Thèse--Université de Paris. / At head of title: ... F. London. Published also as no. 458 in series Actualités scientifiques et industrielles. Translated by Edmond Bauer and J. Winter. "Rèfèrences": p. [79]-80. Includes bibliographical references.

Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 253-260).

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The pulse technique was employed to measure velocities of longitudinal and transverse mechanical waves in solid iodine at ultrasonic frequencies. Pulses were produced and their echoes displayed by a Sperry Ultrasonic Attenuator Comparator. Transducers for generating the waves were: longitudinal waves, X-cut quartz crystals of 5 and 12 Mcs.; transverse waves, Y-cut quartz crystal of. 5 Mcs. For longitudinal measurements the sample was immersed in baths of liquid helium, liquid nitrogen, solid carbon dioxide, ice, ambient room temperature, and boiling water. No dispersion was found. Four samples were used, but as iodine is anisotropic in many of its physical properties most attention was paid to two amorphous samples which were thought to give representative average values of the properties of iodine. From the values of longitudinal velocity, Young's modulus was calculated. Transverse velocity measurements were made in baths of liquid helium, liquid nitrogen, carbon dioxide, and ambient room temperature. The shear, modulus was determined from the transverse velocity; compressibility was found from Young's modulus and the shear modulus. All these quantities behave much as would be expected of a solid material. The Debye characteristic temperature calculated from the longitudinal and transverse velocities was compared with values calculated from specific heat data and Lindemann melting point formula. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This dissertation mainly focuses on the investigation of the magnetic phase diagram of quasi two di- mensional triangular lattice antiferromagnets (TLAFs) Ba₃MNb₂O9 (M = Co2+ (S = 1/2), Mn2+ (S = 5/2)), and their multiferroic properties. Both compounds show a two-step magnetic phase transition at TN1 and TN2 upon cooling from paramagnetic to up-up-down(uud) phase due to the easy-axis anisotropy, and 120 degree ordered phase at zero field. This feature is dissimilar to that of sister a compound Ba₃MNb₂O9 (Ni2+, (S = 1)), in which a single magnetic phase transition occurs due to the easy-plane anisotropy at zero field. Moreover, at low temperature below TN1, successive magnetic phase transitions were observed in both compounds. However, in case of Co compounds, the range of magnetic field where the uud phase stabilizes becomes wider at lower temperature whereas becomes narrower in case of Mn compounds. This different behavior is originated from the nature of the fluctuations that stabilize the uud phase, that is, quantum and/or classical fluctuations. We also found that the spin magnitude and spin structure play a crucial role in stabilizing the multiferroic ground state. Multiferroicity appears in all magnetically ordered phase in the small-spin-system Co compound but only in 120 degree ordered state in the large-spin-system Mn compound. The systematic studies on these compounds provide a highly valuable playground in the investigate of the effect of spin varied from 1/2 to 5/2 in frustrated magnets and multiferroics, attract many interests and in the field. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2016. / March 31, 2016. / Frustrated systems, Magnetism, Magnetoelectricity, Multiferroicity, Triangular Lattice / Includes bibliographical references. / Eun Sang Choi, Professor Co-Directing Dissertation; Efstratios Manousakis, Professor Co-Directing Dissertation; Naresh Dalal, University Representative; Irinel Chiorescu, Committee Member; Volker Crede, Committee Member.

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published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy