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The magnetic and structural properties of pure and doped Rb(sub)2CrCl(sub)4Fyne, P. J. January 1984 (has links)
No description available.
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Fabrication and Characterization of Nano-Sized Magnetic Structures and Their Flux-Pinning Effects on Superconducting Thin FilmsLee, Han Gil 2010 December 1900 (has links)
This dissertation describes experimental studies of how a spatially alternating magnetic field can effectively pin the magnetic flux in a superconducting thin film
(Pb 82 Bi18), thereby enhancing the superconductivity. The spatially alternating magnetic field was provided by a periodic array of nano-sized magnetic structures: 300 nm spacing triangular array of cobalt rods with 100 nm diameter and 300 nm height. The superconducting film deposited on top of the magnetic structures, or an embedded Ferromagnet- Superconductor Hybrids (FSH), showed enhanced critical current and critical magnetic field. The embedded FSH also showed the field matching effect, the field compensation effect, and hysteresis.
This dissertation also explains how to fabricate and characterize magnetic nano- structures. Electron beam lithography and electroplating method were used to fabricate the magnetic nanostructures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the structures of the magnetic rods. Magnetic force microscopy (MFM) was used to study their magnetic properties.
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Green-function theory of anisotropic Heisenberg magnets with arbitrary spinJuhász Junger, Irén 20 July 2011 (has links) (PDF)
In this thesis, anisotropic Heisenberg magnets with arbitrary spin are investigated within the second-order Green-function theory. Three models are considered.
First, the second-order Green-fuction theory for one-dimensional and two-dimensional Heisenberg ferromagnets with arbitrary spin S in a
magnetic field is developed. For the determination of the introduced vertex parameters sum rules, higher-derivative sum rules, and regularity conditions are derived, and the equality of the isothermal and the longitudinal uniform static Kubo susceptibilities is required. Thermodynamic quantities, such as the specific heat, magnetic susceptibility, transverse and longitudinal correlation lengths are calculated. Empirical formulas describing the dependence of the position and height of the susceptibility maximum on the magnetic field are given. An anomal behavior of the longitudinal correlation length is observed. The appearance of two maxima in the temperature dependence of the specific heat is discussed.
Further, as an example of a system with an anisotropy in the spin space, the S=1 ferromagnetic chain with easy-axis single-ion anisotropy is studied. Justified by the up-down symmetry of the model with respect to $S_i^z -> -S_i^z$, $\\langle S_i^z \\rangle=0$ is set. Two different ways of the determination of the introduced vertex parameters are presented. The transverse nearest-neighbor correlation function, spin-wave spectrum and longitudinal correlation length are analyzed. The effects of the single-ion anisotropy on the transverse and longitudinal uniform static
susceptibilities as well as on the appearance of two maxima in the temperature dependence of the specific heat are examined.
Finally, as examples of spatial anisotropic spin systems,layered Heisenberg
ferromagnets and antiferromagnets with arbitrary spin are studied within the rotation-invariant Green-function theory. The long-range order is described by the condensation term, which is determined from the requirement that in the ordered state the static susceptibility has to diverge at the ordering wave vector. For determination of the introduced vertex parameters, the sum rule and the isotropy condition are used and also assumptions regarding the temperature dependence of some parameters are made. The main focus is put on the calculation of the specific heat, the Curie temperature, and the Néel temperature in dependence on the interlayer coupling and the spin-quantum number. Empirical formulas describing the dependence of the transition temperatures on the ratio of interlayer and intralayer couplings are given.
For all three models, the results of the Green-function theory are compared to available results of exact approaches (Quantum Monte Carlo, exact diagonalization, Bethe-ansatz method) and to available experimental data.
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Low temperature calorimetry of spin fluctuations in ZrZnâ†2 and Niâ†3GaGriffiths, Andrew January 1998 (has links)
No description available.
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Green-function theory of anisotropic Heisenberg magnets with arbitrary spinJuhász Junger, Irén 25 May 2011 (has links)
In this thesis, anisotropic Heisenberg magnets with arbitrary spin are investigated within the second-order Green-function theory. Three models are considered.
