| Summary: | Thesis (Ph.D.)--Memorial University of Newfoundland, 2010. Physics and Physical Oceanography Includes bibliographical references (leaves 138-142) To study the critical behavior of the quasi-one-dimensional antiferromagnet CsNiCl3 near the phase transitions, its elastic constants are investigated using high-resolution ultrasonic velocity measurements as function of temperature and magnetic field applied along the c-direction. The experimental data we present with respect to the longitudinal mode, ΔC33/C33, and the transverse modes, ΔC44/C44 and ΔC66/C66 , generate a phase diagram in good agreement with published results by showing two zero-field transitions at TN 1 ≈ 4.75 K, TN2 ≈ 4.35 K, and multicritical point at (Tm ≈ 4.50 K, Hm ≈ 2.29 T). Moreover, the critical exponent β extrapolated from the temperature dependence of ΔC66/C66 shows XY criticality with a constant value 0.35 ± 0.02 for H < Hm . However, field dependent behavior is observed for H > H m . This is the first experimental evidence that the high-field phase transition associated with the 120° spin configuration is weakly first-order and experimentally resolves the controversy about the true criticality of the high-field phase boundary. This result is also strengthened by the step-like variation demonstrated by temperature dependence of ΔC44/C44 at fields higher than the multicritical field (Hm ≈ 2.29 T) and hysteresis phenomena observed in field dependence of ΔC44/C44 at T = 5.00 K. Numerical predictions 44 are made based on the Landau model. The elastic constants of CsNiCl3 in different phases are calculated using the total free energy, which is derived according to the Landau free-energy, the elastic energy, and the magnetoelastic coupling terms invariant under the symmetry operation of the hexagonal group P63/mmc. Meanwhile, the non-mean-field order parameter and quadratic-quadratic (q-q) couplings are considered to optimize the numerical prediction and achieve a good reproduction of the experimental data. Furthermore, by studying the numerical predicted elastic constants and strains, a decrease of the spin structure symmetry, from hexagonal to orthorhombic, is proved in the elliptical phase.
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