemoilmenit-bärande kvartsit (Set2), för att samla information om deras magnetisk susceptibilitet och magnetiska remanens. En miniatyr diamantstädcell (mDAC) konstruerad av en härdad icke-magnetisk legering (40HNU) användes för högtrycksexperiment. Susceptibilitetet av tre prover och av mDAC mättes en MFK1-FA Kappabridge, AGICO. Följaktligen monterade vi ett av proverna i mDAC och vi genomförde susceptibilitetsmätningar under kompression och dekompression. Vi använde en 2G Enterprises SQUID magnetometer för remanensmätningar. Den naturlig remanentmagnetiseringen (NRM) mättes och vi utförde alternerande fält (AF) demagnetisering på tio prover och på mDAC. Dessutom genomfördes isotermisk och anhysteretisk remanentmagnetisering (IRM och ARM) på nio respektive ett prover, och demagnetisering på tre respektive ett prover. Resultaten visade att Set1-prover består av både enkeldomän (SD) och multidomän (MD) magnetit. SD och MD magnetit- och titanomagnetitkorn är de viktigaste magnetiska mineral av Set2-prover, medan finns det indikationer på att hematit och ferrimagnetisk titanohematit också bidrar till signal. Virvelströmmar, som skapats av mDACs ledande material, maskerade provets magnetiska susceptibilitet och det var svårt att kommentera på hur provets susceptibilitet förändras med tryck. Sammantaget var instrumentens prestanda tillfredsställande, även vid små mängder material. Prov och mDACs signalstabilitet var också adekvat. Vår studie gav oss en inblick i högtrycksexperiment och magnetiska egenskaper och den visade möjliga riktningar för framtida studier.
20
Meißner, Thomas. "Exploring Nuclear Magnetic Resonance at the Highest Pressures". Doctoral thesis, Universitätsbibliothek Leipzig, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-115023.
Abstract (sommario):
Die vorliegende Arbeit beschäftigt sich mit der Entwicklung und Anwendung der Kernmagnetresonanz (NMR) unter extremen Drücken bis 101 kbar. Es wird gezeigt, dass die Empfindlichkeit von NMR Experimenten unter hohen Drücken in Diamantstempel- und ähnlichen Hochdruckzellen im Vergleich zu früheren experimentellen Versuchsanordnungen durch Verwendung von Mikrospulen um mehrere Größenordnungen verbessert werden kann. Die neue Versuchsanordnung wird dann zum einen bei der Untersuchung der Druckabhängigkeit elektronischer Eigenschaften von metallischem Aluminium bis 101 kbar verwendet. Zum anderen wird damit der Pseudogap des Hochtemperatursupraleiters YBa2Cu4O8 bei Drücken bis zu 63 kbar untersucht.
21
Larson, Blake E. "Magnetic resonance imaging based percent stenosis and pressure drop measurements in an aortic coarctation model". Thesis, Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/15976.
22
JAIN, ALOK. "A BIOPARTICLE SEPARATION TECHNIQUE THROUGH MICROCHANNELS USING SEQUENTIAL PRESSURE PULSES". University of Cincinnati / OhioLINK, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1085759166.
23
Ebbers, Tino. "Cardiovascular fluid dynamics : methods for flow and pressure field analysis from magnetic resonance imaging /". Linköping : Univ, 2001. http://www.bibl.liu.se/liupubl/disp/disp2001/tek690s.pdf.
24
Albrecht, Carsten. "High pressure studies of magnetic quantum phase transitions in NbFe₂ and Ca₃Ru₂O₇". Thesis, Royal Holloway, University of London, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.479376.
25
Hyunsik, I. M. "X-band related transport in GaAs/AIAs heterostructures under pressure and in magnetic fields". Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302493.
26
Pothini, Venu Madhav. "MAGNETIC RESONANCE IMAGING OF THE HUMAN INFERIOR VENA CAVA DURING LOWER BODY NEGATIVE PRESSURE". UKnowledge, 2004. http://uknowledge.uky.edu/gradschool_theses/197.