First, the second-order Green-fuction theory for one-dimensional and two-dimensional Heisenberg ferromagnets with arbitrary spin S in a
magnetic field is developed. For the determination of the introduced vertex parameters sum rules, higher-derivative sum rules, and regularity conditions are derived, and the equality of the isothermal and the longitudinal uniform static Kubo susceptibilities is required. Thermodynamic quantities, such as the specific heat, magnetic susceptibility, transverse and longitudinal correlation lengths are calculated. Empirical formulas describing the dependence of the position and height of the susceptibility maximum on the magnetic field are given. An anomal behavior of the longitudinal correlation length is observed. The appearance of two maxima in the temperature dependence of the specific heat is discussed.
Further, as an example of a system with an anisotropy in the spin space, the S=1 ferromagnetic chain with easy-axis single-ion anisotropy is studied. Justified by the up-down symmetry of the model with respect to $S_i^z -> -S_i^z$, $\\langle S_i^z \\rangle=0$ is set. Two different ways of the determination of the introduced vertex parameters are presented. The transverse nearest-neighbor correlation function, spin-wave spectrum and longitudinal correlation length are analyzed. The effects of the single-ion anisotropy on the transverse and longitudinal uniform static
susceptibilities as well as on the appearance of two maxima in the temperature dependence of the specific heat are examined.
Finally, as examples of spatial anisotropic spin systems,layered Heisenberg
ferromagnets and antiferromagnets with arbitrary spin are studied within the rotation-invariant Green-function theory. The long-range order is described by the condensation term, which is determined from the requirement that in the ordered state the static susceptibility has to diverge at the ordering wave vector. For determination of the introduced vertex parameters, the sum rule and the isotropy condition are used and also assumptions regarding the temperature dependence of some parameters are made. The main focus is put on the calculation of the specific heat, the Curie temperature, and the Néel temperature in dependence on the interlayer coupling and the spin-quantum number. Empirical formulas describing the dependence of the transition temperatures on the ratio of interlayer and intralayer couplings are given.
For all three models, the results of the Green-function theory are compared to available results of exact approaches (Quantum Monte Carlo, exact diagonalization, Bethe-ansatz method) and to available experimental data.
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Co₂MnSi Heusler alloy thin filmsSingh, Laura Jane January 2005 (has links)
This thesis investigates the growth of intermetallic compounds by co-sputtering from single elemental targets. The preliminary work involved constructing a sputtering set-up to grow abinary alloy (Sm-Co) and investigating how to control the composition spread that was obtained. Having achieved this, a larger sputtering flange was built up to grow the ternary Heusler alloy, Co₂MnSi. Co₂MnSi has been predicted to be a half-metallic ferromagnet, which means that there is an energy gap in the minority spin band at the Fermi energy. This leads to 100% spin polarised conduction electrons, which would enable ideal spin-device performance to be obtained. Co₂MnSi is particularly promising because it is predicted to have a large energy gap in the minority band of ~0.4 eV and has the highest Curie temperature among the known Heuslers of 985 K. Initially, Co₂MnSi was grown on a-plane sapphire and stoichiometric films were single phase and highly (110) textured, without the use of a seed layer. They exhibited the bulk value of the saturation magnetisation, Ms and films grown at the highest deposition temperature (715 K) showed the lowest resistivity (47 μΩcm at 4.2 K) and the lowest room temperature coercivity (18 Oe). The spin polarisation of the transport current, Pt of a 400 nm film grown at this deposition temperature was 54%, consistent with measurements on bulk single crystals. Ms decreased with decreasing film thickness indicating a graded disorder. By growing on GaAs (001), which has a similar lattice parameter to Co₂MnSi it was expected that this disordered region would be confined to the first few atomic layers. However, this was not the case because interfacial reactions resulted in the formation of an epitaxial Mn-As region, and a thin interfacial layer that was Co-Ga rich. This prevented the lattice matching of the Co₂MnSi to the GaAs(001) hence hindering epitaxial growth of the Heusler. The reaction zone also meant that films exhibited a Ms slightly below the bulk value. The expected fourfold anisotropy was not obtained for this cubic material, which is most likely due to the anisotropy of the reconstructed GaAs surface. Inspite of this anomalous behaviour, Pt was 55%, similar to the result obtained on sapphire, indicating that either Pt is independent of orientation or that the Heusler surface reconstructs in the same way. Films showed some improved properties to films grown on a-plane sapphire, indicating the potential of growing on this technologically important substrate. With this in mind, pseudo spin valves involving Co₂MnSi as one of the ferromagnetic electrodes were fabricated in both the CIP and CPP configurations. Clear low-field spin-valve contributions were observed at 15 K but the MR values are much lower than that expected from a PSV with a predicted 100% spin polarised electrode.
