Material Synthesis and Characterization on Low-Dimensional Cobaltates

Authors

Hao Sha, Florida International UniversityFollow

Document Type

Dissertation

Major/Program

Physics

First Advisor's Name

Jiandi Zhang

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

Xuewen Wang

Third Advisor's Name

Wenzhi Li

Fourth Advisor's Name

Yesim Darici

Fifth Advisor's Name

Chunlei Wang

Keywords

Cobaltates, Synthesis, Characterization, Low-dimensional, single crystal, neutron, refinement, magnetic structure

Date of Defense

5-27-2010

Abstract

In this thesis, results of the investigation of a new low-dimensional cobaltates Ba_2-xSr_xCoO₄ are presented. The synthesis of both polycrystalline and single crystalline compounds using the methods of conventional solid state chemical reaction and floating-zone optical furnace is first introduced. Besides making polycrystalline powders, we successfully, for the first time, synthesized large single crystals of Ba₂CoO₄. Single crystals were also obtained for Sr doped Ba_2-xSr_xCoO₄. Powder and single crystal x-ray diffraction results indicate that pure Ba₂CoO₄ has a monoclinic structure at room temperature. With Sr doping, the lattice structure changes to orthorhombic when x ≥ 0.5 and to tetragonal when x = 2.0. In addition, Ba₂CoO₄ and Sr₂CoO₄, have completely different basic building blocks in the structure. One is CoO₄ tetrahedron and the later is CoO₆ octahedron, respectively.

Electronic and magnetic properties were characterized and discussed. The magnetic susceptibility, specific heat and thermal conductivity show that Ba₂CoO₄ has an antiferromagnetic (AF) ground state with an AF ordering temperature T_N = 25 K. However, the magnitude of the Néel temperature T_N is significantly lower than the Curie-Weiss temperature (|θ| ~ 110 K), suggesting either reduced-dimensional magnetic interactions and/or the existence of magnetic frustration. The AF interaction persists in all the samples with different doping concentrations. The Néel temperature doesn’t vary much in the monoclinic structure regime but decreases when the system enters orthorhombic.

Magnetically, Ba₂CoO₄ has an AF insulating ground state while Sr₂CoO₄ has a ferromagnetic (FM) metallic ground state. Neutron powder refinement results indicate a magnetic structure with the spin mostly aligned along the a-axis. The result from a μ-spin rotation/relaxation (μ⁺SR) experiment agrees with our refinement. It confirms the AF order in the ab-plane. We also studied the spin dynamics and its anisotropy in the AF phase. The results from inelastic neutron scattering show that spin waves have a clear dispersion along a-axis but not along c-axis, indicating spin anisotropy.

This work finds the strong spin-lattice coupling in this novel complex material. The interplay between the two degrees of freedom results an interesting phase diagram. Further research is needed when large single crystal samples are available.

Identifier

FI10081001

Recommended Citation

Sha, Hao, "Material Synthesis and Characterization on Low-Dimensional Cobaltates" (2010). FIU Electronic Theses and Dissertations. 248.
https://digitalcommons.fiu.edu/etd/248