The low temperature spin dynamics of the canonical dipolar spin ice materials is examined. The ac susceptibility of dipolar spin ice materials Ho2Ti2O7 and Dy2Ti2O7 is measured to lower temperatures and frequencies than previous studies. This provides a probe of the dynamics of fractionalized magnetic excitations which have been found to exist and interact as monopole-like particles within the spin ice con guration. Low temperatures and low frequencies access the dilute monopole phase, and provide a valuable stress case scenario to theory which has been used to describe the system to date. The relaxation is found to be well described at the lowest temperatures by an Arrhenius law with single energy barrier for both Ho2Ti2O7 and Dy2Ti2O7, with similar barriers to relaxation, 10.7 K and 9.79 K respectively. It is also revealed to be distinctly different from predictions of the dipolar spin ice model, based upon simulations of both a Coulomb gas and dipolar spin ice on the pyrochlore lattice. These simulations, as well as calculations based on Debye-Huckel theory, do not see Arrhenius behaviour in our temperature range, and do not predict the rate at which dynamics freeze out. It is not currently understood what would be required in order to amend this. The implications for thermal methods of probing spin dynamics is discussed, as well as how this measurement impacts the magnetolyte theory of spin ice. Brief reports are presented in the appendices of specific heat measurements of three spin liquid candidates: Yb2Ti2O7, Tb2Ti2O7, and Pr2Hf2O7. In Yb2Ti2O7, measurements of three single crystals, two unique features, a broad anomaly at 195 mK and sharp peak at 265 mK, are found which seem to comprise elements of previous single crystal and polycrystalline measurements. These low temperature features do not correspond to changes in neutron scattering intensity at 400 mK. In Tb2Ti2O7, a second order transition is found, corresponding to the emergence of a mode in inelastic neutron scattering. Absence of an ordering transition in the suspected <111> antiferromagnet Pr2Hf2O7 is also shown, with specific heat measured down to 100 mK with no ordering transition.

The low temperature spin dynamics of the canonical dipolar spin ice materials is examined. The ac susceptibility of dipolar spin ice materials Ho2Ti2O7 and Dy2Ti2O7 is measured to lower temperatures and frequencies than previous studies. This provides a probe of the dynamics of fractionalized magnetic excitations which have been found to exist and interact as monopole-like particles within the spin ice con guration. Low temperatures and low frequencies access the dilute monopole phase, and provide a valuable stress case scenario to theory which has been used to describe the system to date. The relaxation is found to be well described at the lowest temperatures by an Arrhenius law with single energy barrier for both Ho2Ti2O7 and Dy2Ti2O7, with similar barriers to relaxation, 10.7 K and 9.79 K respectively. It is also revealed to be distinctly different from predictions of the dipolar spin ice model, based upon simulations of both a Coulomb gas and dipolar spin ice on the pyrochlore lattice. These simulations, as well as calculations based on Debye-Huckel theory, do not see Arrhenius behaviour in our temperature range, and do not predict the rate at which dynamics freeze out. It is not currently understood what would be required in order to amend this. The implications for thermal methods of probing spin dynamics is discussed, as well as how this measurement impacts the magnetolyte theory of spin ice. Brief reports are presented in the appendices of specific heat measurements of three spin liquid candidates: Yb2Ti2O7, Tb2Ti2O7, and Pr2Hf2O7. In Yb2Ti2O7, measurements of three single crystals, two unique features, a broad anomaly at 195 mK and sharp peak at 265 mK, are found which seem to comprise elements of previous single crystal and polycrystalline measurements. These low temperature features do not correspond to changes in neutron scattering intensity at 400 mK. In Tb2Ti2O7, a second order transition is found, corresponding to the emergence of a mode in inelastic neutron scattering. Absence of an ordering transition in the suspected <111> antiferromagnet Pr2Hf2O7 is also shown, with specific heat measured down to 100 mK with no ordering transition.