Abstract (sommario):
Magnetic Resonance Imaging (MRI) was used to determine changes in the size of the Inferior Vena Cava (IVC) as a result of blood pooling induced by lower body negative pressure (LBNP). Images of the IVC of supine human subjects (10 males, 10 females) were obtained under four conditions: 1) steady-state 0 mmHg LBNP, 2) steady-state –35 mmHg LBNP, 3) ramping from 0 to –35 mmHg LBNP, 4) ramping from –35 to 0 mmHg LBNP. Volumes for a given IVC segment were obtained under the first two conditions during both end inspiration and end expiration breath-holds. Inferior Vena Cava widths were measured under all four conditions at the levels of portal entry and portal exit. The IVC volume for men and women combined decreased 41% due to LBNP (p andlt; 1.02 x 10-9). The IVC was 64.4% wider at portal exit than at portal entry in men (p andlt; 0.0003). Lower Body Negative Pressure induced a decrease in men's vena cava width up to 46% at portal exit and up to 62% at portal entry. Supported by NASA EPSCoR WKU 522611 and NIH GCRC MO1 RR262.
27
Smith, Christopher Stanley. "The application of in-situ high pressure nuclear magnetic resonance spectroscopy to heterogeneous catalysis". Thesis, University of Liverpool, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317244.
28
Meier, Thomas, e Jürgen Haase. "Anvil cell gasket design for high pressure nuclear magnetic resonance experiments beyond 30 GPa". AIP Publishing, 2015. https://ul.qucosa.de/id/qucosa%3A21310.
Abstract (sommario):
Nuclear magnetic resonance (NMR) experiments are reported at up to 30.5 GPa of pressure using radiofrequency (RF) micro-coils with anvil cell designs. These are the highest pressures ever reported with NMR, and are made possible through an improved gasket design based on nano-crystalline powders embedded in epoxy resin. Cubic boron-nitride (c-BN), corundum (α-Al2O3), or diamond based composites have been tested, also in NMR experiments. These composite gaskets lose about 1/2 of their initial height up to 30.5 GPa, allowing for larger sample quantities and preventing damages to the RF micro-coils compared to precipitation hardened CuBe gaskets. It is shown that NMR shift and resolution are less affected by the composite gaskets as compared to the more magnetic CuBe. The sensitivity can be as high as at normal pressure. The new, inexpensive, and simple to engineer gaskets are thus superior for NMR experiments at high pressures.
29
Pressler, Rebecca E. "AN INTEGRATED PETROFABRIC STUDY OF THE HIGH-PRESSURE ORLICA-SNIEZNIK COMPLEX, CZECH REPUBLIC AND POLAND". Ohio University / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1149180445.
30
Zanjani-pour, Sahand. "Intervertebral disc stress and pressure in different daily postures : a finite element study". Thesis, University of Exeter, 2016. http://hdl.handle.net/10871/26682.
Abstract (sommario):
Low back pain is the most common cause of disability in the United Kingdom with health care costs of more than 1 billion pounds per year. One reason associated with low back pain is the degeneration of intervertebral discs due to loads on the spine. Daily postures such as standing and sitting produce different loads on the discs. Previously, many studies investigated the stress and pressure within the disc in these postures. The results do not agree with each other and the experiments have many limitations. The aim of this project was to assess the feasibility of incorporating magnetic resonance (MR) imaging and finite element (FE) analysis to predict the pressure and stresses developed by different daily postures in an individual. Transient and non-transient subject specific 2D models of nine individuals in standing and sitting were created based on previously acquired MR images. The geometry of these FE models was based on supine MR images. The sitting and standing boundary conditions were calculated by comparing their MR images with the supine posture. The results showed that for six subjects sitting created more intradiscal pressure compared to standing and in one subject standing more than sitting. For two of the subjects the pressure was nearly the same in sitting and standing. Because of the 2D model’s limitations, 3D models of an individual were developed. Both transient and non-transient models of the individual were created. The intradiscal pressure results were three times lower compared to the 2D models. This was due to consideration of out of plane deformation in the 3D models. These results were in the range of in-vivo and in-vitro measurements available in the literature. In conclusion, it was possible to create kinematic transient subject specific FE models based on the MR images in different postures. 2D models provide a method for comparing the postures but 3D models may be more realistic.
31
Saxey, David W. "A high-resolution superconducting pressure gauge and irreversible magnetic effects in Nb and NbTi wires". University of Western Australia. School of Physics, 2005. http://theses.library.uwa.edu.au/adt-WU2005.0007.