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Unconventional Superconductivity Mediated by the Higgs Amplitude Mode in Itinerant Ferromagnets:Forestano, Roy Thomas January 2021 (has links)
Thesis advisor: Kevin Bedell / Over 20 years ago, Blagoev et. al. predicted an s-wave pairing instability in a ferromagnetic Fermi liquid (FFL) as a consequence of spin fluctuations [5]. Shortly after, it was discovered that, when magnetic interactions in the ferromagnetic superconductor UGe2 dominate, quasiparticles with parallel spin form pairs in odd-parity orbitals; i.e., a form of spin-triplet p-wave superconductivity emerges, in contrast to Blagoev et. al.'s prediction [6]. In this work, we return to this issue by introducing the effects of a gapped amplitude (or "Higgs") mode on the vertex corrections and subsequent form of Cooper pairing. As the Higgs mode only propagates in the presence of a finite spin current, such an amplitude mode results in strong momentum-dependence in the many-body vertex. This results in the emergence of an unconventional form of superconductivity mediated by unconventional low-energy modes in a weak itinerant ferromagnet. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Physics. / Discipline: Mathematics.
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Study of the Dipolar Ising System LiHoxY1-xF4 Using Muon Spin Relaxation/RotationRodriguez, Jose 08 1900 (has links)
<p> LiHoxY1-xF4 is an insulating system where the Ho ions are magnetic. The crystal field gives an Ising character to the Ho ions, and the dominant interaction between them is through magnetic dipolar fields. For x=1, the system is a ferromagnet with a critical temperature of 1.54 K. The critical temperature decreases as x is decreased until x~0.25. At that point the long range order is destroyed, giving rise to a state with some characteristics of spin glass behavior.</p> <p> The study of this system is important because its Ising nature makes it a test ground for theoretical models. Interestingly, not only experiments and theory diverge in many points, but experimental data from different research groups are contradictory. In order to contribute to a clearer experimental picture, we performed μSR measurements in this system and we present our results in this thesis.</p> <p> In the ferromagnetic samples (x=1 and 0.45) we found that the internal field distribution at the muon site is quite broad, even in the x=1 system. Then it is possible that the magnetic field distribution at the Ho ions is broad as well. We show qualitatively how this broadening could explain a feature of the field-temperature phase diagram which is still not understood.</p> <p> For the more diluted samples (x=0.25, 0.12, 0.08, 0.045 and 0.018), we
found that they all have the same qualitative behavior as a function of temperature and magnetic field. Our analysis of the data did not show any feature of canonical spin glass behavior. Instead, an analysis using dynamical Kubo-Toyabe functions show a temperature independent fluctuation rate of the Ho moments below ~0.5 K for all these samples. This behavior is characteristic of cooperative paramagnetic systems. We also found that the low temperature fluctuation rate decreases as x is decreased.</p> <p> To analyze the data from LiHoxY1-xF4 we developed microscopic models of μSR signals. These models are presented in this thesis and they correspond to stochastic models of signals using stretched magnetic field distributions; and the modeling of signals from systems with F-μ-F bond formation where the set of bonds does not have cubic symmetry.</p> / Thesis / Doctor of Philosophy (PhD)
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Chemical, Magnetic, and Orbital Order of Polycrystalline and Thin film Double PerovskitesRicciardo, Rebecca Ann 24 September 2009 (has links)
No description available.