Thesis advisor: Michael J. Graf / In this work I present a characterization of spin dynamics in LiY1-xHoxF4 over a wide region of frequency - temperature - magnetic field - concentration phase space to probe the onset of magnetic correlations. Specifically, measurements were made of the T = 1.8 K magnetic field and frequency dependence of AC susceptibility, and temperature and field dependence of the longitudinal field positive muon spin relaxation (μSR) for LiY1-xHoxF4 with x = 0.0017, 0.0085, 0.0408, and 0.0855. To determine the concentration range over which the spin dynamics are determined primarily by the Ho3+-μ interaction rather than by the F-μ interaction I characterize the dynamics associated with the formation of the (F-μ-F)−complex by comparing data with Monte Carlo simulations. Numerical simulations of the susceptibility for the x = 0.0017 and 0.0085 are fit to my data and show that Ho-Ho cross-relaxation processes become important at higher concentration, signaling the crossover from single-ion to correlated behavior. The muon spin depolarization is simulated using the parameters extracted from the susceptibility, and the simulations agree well with data for these two samples. It is found that the susceptibility and μSR data for samples with x = 0.0408 and 0.0855 cannot be described within a single-ion picture, possibly due to the onset of collective phenomena. An unusual peak is also discovered in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 – 20 K, and ascribed to a modification of the Ho3+ fluctuation rate due to a field induced shift of the energy splitting between the ground and first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm-1. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

AC susceptibility is an important characterisation technique measuring the time dependent magnetisation and dynamics of a magnetic system. It is capable of yielding information on thermodynamic phase transitions, relaxation processes and losses in a variety of interesting magnetic and superconducting materials. In particular it is a powerful probe of the mixed state of superconductivity providing insight into the ux dynamics at play and determination of a number of physical properties such as the critical temperature Tc, field Hc and characteristic length scales. Application of pressure can tune materials through multiple phases and interesting phenomena. The thesis describes the design of a calibratable susceptometer in a piston cylinder pressure cell, achieving AC susceptibility measurements of the same accuracy as a SQUID magnetometer but under pressure. This is used to make measurements on an electrostatically doped capacitance device, a single chain magnet and a heavy fermion superconductor. These studies are summarised below. Electric double layer (EDL) devices provide a means of continuous tuning through a materials phase diagram by applying an electric field, including inducing superconductivity. Application of pressure in tandem with electrostatic doping could improve the efficiency of these devices and provide a second tuning parameter. An EDL capacitor was constructed and measured with the above susceptometer aiming to shift the Tc of a doped high temperature superconducting cuprate La1:9Sr0:1CuO4. The Tc shifts proved irreproducible already at ambient conditions. Indeed during the course of this research further experimental evidence emerged in the literature indicating EDL devices may very well work due to electrochemical doping rather than electrostatic, possibly accounting for the lack of repeatability. Work therefore focused on mapping the ionic liquid DEME-TFSI's glass-liquid phase diagram over the 1 GPa pressure range, rather than extending the study of the EDLC device to high pressure. Single chain magnets (SCM) are an interesting class of material consisting of a one-dimensional molecular magnet chain manifesting magnetic hysteresis and slow relaxation best characterised by AC susceptibility. The susceptometer was used to study the SCM [Co(NCS)2(pyridine)2]n to investigate the effect of pressure on its characteristic magnetic relaxation time and energy barrier. A secondary signal appears at ~0.44 GPa which is attributed to the development of an additional structural phase that has been independently observed in X-ray crystallographic measurements. The heavy fermion superconductor U6Fe has the highest Tc ~4 K of all the U-based compounds and large critical fields of ~10-12.5 T, depending on direction, which increase on initial application of pressure. It exhibits a coexisting charge density wave (CDW) below 10 K making it a promising candidate for the modulated superconductivity of the theorised Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. A feature at 110 K is also evident in Mossbauer, resistivity and specific heat measurements, the origin of which has not yet been clearly identified. Evidence for the FFLO state was sought by mapping the upper critical field Hc2 along with the peak effect through AC susceptibility measurements up to pressures of 1 GPa. The data is accounted for by an evolution of collective pinning and superconducting parameters, with no clear evidence for an FFLO state although an enhancement of the reduced field is observed.