Abstract (sommario):
A high resolution pressure gauge has been developed for use in thermodynamic measurements along the lambda line in liquid helium. The gauge was designed to operate at cryogenic temperatures and provide pressure measurements up to 30bar, with an accuracy of 3 × 10¯¹ºbar in a 1Hz bandwidth. Experiments reported here show the gauge to have met these specifications; at least for measurements close to zero pressure, at temperatures close to 4.5K. It is expected that operation at higher pressures, and at temperatures closer to the lambda transition, will result in similar or even improved performance. The gauge consists of a titanium-alloy diaphragm with a superconducting position transducer read-out. Compensation techniques internal to the superconducting circuit were used to eliminate any significant sensitivity to temperature fluctuations and in-line acceleration. For high values of common-mode rejection, thermal compensation revealed a non-linear temperature characteristic which was exploited to provide a further reduction in the temperature sensitivity. Acceleration compensation was achieved up to a common-mode rejection of more than 78dB. Present performance appears to be limited by thermal gradient fluctuations at low frequencies and at higher frequencies by a noise source which appears to originate beyond the superconducting transducer. It is expected that some further improvement may be gained in this higher frequency band simply by trapping a larger persistent current in the superconducting circuit. In the course of development and characterization of the gauge several anomalous effects were discovered and investigated. In response to changes in temperature, the gauge was found to exhibit irreversible behaviour in a variety of ways. These phenomena were fully investigated and found to be complex in nature. A critical state model was employed which was successful in explaining many of the observed effects. Other authors have observed apparently related behaviour in samples of niobium and some have developed similar critical state models which give results generally consistent with those reported here. However, these latter works have not investigated the presence of such effects within superconducting wires; neither have they considered the implications for devices based upon superconducting wire circuits. It appears this anomalous behaviour may be relevant to a broad range of instruments employing superconducting wire circuits similar to that used here. If this is the case, the results presented here have significant consequences for the performance of such devices
32
Ohia, Obioma Ogonna Chinyerem. "Two-fluid simulations of magnetic reconnection with a kinetic closure for the electron pressure anisotropy". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/95854.
Abstract (sommario):
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 189-206).
Magnetic reconnection is a rapid rearrangement of magnetic line topology in a plasma that can allow magnetic energy to heat, drive macroscopic flows, or accelerate particles in space and laboratory plasmas. Though reconnection affects global plasma dynamics, it depends intimately on small-scale electron physics. In weakly-collisional plasmas, electron pressure anisotropy resulting from the electric and magnetic trapping of electrons strongly affects the structure surrounding the electron diffusion region and the electron current layer. Previous fluid models and simulations fail to account for this anisotropy. In this thesis, new equations of state that accurately describe the electron pressure anisotropy in cases of sufficiently strong guide magnetic field are implemented in fluid simulations and are compared to previous fluid models and kinetic simulations. Elongated current layers in the reconnection region, driven, in part, by this pressure anisotropy, appear as part of a self-regulating mechanism of electron pressure anisotropy. The structure depends on plasma parameters, with low guide fields yielding longer layers.
by Obioma Ogonna Chinyerem Ohia.
Ph. D.
33
Keong, Nicole Chwee Har. "The characterization of white matter injury patterns in normal pressure hydrocephalus using magnetic resonance imaging". Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608298.
34
Papandrew, Alexander B. Fultz B. "The effects of high pressure on the vibrational and magnetic properties of iron-based materials /". Diss., Pasadena, Calif. : Caltech, 2006. http://resolver.caltech.edu/CaltechETD:etd-03022006-151019.
35
Hirsch, Sebastian. "Compression-sensitive Magnetic Resonance Elastography". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://dx.doi.org/10.18452/17205.