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3d- und 4f-Korrelationen in quaternären Eisenpniktiden: der Sonderfall CeFeAs1-xPxOJesche, Anton 22 August 2011 (has links) (PDF)
Die Legierungsserie CeFeAs1−xPxO bietet die Möglichkeit, eine außergewöhnliche Vielfalt unterschiedlicher Grundzustände mit starken Korrelationen der 3d- und der 4f-Elektronen zu untersuchen. CeFePO ist an der Grenze zwischen einem paramagnetischen und einem ferromagnetischen Ce-Zustand und zeigt starke 4f-Korrelationen, die zu Schwere-Fermionen-Verhalten führen, während Fe unmagnetisch ist. Im Gegensatz dazu sind die Eigenschaften von CeFeAsO durch die 3d-Korrelationen des Fe dominiert, die zu antiferromagnetischer Ordnung unterhalb von T_N(Fe) = 145K führen, während sich Ce in einem stabilen dreiwertigen Zustand befindet und unterhalb von T_N(Ce) = 3.7K ebenfalls antiferromagnetisch ordnet. Man erwartet deshalb mindestens zwei kritische Punkte, an denen die magnetische Ordnung unterdrückt wird. Hier sollte insbesondere geklärt werden, ob bei diesen kritischen Konzentrationen Quantenphasenübergänge auftreten, bei denen die Ordnungstemperatur zu T = 0K verschoben ist und in denen die Ursache von Nicht-Fermi-Flüssigkeitsverhalten und unkonventioneller Supraleitung gesehen wird.
Grundlage für die Untersuchungen war zunächst die Züchtung qualitativ hochwertiger Einkristalle hinreichender Größe, was im Rahmen dieser Arbeit erstmalig gelungen ist. Hierzu wurde eine Sn-Flux Methode optimiert, mit der plättchenförmige Einkristalle mit Abmessungen von typischerweise 1mm x 1mm x 0.1mm und Massen bis 0.6mg erhalten werden konnten. Zur Bestimmung struktureller Parameter kamen Röntgenbeugung, energiedispersive Röntgenspektroskopie und chemische Analyse zum Einsatz. Physikalische Eigenschaften wurden vor allem durch Messungen der Spezifischen Wärmekapazität, der Magnetisierung und des elektrischen Widerstandes im Temperaturbereich T = 0.35 − 300K untersucht.
Die antiferromagnetische Ordnung von Fe in CeFeAsO ist mit einer orthorhombischen Verzerrung verbunden, die bei einer etwas höheren Temperatur von T_0 = 151K stattfindet. Diese Phasenübergänge sind von besonderem Interesse, da ihre Unterdrückung zur Ausbildung von Hochtemperatur-Supraleitung in den Eisenpniktiden führt, ihr Wechselspiel aber nicht vollständig verstanden ist. Sie unterteilen die Temperaturabhängigkeit des elektrischen Widerstandes ρ(T) von CeFeAsO in zwei Bereiche. In der paramagnetischen tetragonalen Phase nimmt ρ(T) beim Abkühlen von Raumtemperatur aus bislang ungeklärter Ursache zunächst leicht zu. Erst mit Einsetzen der orthorhombischen Verzerrung bei T_0 kehrt sich die Temperaturabhängigkeit um und ρ(T) nimmt mit sinkender Temperatur ab, wobei die Abnahme bei T_N(Fe) nochmals stärker wird und bis zu tiefsten Temperaturen metallisches Verhalten beobachtet wird. Dass sich CeFeAsO somit nicht unmittelbar an der Grenze zu einem Mott-Isolator befindet, wie es in Anlehnung an die Kuprat-Supraleiter zunächst vermutet wurde, und Restwiderstandsverhältnisse von RRR > 10 überhaupt möglich sind, konnte im Rahmen dieser Arbeit erstmalig gezeigt werden. Durch sorgfältige Untersuchung des Temperaturunter-
schiedes zwischen T_N(Fe) und T_0 und dem Vergleich mit dotierten und undotierten AFe2As2-Verbindungen konnte ein vereinheitlichtes Bild der Ausgangsverbindungen aller Fe-basierten Supraleiter geschaffen werden. In diesem tritt im Temperaturbereich T_N(Fe) < T < T_0 eine elektronische nematische Phase hervor, deren Existenzbereich durch die magnetische Kopplung entlang der kristallographischen c-Achse und deren Defektabhängigkeit bestimmt ist.
