Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Garmestani, Hamid; Committee Member: Gall, Ken; Committee Member: Thadhani, Naresh.

Includes bibliographical references (p. 79-82).

Thesis (Ph. D.)--University of California, San Diego, 2000. / On t.p. "2̳", "x̳", "1̳-̳x̳" are subscripts. Vita. Includes bibliographical references.

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A study of the martensitic phase transition in the shape memory alloy Ni2MnGa has been carried out. Ni2MnGa is one of the group of "shape memory effect" alloys which are currently exciting considerable interest. The origin of this effect in the compound is in the phase change which takes place on cooling through T = 200 K from the cubic L21 Heusler structure to a tetragonal phase. Recently the results of band structure calculations have been used to conclude that in Ni2MnGa the structural phase transition is driven by a band Jahn-Teller distortion.

L'auteur n'a pas fourni de résumé en français / The superconductivity had been thought to be incompatible with the magnetism, because the former originates from the weak attractive leading to the formation of Cooper pairs, whereas the latter is based on the strong Coulomb repulsive force, leading also to strong electronic correlations. Unconventional superconductors,which include heavy fermion systems, high-Tc cuprates, organic superconductors, and iron-pnictides, is a major topic of condensed matter physics. In all these systems, it has been understood that magnetism can even plays an important role for the pairing mechanism, so that both phenomena can coexist and even favour each other.Our target is on heavy fermion systems, namely uranium and rare earth compounds, where the 5f or 4f electrons which have a dual nature (itinerant/localized), play an important role. More precisely, we will focus on the ferromagnetic superconductors and their quantum criticality. In this field, new materials open new frontiers of research. The student will participate in this stream. He will learn and develop the fundamental crystal growth techniques, such as Czochralski, flux, and Bridgeman method. Since high quality single crystals are essential to elucidate the superconducting properties, a lot of efforts will be devoted to improve the quality of the samples. The next target is the quantum oscillation measurements, which allow a detailed microscopic observation of the heavy electronic state and of the topology of the Fermi surface. They require both very low temperature and high fields, like the study of the field induced superconducting phases in these compounds (like URhGe or UCoGe). The student will perform the measurements under extreme conditions, namely high fields up to 15T in SPSMS, or up to 30T in LNCMI, at low temperatures down to 30 mK, and high pressure up to 3 GPa.From the educational point of view, it is ideal that the student starts to synthesize a material, characterizes it, performs the low temperature measurements by him/herself throughout the PhD period, and get used to the exciting measurements under extreme conditions in a large scale facility like the LNCMI. Such a wide spectrum is rather rare in Europe, but our group („SPSMS/LNCMI) can provide such a unique opportunity, helping the student to become an independent researcher. It should be noted that the experiments in SPSMS and LNCMI are quite complementary each other. For quantum oscillation study, high fields, low temperatures and high quality singles are inevitably important. In general, the precise measurements at high fields up to 15T would be enough in order to determine the Fermi surface topology and the effective mass, which canbe done in SPSMS. However, the specific case, such as Lifshitz transition, field induced quantum critical phenomena, requires higher fields than 15T, which can be achieved by the resistive magnet in LNCMI.This project is supported by the ANR (CORMAT, SINUS) and the ERC starting grant “NewHeavyFermion”.Recently in SPSMS we purchased a top-loading dilution refrigerator for the quantum oscillation measurements, and started the installation. By the end of this summer, hopefully we detect the first de Haas-van Alphen signal at high fields up to 15T and at temperatures down to 30mK. Furthermore, we started to install the flux crystal growth equipment this month, involving the reconstruction of the room for the safe treatment of uranium compounds.

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Recent theoretical studies have suggested that the observed suppression of superconductivity in superconductor/ferromagnet (S/F) heterostructures could be modulated by controlling the ferromagnetic exchange interaction in the superconductor. The exchange interaction in the superconductor is the sum of the exchange interaction from the ferromagnetic regions, which has a phase and magnitude that depends upon the direction that the ferromagnet is magnetised and the distance. As the exchange interaction has a phase it is possible that the contribution from two regions will cancel out. The exchange interaction, which can be viewed as an imbalance in the spin populations, suppresses superconductivity so any reduction in the exchange interaction will increase the superconducting transition temperature (Tc) of the heterostructure. Thus by changing the magnetisation of the ferromagnetic regions it is possible to control the exchange interaction in the superconductor and so the superconducting properties of the heterostructure. I have measured the superconducting properties of niobium/cobalt bilayers as a function of the applied magnetic field. I have observed that one component of the superconducting properties is controlled by the net magnetisation rather than the magnitude of the applied field. I have been able to show that this component of the observed change in the superconducting properties is not simply due to stray magnetic field but that the superconductivity is being actively controlled by means of the exchange interaction. This is the first experimental evidence that the superconducting properties of a S/F heterostructure can be controlled in this way, which opens up the possibilities for the construction of future devices.

<p> Magnetic susceptibility data indicate that Tb2Mo2O7 undergoes a spin glass transition at 25 K. The crystal structure is consistent with the fully ordered pyrochlore model. Short range order, involving ferromagnetic and antiferromagnetic correlations, is observed down to 8 K by neutron scattering. </p> <p> Sr2Cr03F and SrLaCr04 have the same magnetic structure. The neutron diffraction data are consistent with two different models. In both cases the magnetic cell is √2a and c and the magnetic moments lie out of the plane. However, in one model the in-plane projection is canted and in the other it is colinear. Sr2Cr03F has a Tc value of 132(2) K and a susceptibility maximum at 280 K. SrLaCr04 has a Tc value of approximately 200 K and a susceptibility maximum near 400 K. The critical· exponent β for Sr2Cr03F is 0.26(4). The (100) magnetic reflection of both SrLaCr04 and Sr2Cr03F shows short range order correlations above Tc. </p> / Thesis / Master of Science (MSc)

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