Abstract (sommario):
Diese Arbeit stellt das Konzept der kompressionssensitiven Magnetresonanzelastographie vor. Kompressionssensitive MRE analysiert die Ausbreitung von Kompressionswellen und liefert dadurch Erkenntnisse über die Kompressionseigenschaften eines Mediums auf Grundlage eines poroelastischen Modells. Anomalien bei der Regulation des Gewebedrucks stehen in Zusammenhang mit verschiedenen Krankheitsbildern, wie Normaldruck-Hydrozephalus und Pfortader-Hypertonie. Statischer Druck spielt als Porendruck eine zentrale Rolle in den poroelastischen Wellengleichungen; die kompressionssensitive MRE könnte daher ein nichtinvasives Diagnoseinstrument darstellen, das die durch konventionelle Scherwellen-Elastographie gewonnenen Informationen um weitere Aspekte ergänzt. Diese Arbeit beschreibt die Entwicklung einer schnellen Singleshot-EPI-Bildgebungssequenz, mit deren Hilfe die durch propagierende Druckwellen hervorgerufene volumetrische Verzerrung quantifiziert werden kann. Die Validierung der kompressionssensitiven MRE erfolgte an verschiedenen Systemen: an porösen Gelphantomen, an der menschlichen Lunge in zwei Atemzuständen, in einer ex-vivo Schafsleber bei unterschiedlichen hydrostatischen Drücken und schließlich am menschlichen Gehirn. Die Ergebnisse belegen, dass die Stärke der induzierten volumetrischen Verzerrung sensitiv gegenüber Druckänderungen ist, wohingegen die Scherverzerrung keine derartige Abhängigkeit aufweist. In einer weiteren Studie wurde intrinsische Pulsation des menschlichen Hirns anstelle einer externen Vibrationsquelle ausgenutzt. Dabei erzeugte die arterielle Pulswelle eine kurze lokale Expansion des Hirnparenchyms; in der sich anschließenden diastolischen Phase erfolgte eine langsame Rückkehr zum Ausgangszustand. Aus den gemessenen volumetrischen Verzerrungen wurden durch Inversion der Druckwellengleichung numerische Werte für den Druckwellenmodul M berechnet; Rauschen wurde als primäre Ursache für die systematische Unterschätzung von M identifiziert.This thesis introduces the concept of compression-sensitive Magnetic Resonance Elastography. Compression-sensitive MRE detects the propagation of pressure waves, providing insight into the compressibility of a material based on a poroelastic tissue model. Poroelastic models incorporate compressibility through interaction of compartments, even as each individual compartment remains incompressible. Hydrostatic tissue pressure abnormalities are associated with a number of diseases, such as normal pressure hydrocephalus or hepatic portal hypertension. Since pore pressure plays a central role in the poroelastic wave equations, compression-sensitive MRE could potentially serve as a diagnostic tool, providing information complimentary to shear-wave MRE data. This thesis describes the development of a fast single-shot EPI MR sequence capable of quantifying volumetric strain induced by external vibrations. Compression-sensitive MRE was validated in porous gel phantoms, in the human lung at two different respiratory states, in an ex vivo sheep liver at varying levels of hydrostatic pressure, and finally in human liver and brain. Results illustrate that compression-sensitive MRE is capable of quantifying volumetric strain in phantoms and in human organs. It was found that volumetric strain was sensitive toward pressure changes associated with different physiological states, whereas shear strain remained constant. In an additional study, pulsation of the human brain, driven by the heart cycle, was used as the actuation source instead of the external vibration generator. Results indicate local expansion of brain parenchyma upon the arrival of the arterial pulse wave, followed by a slow return to the initial state during the diastolic phase. Numerical values for the pressure wave modulus M were calculated from measured volumetric strain through inversion of the pressure wave equation. Measurement noise was identified as the primary effect causing a severe underestimation of M.
36
Korneta, Oleksandr B. "A systematic study of transport, magnetic and thermal properties of layered iridates". UKnowledge, 2012. http://uknowledge.uky.edu/physastron_etds/2.
Abstract (sommario):
A unique feature of the 5d-iridates is that the spin-orbit interaction (SOI) and Coulomb interactions U are of comparable strength and therefore compete vigorously. The relative strength of these interactions stabilizes new exotic ground states that provide a fertile ground for studying new physics. SOI is proportional to Z^4 (Z is the atomic number), and it is now recognized that strong SOI can drive novel narrow-gap insulating states in heavy transition metal oxides such as iridates. Indeed, strong SOI necessarily introduces strong lattice degrees of freedom that become critical to new physics in the iridates. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of U and SOI interactions via slight chemical doping and application of hydrostatic pressure in the layered iridates, particularly, BaIrO3 and Sr2IrO4.
37
Huang, Lunmei. "Computational Material Design : Diluted Magnetic Semiconductors for Spintronics". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-7800.
38
Volk, Michael [Verfasser], e Stuart [Akademischer Betreuer] Gilder. "Influence of pressure, temperature and composition on magnetic recording in meteorites / Michael Volk. Betreuer: Stuart Gilder". München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2016. http://d-nb.info/1113534699/34.
39
Wei, Qingguo. "Magnetic properties of iron-nickel metals and alloys under high pressure with relevance to planetary cores". Diss., Ludwig-Maximilians-Universität München, 2015. http://nbn-resolving.de/urn:nbn:de:bvb:19-184441.
Abstract (sommario):
This dissertation explores the effects of pressure on the magnetic remanence of iron-nickel and iron-silicon alloys relevant to the solid inner cores of the terrestrial planets and Earth’s moon. The Earth’s inner core likely comprises mostly pure iron in a hexagonal close packed (hcp) structure. Experiments on pure iron powder and foil were carried out up to 21 GPa at room temperature. The most important conclusion from this work is that either hcp-iron is ferromagnetic or that a poorly understood, intermediate hcp phase of iron is ferromagnetic. It was also determined that the results must be corrected for magnetic shape anisotropy, which is related either to the original sample material (foil) or how the bulk sample volume changes shape due to increasing oblateness of the chamber during pressurization. Fe-Ni alloys in the face centered cubic (fcc) phase with compositions around Fe64Ni36, called Invar, exhibit near-null thermal expansion, making them useful for technological applications. Models explaining the Invar effect evoke magnetovolume effect that compensate for thermal expansion. Previous work suggested that the Curie temperature of Fe64Ni36 decreases 35 K per GPa, which predicts that around 5 GPa, Fe64Ni36 will turn paramagnetic. Our experiments on Fe64Ni36 found a marked decrease in magnetization between 5-7 GPa, consistent with former studies, but that it remains ferromagnetic until 16 GPa. The magnetic remanence of low Ni Invar alloys increases faster with pressure than for other body-centered-cubic compositions due to the higher magnetostriction of the low Ni Invar metals. Experimental results on body centered cubic (bcc) Fe-Ni alloys match well with those for pure iron-- again leading to the conclusion that either an intermediate hcp phase, or that the hcp phase itself, is ferromagnetic. The ubiquitous enhancement in magnetization under pressure, or during pressure release, of the Fe-Ni and Fe-Si alloys is associated with strain-induced martensitic effects. Finally, a defocused laser heating technique was developed to measure the Curie temperature in diamond or moissanite anvil cells. Preliminary results on titanomagnetite (Fe2.4Ti0.6O4) are broadly consistent with previous work.
40
Virhammar, Johan. "Idiopathic Normal Pressure Hydrocephalus : Cerebrospinal Fluid Tap Test and Magnetic Resonance Imaging as Preoperative Prognostic Investigations". Doctoral thesis, Uppsala universitet, Neurologi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-222779.
Abstract (sommario):
Idiopathic normal pressure hydrocephalus (iNPH) is a condition with dilated cerebral ventricles but intracranial pressure within normal limits. The symptoms of gait impairment, cognitive decline and urinary incontinence develop gradually. Treatment with shunt insertion results in improvement in eight out of ten patients. The cerebrospinal fluid tap test (CSF TT) and preoperative magnetic resonance imaging (MRI) are methods used to select patients who may benefit from shunt surgery, but they are performed and interpreted differently in different centers throughout the world. The aim of this thesis was to evaluate the performance of the CSF TT and the underlying mechanisms of improvement in gait function after CSF removal, and to investigate the prognostic value of preoperative MRI scans. Improvement in gait and changes in cerebral blood flow (CBF) after a CSF TT were investigated in two prospective studies that included 39 and 20 patients, respectively. Gait assessment and perfusion MRI were done before and several times during the first 24 hours after a CSF TT. Perfusion was investigated with pseudo-continuous arterial spin labeling. At the group level, gait function was significantly improved at all investigation times, but only one-third of individual CSF TT responders were improved at all investigation times. In patients with increased CBF in lateral and frontal white matter after the CSF TT, gait function improved more than it did in patients with decreased CBF in these regions. However, in the whole sample, there was no significant increase in CBF after CSF removal. Preoperative MRI scans were retrospectively evaluated in 109 patients with iNPH who had undergone shunt surgery. The callosal angle was smaller in shunt responders compared with non-responders. The following findings showed the highest association with a positive outcome after shunting: a small callosal angle, wide temporal horns, and occurrence of disproportionally enlarged subarachnoid space hydrocephalus. In conclusion, CBF in white matter close to the lateral ventricles may play a role in the reversibility of symptoms after CSF removal in patients with iNPH. The CSF TT should be reevaluated if the patient does not initially improve, and preoperative MRI investigations can add prognostic information regarding the selection of shunt candidates.
41
Li, Yujie, German Farrherr e Rainer Kimmich. "Hydrodynamic dispersion of pressure-induced and electroosmotic flow in porous glasses probed by Nuclear Magnetic Resonance". Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-193043.
Abstract (sommario):
Fluid transport by flow in random porous media is subject to hydrodynamic dispersion. In a series of pulsed field-gradient NMR experiments, we have compared flow induced by pressure gradients on the one hand and by electroosmosis on the other.