Wie alle Substitutionen in RFeAsO-Verbindungen führt die Ersetzung von As durch P auch in CeFeAs1−xPxO zu einer Verringerung von T_N(Fe). Ein quantenkritischer Punkt mit T_N(Fe) --> 0K ist jedoch unwahrscheinlich, da ab einer kritischen Konzentration von x = 0.30 die Signatur der Eisen-Ordnung in ρ(T) zwar merklich schwächer wird, T_N(Fe) ≈ 40K bei weiterer Erhöhung von x aber nicht mehr zu tieferen Temperaturen schiebt. In Proben mit der kritischen Konzentration von x = 0.30 - und nur in diesem Konzentrationsbereich - konnte reproduzierbar ein verschwindender elektrischer Widerstand und damit ein Hinweis auf Supraleitung mit einer Sprungtemperatur von T_SL= 4K gefunden werden. Im Gegensatz zur ’Dom-förmigen’ Abhängigkeit der Sprungtemperatur von der Konzentration eines Fremdatoms in den Phasendiagrammen anderer Fe-basierter Supraleiter nimmt jedoch T_SL in CeFeAs1−xPxO bei weiterer Erhöhung von x nicht zu. Stattdessen wird bei x > 0.30 ein ferromagnetisch geordneter Grundzustand (des Ce) stabilisiert, der mit Supraleitung konkurriert.
Die antiferromagnetische Ordnung von Cer in undotiertem CeFeAsO weist typische Merkmale magnetischer Ordnung lokaler Momente auf und impliziert eine Dominanz der RKKY-Wechselwirkung gegenüber einem schwachen Kondo-Effekt.
Die Ersetzung von As durch P wirkt als chemischer Druck und stabilisiert somit den unmagnetischen Valenzzustand Ce4+. Trotzdem ist die Ce-Ordnung bei kleinen P-Konzentrationen - im Gegensatz zur Fe- Ordnung - nahezu unverändert vom Verhalten in undotiertem CeFeAsO. Bei der kritischen Konzentration von x = 0.30 tritt überraschend ein plötzlicher Übergang von antiferromagnetischer zu ferromagnetischer Ordnung mit einer Curie-Temperatur von T_C(Ce) = 4K auf, der offensichtlich mit der Unterdrückung der Fe-Ordnung korreliert ist und nicht nur aus einem reinen Volumeneffekt resultiert. Als mögliche Ursache wird eine Umstrukturierung der Fermi-Fläche bei Unterdrückung der Fe-Ordnung betrachtet, die zu einem Vorzeichenwechsel der Austauschkopplung J_ij bei RKKY-Wechselwirkung führt.
Bei hohen Phosphor-Konzentrationen sinkt T_C(Ce) und geht bei x = 0.90 von ferromagnetischer zur antiferromagnetischer Ordnung über, wie es bei Annäherung an einen quantenkritischen Punkt bereits in einer Vielzahl ferromagnetischer Systeme beobachtet wurde.
In stöchiometrischem CeFePO wurde magnetisch kurzreichweitige Ordnung und Spin-Glas-Verhalten mit einer charakteristischen Temperatur von T_SG= 0.75K gefunden. Der Unterschied zur früher gemachten Beobachtung eines paramagnetischen Grundzustandes resultiert einzig aus Variationen bei der Synthese und ist in strukturellen Parametern nicht nachweisbar - eine für Schwere-
Fermionen-Systeme typische Sensitivität!
Der zweite kritische Punkt, an dem die Ce-Ordnung verschwinden sollte, liegt also nicht wie zu Beginn erwartet bei hohen P-Konzentrationen, sondern in stöchiometrischem CeFePO. Eine Wärmebehandlung von CeFePO bei T ∼ 800◦C kann zur gezielten Manipulation des Grundzustandes genutzt werden und hat zur Ausbildung von logarithmischer Divergenz der spezifischen Wärmekapazität C/T und damit einem ersten direkten Hinweis auf Quantenkritikalität geführt.
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