| The honeycomb structure formed by magnetic atoms has complex magnetic ground states and potential quantum characteristics,such as Berezinsky-KosterlizThouless phase transition,quantum spin liquid state,non-trivial topological properties,etc.,which have attracted intensive attention in the field of condensed matter physics.Especially in honeycomb lattice antiferromagnetic materials represented by Ni2+ions with small spin S=1 and Co2+ ions with effective spin S=1/2,special frustrated effects can produce more interesting physical characteristics.Therefore,it is of great significance to deeply study the magnetic materials with honeycomb lattice.This thesis mainly studied the growth conditions of single crystal required for the honeycomb material Ba2Ni(PO4)2 and the low-temperature physical properties of the single crystal.In addition,the low-temperature magnetism of Ba2Co(PO4)2 polycrystalline and the growth conditions of Ba2Co(PO4)2 single crystal were also studied in detail.The contents of the thesis are as follows.In chapter one,firstly,we review the different magnetic ground states of honeycomb lattice frustrated magnetic materials and the quantum properties under different magnetic ground states.Secondly,the main progress of stripe order ground state material Ba2Ni(PO4)2 and isostructural compound Ba2Co(PO4)2 are introduced.Finally,the research of thermal transport properties and the optical floating-zone method and flux method for the crystal growth are briefly introduced.In Chapter two,Ba2Ni(PO4)2 single crystals were grown by optical floating-zone method and the growth conditions required for this material were comprehensively studied.It was found that the growth speed and atmosphere are key factors in determining the crystal quality.Slow growth speed can reduce cracks caused by thermal stress and the pure argon atmosphere can greatly reduce bubbles in the molten zone.Through experimental characterizations such as X-ray energy-dispersion spectrometer,X-ray diffraction and Laue photograph,it was confirmed that the obtained single crystals have good quality.In chapter three,based on the high-quality Ba2Ni(PO4)2 single crystals,the magnetic susceptibility,specific heat and thermal conductivity were studied in detail.According to the magnetic susceptibility and specific heat data,it was found that Ba2Ni(PO4)2 enters the antiferromagnetic order state below 5 K and has a strong magnetic anisotropy.The temperature dependence of thermal conductivity indicates that there is a strong spin fluctuation in the system that can scatter phonons.The magnetic field dependencies of thermal conductivity show that there are a variety of magnetic transitions.At the same time,combined with the magnetic field dependencies of specific heat,it was confirmed that the magnetic excitation scatters the phonon in the system.In chapter four,Ba2Co(PO4)2 polycrystalline samples were synthesized,and the magnetic susceptibility and magnetization were measured.It was found that there were two phase transitions and magnetic-field-induced magnetic transitions in Ba2Co(PO4)2,displaying rich magnetic properties.In order to study the magnetic ground state of Ba2Co(PO4)2 more comprehensively,we tried to grow single crystals of this material.KCl was used as a flux to successfully grow 1-2 mm long-strip single crystals,and a larger size single crystal was grown by optical floating-zone method.In chapter five,the main works of this paper are summarized. |
PDF Full Text Request