The media were porous glasses with pore dimensions from 1 to 100 μm. With increasing flow rates, a crossover from subdiffusive to superdiffusive mean-squared displacement laws was observed in both cases. This demonstrates the competition between molecular diffusion and convection, and is a typical example of anomalous transport.
42
Li, Yujie, German Farrherr e Rainer Kimmich. "Hydrodynamic dispersion of pressure-induced and electroosmotic flow in porous glasses probed by Nuclear Magnetic Resonance". Diffusion fundamentals 7 (2007) 8, S. 1-12, 2007. https://ul.qucosa.de/id/qucosa%3A14165.
Abstract (sommario):
Fluid transport by flow in random porous media is subject to hydrodynamic dispersion. In a series of pulsed field-gradient NMR experiments, we have compared flow induced by pressure gradients on the one hand and by electroosmosis on the other.
The media were porous glasses with pore dimensions from 1 to 100 μm. With increasing flow rates, a crossover from subdiffusive to superdiffusive mean-squared displacement laws was observed in both cases. This demonstrates the competition between molecular diffusion and convection, and is a typical example of anomalous transport.
43
Petre, Marc Thomas. "INVESTIGATING THE INTERNAL STRESS/STRAIN STATE OF THE FOOT USING MAGNETIC RESONANCE IMAGING AND FINITE ELEMENT ANALYSIS". Case Western Reserve University School of Graduate Studies / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=case1181240611.
44
Bayramoglu, Sadik. "Characterization Of Ultra-fine Grained Steel Samples Produced By High Pressure Torsion Via Magnetic Barkhausen Noise Analysis". Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/2/12610637/index.pdf.
Abstract (sommario):
High Pressure Torsion (HPT) is one of the most widely used severe plastic deformation methods which enable to obtain a crack free ultra-fine grained bulk material with improved mechanical properties like increased strength and toughness. In the process, a disc shaped sample is pressed between two anvils and deformed via surface friction forces by rotating one of the anvils. The aim of this study is to nondestructively characterize the variations in the deformation uniformity of the severely deformed steel disks. Two sets of low carbon steel samples were obtained by applying the unconstrained and constrained HPT process up to 6 turns. Magnetic Barkhausen Noise (MBN) method was used in order to evaluate the samples in a nondestructive manner via a commercial device. The results of the MBN measurements were verified with those of conventional methods such asx-ray diffraction (XRD), metallographic examination and hardness measurements. The initial stages of HPT revealed the effects of conventional plastic deformation on MBN
however with further straining, grain size refinement prevailed and caused increase in MBN signals.
45
Smith, Jason Michael. "Resonant tunnelling in GaAs / AlAs double barrier heterostructures under elevated hydrostatic pressure and in high magnetic fields". Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337539.
46
Manago, Masahiro. "Enhancement of Spin-Triplet Superconductivity by Pressure-Induced Critical Ferromagnetic Fluctuations in UCoGe". Kyoto University, 2019. http://hdl.handle.net/2433/242592.
47
Wang, Weiwei. "Instrumentation development for magneto-transport and neutron scattering measurements at high pressure and low temperature". Thesis, University of Edinburgh, 2013. http://hdl.handle.net/1842/7676.