Effects of Nb2O5 addition on the formation and properties of the Bi-based superconductors were studied. The superconducting samples were synthesized by two different methods, the solid state reaction (SSR) and ammonium nitrate melt route (ANM) to obtain information about the growth techniques. Ten samples (five with each method) with the chemical compositions of Bi1.6Pb0.4Sr2Ca2Cu3NbxOy, with x= 0.00, 0.05, 0.10, 0.20, and 0.30 were prepared. Phase identification, structural and microstructural analysis were carried out using powder X-ray diffraction method, scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The transition temperature and the critical current measurements were carried out using the electromagnetic measurement system. The critical parameters of the samples were also determined by AC susceptibility and magnetization measurements. Improvements in the formation of the Bi-2223 phase with Nb addition have been observed by both methods. The ANM method leads mixtures of Bi-2223 and Bi-2212 phases, but with the SSR method pure Bi-2223 phase was obtained. With small amounts of Nb addition the critical temperature, Tc, remains the same for ANM samples and it increases for the SSR samples. For both series of samples Tc decreases with further increase of Nb content. With Nb addition the critical currents of the ANM sample almost remain the same and those of the SSR samples increases. Observations based on AC susceptibility measurements showed that Nb addition improve inter-grain coupling. Inductive critical current densities obtained from magnetization measurements agree with the critical currents stated above and show that Jc increase with increase Nb content.

Dans notre travail, nous avons utilisé la technique Monte Carlo afin de simuler l'effet des interactions dipolaires sur le retournement de l'aimantation d'une assemblée de nanoparticules ferromagnétiques. Dans notre modèle, nous avons considéré des nanoparticules à anisotropie uniaxiale, plus précisément le Cobalt. Avec la condition KV/kBT >> 1, le moment magnétique de chaque nanoparticule est figé dans ses positions d'équilibre et il ne peut pas fluctuer autour. Nous avons choisi de faire notre étude par le biais de la susceptibilité alternative car elle apporte l'avantage de pouvoir faire l'étude sur une large gamme de fréquences sans avoir à changer de technique de mesure puisque le temps de mesure est égale à l'inverse de la fréquence du champ appliqué. Nous avons étudié douze types d'assemblées chacune diffère de l'autre soit par sa forme, la distribution spatiale des nanoparticules ou la distribution des axes d'anisotropie. Et nous avons discerné l'influence de chaque paramètre sur l'effet des interactions dipolaires sur le retournement de l'aimantation. / In our work, we used Monte Carlo technique to simulate the effect of dipolar interactions on the magnetization reversal of a ferromagnetic nanoparticles assembly. In our model, we considered nanoparticles with uniaxial anisotropy and specifically the Cobalt. With the condition KV/kBT >> 1, the magnetic moment of each nanoparticle is fixed in its equilibrium positions and it can not fluctuate around. We chose to our investigation the ac susceptibility because it provides the advantage of making the study with a wide frequency range without changing measurement technique, because the measurement time is equal to the inverse of the frequency of the alternating magnetic field applied. We studied twelve types of assemblies each one differs from the other by some properties : The shape of the assembly, the spatial distribution of the nanoparticles or the distribution of the anisotropy axes. And we discerned the influence of each of theses parameters on the effect of dipolar interactions on the magnetization reversal.

Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen. / The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured.

