Magnetic Phase Diagram of Triangular Lattice Antiferromagnet Ba₃MNb₂O9 (M = Co, Mn) and Its Multiferroicity

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.