Abstract (sommario):
High pressure, high magnetic field and low temperature techniques are required to investigate magnetic transitions and quantum critical behaviour in different ferromagnetic materials to elucidate how novel forms of superconductivity and other new states are brought about. In this project, several instruments for magneto-transport and neutron scattering measurements have been designed and built. They include inserts for a dilution refrigerator and pressure cells for resistivity, magnetic susceptibility and inelastic neutron scattering measurements. The technical drawings of the low temperature inserts and pressure cells were produced with Solid Edge computer-aided software and the performance and safety assessments were evaluated with the ANSYS finite element analysis package. The pressure cells developed include diamond anvil cells, piston cylinder cells and some auxiliary equipment. Pressure effects on the physical properties such as the electrical resistivity and magnetic ordering of some ferromagnetic materials were studied with the equipment developed. A two-axis rotating stage was developed and deployed with a dilution refrigerator combined within a superconducting magnet to measure various physical properties as a function of the orientation of the sample with respect to applied field at sub-Kelvin temperature. The rotating stage is made of Beryllium Copper (BeCu) alloy. In order to avoid the entanglement of the wires, custom-designed “flexi cables” - copper tracks printed on a Kapton foil with a yield of nearly 100% - to work with the rotating stage were manufactured. The performance of the rotating stage has been demonstrated by a quantum oscillation in the electrical resistivity study of a high field ferromagnetic superconductor URhGe. A miniature diamond anvil cell based on the turnbuckle principle has been designed. The cell, made of BeCu alloy, is 7mm in length and 7mm in diameter. It has been shown to reach a maximum pressure of 10 GPa with diamond anvils with 800 μm culets. The small dimensions of the cell allow it to fit into the existing sample environment such as Physical Properties Measurement System (PPMS) and Magnetic Properties Measurement System (MPMS) from Quantum Design, USA, and onto the customized two-axis rotating stage built for the dilution fridge. It also thermalizes rapidly allowing rapid cooling and heating during the experiments. The cell can be used to make both resistivity and magnetic susceptibility measurements. To ensure the hydrostaticity of the pressure around the sample in the turnbuckle cell, a gearbox was designed for cryogenic loading of liquid argon and room temperature gas loading of either helium or argon at a loading pressure of up to 0.3 GPa. Pressure effects on the Curie temperature of a PrNi ferromagnet were studied in a diamond anvil cell. Four-probe resistance measurements under pressures up to 9 GPa were carried out in a PPMS. The possibility of tuning the physical properties of the material by altering the pressures has been demonstrated. By analysing the results of the electrical resistivity measurements under pressures, it was concluded that the Curie temperature of PrNi increases with pressure at the rate of 0.85 K per GPa. The quantity ∆(δρ/δτ)which reflects some part of the entropy change also increases with pressure. The expected quantum critical point has not been observed in this material up to 9 GPa. A large volume high-pressure piston-cell for inelastic neutron scattering measurements has been designed and can reach a pressure of up to 1.8 GPa with a sample volume in excess of 400 mm3. The dimension of the part of the cell exposed to the neutron beam has been optimized to minimize the attenuation of the neutron beam. The novel design of the piston seal also eliminates the use of a sample container, which makes it possible to accommodate larger samples and reduces the absorption. The pressure in the cell is measured by a manganin pressure gauge placed next to the sample. The performance of the cell was illustrated by an inelastic neutron scattering study of UGe2.
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Qvarlander, Sara. "Analysis of ICP pulsatility and CSF dynamics : the pulsatility curve and effects of postural changes, with implications for idiopathic normal pressure hydrocephalus". Doctoral thesis, Umeå universitet, Radiofysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-82784.
Abstract (sommario):
The volume defined by the rigid cranium is shared by the brain, blood and cerebrospinal fluid (CSF). With every heartbeat the arterial blood volume briefly increases and venous blood and CSF are forced out of the cranium, leading to pulsatility in CSF flow and intracranial pressure (ICP). Altered CSF pulsatility has been linked to idiopathic normal pressure hydrocephalus (INPH), which involves enlarged cerebral ventricles and symptoms of gait/balance disturbance, cognitive decline and urinary incontinence that may be improved by implantation of a shunt. The overall aim of this thesis was to investigate the fluid dynamics of the CSF system, with a focus on pulsatility, and how they relate to INPH pathophysiology and treatment. Mathematical modelling was applied to data from infusion tests, where the ICP response to CSF volume manipulation is measured, to analyse the relationship between mean ICP and ICP pulse amplitude (AMP) before and after shunt surgery in INPH (paper I-II). The observed relationship, designated the pulsatility curve, was found to be constant at low ICP and linear at high ICP, corresponding to a shift from constant to ICP dependent compliance (paper I). Shunt surgery did not affect the pulsatility curve, but shifted baseline ICP and AMP along the curve towards lower values. Patients who improved in gait after surgery had significantly larger AMP reduction than those who did not, while ICP reduction was similar, suggesting that improving patients had baseline ICP in the linear zone of the curve before surgery. Use of this phenomenon for outcome prediction was promising (paper II). The fluid dynamics of an empirically derived pulsatility-based predictive infusion test for INPH was also investigated, with results showing strong influence from compliance (paper III). Clinical ICP data at different body postures was used to evaluate three models describing postural effects on ICP. ICP decreased in upright positions, whereas AMP increased. The model describing the postural effects based on hydrostatic changes in the venous system, including effects of collapse of the jugular veins in the upright position, accurately predicted the measured ICP (paper IV). Cerebral blood flow and CSF flow in the aqueduct and at the cervical level was measured with phase contrast magnetic resonance imaging, and compared between healthy elderly and INPH (paper V). Cerebral blood flow and CSF flow at the cervical level were similar in INPH patients and healthy elderly, whereas aqueductal CSF flow differed significantly. The pulsatility in the aqueduct flow was increased, and there was more variation in the net flow in INPH, but the mean net flow was normal, i.e. directed from the ventricles to the subarachnoid space (paper V). In conclusion, this thesis introduced the concept of pulsatility curve analysis, and provided evidence that pulsatility and compliance are important aspects for successful shunt treatment and outcome prediction in INPH. It was further confirmed that enhanced pulsatility of aqueduct CSF flow was the most distinct effect of INPH pathophysiology on cerebral blood flow and CSF flow. A new model describing postural and hydrostatic effects on ICP was presented, and the feasibility and potential importance of measuring ICP in the upright position in INPH was demonstrated.Forskningsfinansiär:
European Union, ERDF: Objective 2, Northern Sweden (grant no. 158715-CMTF).
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Verseils, Marine. "Interplay between magnetic ordering and ferroelectricity in multiferroics with quadruple perovskite structure". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066457/document.
Abstract (sommario):
Cette Thèse traite de la ferroélectricité magnétiquement induite dans deux pérovskites quadruples de manganèse: (LaMn3)Mn4O12 et (YMn3)Mn4O12. Tous deux possèdent une monovalence et une structure antiferromagnétique commensurable des sites B. Ces caractéristiques simples, font de ces composés métastables et stabilisés sous haute pression, des systèmes modèles pour identifier la contribution des interactions d’échange symétrique et antisymétrique à la polarisation. YMO est une nouvelle phase dans laquelle le faible rayon de l’ion Y3+ augmente la pression chimique et donc l’interaction d’échange. L’orientation magnétique des sites B à lieu 30 degrés plus haut que dans LMO, à TN,B=108 K. En revanche, nous mesurons une polarisation, P = 0.54 μC cm-2, identique dans les deux composés. Il s’agit d’une valeur record dans les ferroélectriques magnétiques. De façon inattendue, la nature de la ferroélectricité magnétique est très différente dans chaque composé. Dans LMO, la ferroélectricité apparaît à la transition magnétique des sites B, à TN,B = 78 K, mais nous ne reportons pas de brisure du centre d’inversion par diffraction de rayons X ou par spectroscopie Raman et IR. Nous tentons d’expliquer ce résultat inattendu dans le cadre de la théorie phénoménologique des ferroélectriques impropres. D’un autre côté, dans YMO, nous observons l’apparition de la ferroélectricité à T* = 70 K bien que T* ne corresponde pas à une transition magnétique. En effet, à T* nous reportons seulement une anomalie magnétique suggérant un ordre magnétique latent. La transition structurale, qui a lieu à Ts, pourrait être responsable de l’alignement des domaines ferroélectriquesIn the present Thesis, we study the large ferroelectricity induced by magnetism in two quadruple perovskite compounds: (LaMn3)Mn4O12 and (YMn3)Mn4O12, which both display single-valent properties and a commensurate C-type antiferromagnetic structure of the B-sites. These simple features offer a playground to elucidate the contribution of the symmetric and antisymmetric exchange interactions to the polarization. Both compounds are metastable and stabilized under high-pressure. YMO is a new phase, where the small Y3+ ion exerts a large chemical pressure, which is expected to enhance the exchange interaction and, thus, the spontaneous polarization. We find an ordering temperature of the B-sites, 30 K higher than in LMO. On the other hand, we surprisingly find identical values of the spontaneous polarization, P = 0.54 μC cm-2, in both compounds. It is a record value for magnetic ferroelectrics. In spite of the similarities, the nature of magnetic ferroelectricity appears to be very different in the two compounds. In LMO, ferroelectricity is induced by the magnetic ordering of B-sites, although no indication of inversion symmetry breaking is detected. We argue that this puzzling observation is consistent with prediction of domain structure in improper ferroelectrics. On the other hand, in YMO, the occurrence of ferroelectricity at T*=70 K is consistent with a polar structural modulation below Ts=200 K, however T* does not correspond to any long-range magnetic transition. Indeed, T* marks a magnetic anomaly suggesting a latent magnetic phase. We put forward the hypothesis that the above polar distortion of the crystal structure may force the alignment of polar domains
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Wei, Qingguo [Verfasser], e Stuart [Akademischer Betreuer] Gilder. "Magnetic properties of iron-nickel alloys under high pressure with relevance to planetary cores / Qingguo Wei. Betreuer: Stuart Gilder". München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2015. http://d-nb.info/1074358791/34.