Orientador: Mario Antonio Bica de Moraes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T18:58:49Z (GMT). No. of bitstreams: 1 Rouxinol_FranciscoPauloMarques_D.pdf: 6433084 bytes, checksum: 84d703ca8da3620b077c3b1335c59965 (MD5) Previous issue date: 2008 / Resumo: Técnicas de condensação de vapor são úteis na preparação de ligas magnéticas cujos componentes têm pouca, o mesmo nenhuma, solubilidade em condições de equilíbrio Neste trabalho, uma dessas técnicas ¿ sputtering ¿ foi empregada para fabricar ligas metaestáveis de GdXCr1-X, cujas propriedades magnéticas foram investigadas em função da concentração de Gd, x. Difratometria de raios-X de baixo ângulo (GAXRD) e espectroscopia de retroespalhamento Rutherford foram utilizados para determinar a estrutura do filme e sua composição elementar, respectivamente. As análises de GAXRD mostraram que a estrutura da fase de Gd, é hcp quando x ³ 0,88; e amorfa quando 0,16 £ 0,76. Uma estrutura bcc , para a fase de Cr, foi observada nos difratogramas quando x < 0,16, e amorfa quando x ³ 0,16 Para investigar as propriedades magnéticas utilizamos um magnetometro SQUID e m PPMS. O primeiro foi utilizado para as medidas de momento magnético em função do campo estático e temperatura. O PPMS foi tilizado nas investigações de susceptibilidade-AC em função da freqüência de oscilação do campo, temperatura e campo estático. A complexa natureza magnética dos filmes de Gd-Cr foi observada através das isotermas MxH, que não apresentaram saturação em baixas temperaturas, nem comportamento linear em altas temperaturas. Pela análise dos dados magnéticos, observamos que as amostras admitem um comportamento ferromagnético para x ³ 0,5 e paramagnético para as outras concentrações de Gd. A temperatura de Curie (TC) apresenta um aumento monotônico de 170 para 290 K quando x aumenta de 0,5 para 1,0. A temperatura de Curie-Weiss (q C) mostra um aumento monotônico com x. A partir das isotermas MxH a 2 K, o momento de saturação foi calculado, sendo independente de x e aproximadamente constante com um valor médio de 7,3 µB. Medidas de susceptibilidade em campos estáticos e dinâmicos revelaram a existência de comportamentos de vidros magnéticos em todas as amostras abaixo da temperat ra de freezing (Tf). Observamos, nas ligas com altas concentrações de Gd, a presença de comportamentos ferromagnéticos e cluster-glass em baixas temperaturas. Concluímos que a interação de troca entre os átomos de Gd dentro dos clusters de Gd não é do tipo RKKY, e sim do tipo supertroca. O efeito magnetocalórico (MCE) foi investigado através da variação de entropia magnética ( D SM) em função da temperatura, para a remoção de um campo de 50 kOe Curva de D SMxT para as amostras com x < 0,2 apresentaram um formato típico de superferromagneto, consistente com a existência de clusters Gd nos filmes. Nas outras amostras em que o EMC foi analisado, a presença de clusters é observada pelo comportamento dessas curvas a baixa temperatura; a altas temperaturas o comportamento de D SM com T indica fortemente a presença de mais fases magnéticas no filme. O diagrama de fase baseado em TC e Tf, e sua dependência com x é apresentado / Abstract: Vapor condensation techniques are very useful for preparation of alloys whose components have no mutual solubility under equilibrium conditions In this work, one of these techniques ¿ sputtering ¿ has been used to fabricate metastable GdXCr1-X alloys whose magnetic properties were investigated as a function of the Gd concentration, x. Grazing incidence angle X-ray diffraction (GAXRD) and Rutherford backscattering spectroscopy were employed to characterize the film structure and elemental composition, respectively. The GAXRD measurements revealed, for the Gd fraction, a hcp structure for x ³ 0,88; for 0,16 £ x £ 0,76 the Gd fraction was amorphous. The existence of a bcc structure for x < 0,16 was observed in the diffractograms for the Cr phase, which was amorphous for x ³ 0,16. To investigate the magnetic properties, a SQUID magnetometer and a PPMS were used. The former was employed for magnetic moment measurements as a function of applied static field and temperature. The PPMS was used for ac-susceptibility determinations as a function of the frequency of the ac driving field, temperature, and applied static field. The complex magnetic nature of the Gd-Cr films was revealed from the MxH isotherms which did not show saturation even at the lowest temperatures, and did not exhibit a linear behavior at higher temperatures. Processing of the magnetic data has shown that the films exhibit a ferromagnetic behavior for x ³ 0,5 and paramagnetic one for all other Gd concentrations. The Curie temperature (TC) increased monotonically from 170 to 290 K as x increased from 0,5 to 1,0. A monotonical increase in the Curie-Weiss temperature ( q C ) with x was also observed for all films. From the extrapolated MxH isotherm at 2 K (saturation magnetization), the saturation moments were calculated and found to be nearly constant at about 7.3 µB. Both static and dynamic susceptibility measurements revealed the existence of a magnetic glassy behavior in all alloys, occurring below a freezing temperature Tf . For the higher concentration alloys, the ferromagnetic and the cluster glass state were observed at low temperatures. It was thus concluded that the exchange interactions within Gd atoms in the clusters were not of the RKKY but of the superexchange type. The magnetocaloric effect (MCE) was investigated from the magnetic entropy change ( DSM) as a function of temperature, for the removal of a 50 kOe field. Samples with x < 0,2 exhibited DSMxT curves whose shapes are typical of a superferromagnet, consistently with the existence of Gd clusters in the films. For all the other alloys whose MCE was investigated, the presence of clusters is manifested from the behavior of these curves at low temperatures; at higher temperatures, the evolution of DSM with T strongly indicated the presence of more than one magnetic phase in the alloys A magnetic phase diagram based on the Tf and Tc transition temperatures and their dependence on x is presented in this thesis / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen. / The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured.