Relaxation (NMR)

Résumé

In MRI and NMR spectroscopy, an observable nuclear spin polarization (magnetization) is created by a homogeneous magnetic field. This field makes the magnetic dipole moments of the sample precess at the resonance (Larmor) frequency of the nuclei. At thermal equilibrium, nuclear spins precess randomly about the direction of the applied field. They become abruptly phase coherent when they are hit by radiofrequency (RF) pulses at the resonant frequency, created orthogonal to the field. The RF pulses cause the population of spin-states to be perturbed from their thermal equilibrium value. The generated transverse magnetization can then induce a signal in an RF coil that can be detected and amplified by an RF receiver. The return of the longitudinal component of the magnetization to its equilibrium value is termed spin-lattice relaxation while the loss of phase-coherence of the spins is termed spin-spin relaxation, which is manifest as an observed free induction decay (FID). For spin=½

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https://en.wikipedia.org/wiki/Relaxation_(NMR)

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The lattice dynamics of solid He-4 has been explored using pulsed NMR methods to study the motion of He-3 impurities in the temperature range (0.05-0.20 K) where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid He-4 lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the He-3 motion that results from quantum tunneling in the He-4 matrix. The measurements were made for He-3 concentrations 16 < x(3) < 2000 ppm. For He-3 concentrations x(3) = 16 and 24 ppm, large changes are observed for both the spin-lattice relaxation time T-1 and the spin-spin relaxation time T-2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher He-3 concentrations.

American Physical Society2013

Chargement

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

This thesis is dedicated to the study of the magnetization reversal dynamics in compressively strained (Ga,Mn)As and differently functionalized (Ga,Mn)As materials. In the first part the domain wall dynamics of pure (Ga,Mn)As/GaAs materials is in the focus. The changes in the magnetic anisotropy energy landscape occurring as a function of temperature are monitored in detail by different experimental techniques. These measurements provide the necessary information for the identification of the temperature ranges corresponding to different magnetic anisotropy regimes. Knowing the biaxial anisotropy and uniaxial anisotropy dominated regimes the dependence of the domain wall dynamics on the magnetic anisotropy landscape is studied. Critical changes in domain wall alignment observed by Kerr microscopy are found upon changing from biaxial to uniaxial anisotropy. These changes could be partially attributed to the tendency of the system to minimize magnetic free poles at the domain boundaries. To complement the space resolved studies of the magnetization reversal, magnetic time relaxation effects are addressed. In particular the magnetic aftereffect in (Ga,Mn)As/GaAs is studied in detail. The irreversible aftereffect is evidenced as a critical reduction of the domain wall velocity with time under a constant applied magnetic field below coercivity. This time relaxation is tracked as a function of both time and magnetic field. The measurements show that the overall relaxation is composed of two relaxation processes acting in parallel at short and long time scales. By fitting these experimental results the values of the relaxation times of both relaxation processes were obtained. By this modeling also the values of the activation volumes for two independent (Ga,Mn)As materials are estimated. In the second part of this thesis the possibilities of tuning the electrical and magnetic properties of (Ga,Mn)As by volume and surface treatments are explored. As for the volume modification, oxygen species were incorporated into the (Ga,Mn)As films by means of exposure to an oxygen plasma. The incorporation of oxygen was evidenced by depth profile X-ray photoelectron spectroscopy. This treatment is found to weaken the ferromagnetism, visible as a reduction in the Curie temperature, and to hinder the electrical transport evidenced as an increase in the electrical resistance. In agreement with theories accounting for the hole mediated ferromagnetism in (Ga,Mn)As this behaviour was related to a hole compensation mechanism arising from the presence of oxygen impurities sitting in interstitial positions of the (Ga,Mn)As Zinc blende structure. In the last part, the modulation of the electrical and magnetic properties via surface functionalization is presented. The effects produced by the adsorption of molecular species on (Ga,Mn)As are similar to those found after oxygenation, namely a strong weakening of the magnetic properties and an increase in the values of the electrical resistance. However, in this case the distinctive feature is provided by the possibility of regulating the hole quenching effect by exploiting the additional degree of freedom provided by the chemical properties of the adsorbates. The adsorbate chosen for these experiments are dye molecules that absorb light in the visible range, in this way allowing for a light modulated interaction with the substrate. Using this concept is was possible to observe a modulation of the ferromagnetic transition temperature, the coercive field and the electrical resistance upon illumination. These observations provide a proof of principle for the realization of photo-sensitized (Ga,Mn)As devices where the magnetic properties can be regulated by light.

EPFL2011

Chargement

This study concerns the evolution of the thermomechanical properties of new high-performance materials. In particular, we focus on the relationship between the microstructure and the mechanical properties of a superalloy based on Co-Ni-Cr. Superalloys are metallic alloys with superior characteristics to conventional alloys, especially in extreme conditions, and find application in all that areas requiring materials with high elastic modulus, yield strength and resistance. The investigated alloy is used with satisfaction since several years, but the origin of its outstanding mechanical properties is still poorly understood, as well as the role of small alloying elements additions on mechanical properties. To understand the influence, on the mechanical properties, of the elements added to the base composition, the study is carried out simultaneously on two variants of the material, differing for the presence of a small amount of beryllium in one them. The variant containing beryllium exhibits superior mechanical properties with respect to the base variant. The microstructural evolution and mechanical properties was monitored by using several complementary techniques. Series of heat treatments were performed on samples from the two variants, and both TEP and hardness were systematically measured. This has allowed us to establish the different stages of microstructural changes with the temperature. The TEP is a parameter very sensitive to the change in matrix's composition. It indicates that exposure of this material to a temperature between 500°C and 600°C produces a precipitation stage (A), involving titanium. Such a precipitation is responsible for a remarkable hardening, as well as an increase in the elastic limit. Mechanical spectroscopy allows us to obtain information about the mobility of structural defects which dissipate energy when undergone to a periodic stress. By developing a phenomenological model based on the ADIF curves, we can affirm that the hardening comes from the pinning of dislocations by A precipitates. A second precipitation stage (B) occurs only on the variant containing beryllium, undergone a complex sequence of heat treatments up to 700°C. The analysis of samples by TEM and APT provided essential support to the interpretation of results, and to characterization of the precipitation B. Precipitates B are nanostructured intermetallic compounds NiBe, analogous to GP zones. Their growth is favored by the segregation of titanium at the precipitate-matrix interface to relieve the coherency strains. The hardening provided by B precipitates, together with the structural hardening due to the cold working and the precipitation hardening due to the heating at 550°C, allows to obtain the maximum hardness values. The peaks on the internal friction spectra, also provide informations about the influence of beryllium on the mobility of grain boundaries. This parameter determines the recrystallization and the relaxation mechanism at high temperature, which can be observed by mechanical spectroscopy. Finally, the work here presented yields an important contribution to the understanding of the performance's origin of a Co-Ni-Cr superalloy. It also points the way for other studies aiming the optimization of production processes, in terms of temperature and time of annealing, to obtain the desired properties. This thesis allowed to discover a new hardening phase (B) previously unknown, which could lead to even higher hardness values.

EPFL2016

Chargement

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

EPFL2011

EPFL2016