Quantum Magnetism in Effective Spin-1/2 Co2+-Triangular Lattice Antiferromagnets

The geometrically frustrated magnetism(GFM) is an energetic field of research in condensed matter magnetism. The frontier of the GFM research has been greatly expanded from the prediction of the exotic quantum spin liquid to the ubiquitous topological physics. As one of the simplest geometrically frustrated systems, the spin 1/2 triangular antiferromagnet(TLAF) systems have recently attracted lots of attention. In this thesis, a series of Co2+-based spin 1/2 TLAFs were systematically studied with specific heat, neutron diffraction, thermal conductivity, magnetic susceptibility, magnetic torque measurements and other techniques. In chapter 1 and 2 of this thesis, I will briefly introduce the background material of TLAFs and the experiment methods. From Chapter 3 to 6, I will report the research results of four Co2+-based spin 1/2 TLAFs. In chapter 3, the phase diagrams as functions of the magnetic field, temperature, and orientation of field were mapped out on the TLAF with easy plane anisotropy Ba3CoSb2O9(BSCO). In chapter 4, we probed the effect of randomness induced by site disorder in Sr-doped BSCO Ba2.87Sr0.13CoSb2O9. In chapter 5, an isosceles triangular lattice system Ca3CoNb2O9 was studied to reveal the effect of lattice distortion in TLAFs. And In chapter 6, a quantum spin liquid candidate TLAF Na2BaCo(PO4)2 is reported. We observed possible itinerant excitations and quantum spin state transitions in this system. In the end, I will summarize our current research and offer perspectives for future work in the last chapter.