Study Of Magnetic Phase Transition And ESR In Frustrated Triangular-lattice Antiferromagnetic Ba₃MnM'₂O₉?M'=Sb,Nb? Under High Magnetic Field

Geometrically frustrated magnet is one of important research topic in condensedmatter physics. Due to the strong geometrical spin frustration and quantum fluctuations, this system exhibit many exotic quantum states and unconventional quantum effect. Triangularlattice magnet as one of typical frustrated magnets, belong to the candidate materials to exhibit exotic quantum phenomena such as unconventional magnetic ground states and quantum phase transition behaviors. Recently, the 6H-Ba₃MM'₂O₉?M=Cu, Co, Ni, Mn: M'=Sb or Nb? family of complex perovskites have received particular attention, a new type of triangular lattice magnet. In this dissertation, we mainly investigate the possible fieldinduced magnetization steps and electron spin resonance?ESR? behavior for Ba₃ Mn Sb₂O₉, Zn doped Ba₃ Mn Sb₂O₉ and Ba₃ Mn Nb₂O₉ polycrystals.?1? The basic concepts of geometrical frustration is introduced, including the classification of geometrically frustrated magnet based on the dimensions, physical behavior and related materials. We briefly discuss the new exotic quantum magnetic ground states and multi-interaction coupling such as, the spin-lattice coupling and the spin-orbital coupling. The structure of 6H-Ba₃MM'₂O₉, field-induced magnetic phase transition and the motivation of this article is briefly introduced.?2? The details for sample synthesis, experimental procedure for physical property measurements is presented, including the parameters of the measuring instrument and measuring methods under high magnetic field.?3? The Ba₃ Mn Sb₂O₉ polycrystals ware prepared by the solid-state reaction method. Based on the magnetic susceptibility and specific heat measurement, triangle antiferromagnetic ground state is determined with magnetic transition temperature TN 10.2K. We discovery a field-induced 1/3 magnetization steps around 16.5T under pulsed high magnetic field, indicated the existence of field-induced magnetic transition from “120°”spin structure to “up-up-down” state. The ESR spectra under magnetic field suggests the formation of short-range AFM spin correlation state below 20 K. Then transforms into a long-ranged AFM ordered state with temperature below 10.2K. For Zn-doped Ba₃ Mn Sb₂O₉ system, the antiferromagnetic exchange coupling between Mn2+ spins are suppressed by partial Zn substitution of Mn ions, which also weaken the spin fluctuation of magnetic ions. But, the antiferromagnetic ground state are stable against the dilution of Zn ions.?4? The magnetism and ESR behaviors of Ba₃ Mn Nb₂O₉ polycrystals were systematically investigated. Ba₃ Mn Nb₂O₉ was firstly proved to be a triangular lattice antiferromagnet with Neel transition temperature TN³.2K, frustrated parameter f=6.7. No 1/3 magnetization step was observed under high magnetic field, which can be ascribed to the partical cation disorder at B sites. The ESR spectra suggests the formation of short-range AFM spin correlation state below 18 K.