| Frustrated spin systems have prominent quantum spin fluctuations,which cause numerous novel magnetic properties and quantum states,e.g.,quantum spin liquids(QSL),quantum spin state transitions,spin ices.Therefore,spin frustrated materials have been one of the focuses in condensed matter physics.The most representative one is the QSL first predicted in the two-dimensional triangular antiferromagnet.In this case,the strongly interacting spins do not develop a long-range magnetic order and no spontaneous symmetry breaking even at the absolute zero temperature,and form a novel disordered quantum state with long range entanglement between the spins.Moreover,QSLs have some special elementary excitations,including spinons,Majorana fermions and topological visons,which are closely related to quantum information,quantum computing and high temperature superconductivity.In addition,many spin frustrated materials exhibit a series of quantum spin state transitions with external magnetic field along the characteristic direction,which means the rich magnetic phase transition behaviors.The spin ice materials are mainly found in rareearth pyrochlore systems,but the more specific kagome spin ices have been generally realized in artificial spin ice systems formed by nanorods of ferromagnets organized into honeycomb networks.However,it is a big challenge to further explore the abundant phase diagram of kagome spin ices due to the small system sizes and the large magnetic energy scales,and it is hoped that natural kagome spin ice compounds will help better for breaking through the exploration of the kagome spin ice systems.In this thesis,we study the special quantum magnetism behaviors of several twodimensional spin frustrated materials with triangular lattice,hexagonal honeycomb lattice and kagome lattice by using the ultralow-temperature heat transport,magnetic susceptibility and specific heat measurements.The main contents of the thesis are summarized as follows:An overview on the exotic magnetism properties of spin-frustrated materials,including quantum spin liquids,quantum spin state transitions,spin ices.Firstly,the geometrical frustration and the new frustration type within two-dimensional honeycomb lattice are reviewed.Secondly,various experimental identifications of QSLs are discussed and some research progress on some typical QSL candidate materials with different frustration lattices are summarized.Furthermore,the research status of quantum spin state transitions is given and finally spin ices are briefly introduced.The magnetism,magnetic transitions and magnetic excitations of a QSL candidate Na2BaCo(PO4)2 with a Co-based triangular lattice were systematically studied by measuring thermal conductivity,ac magnetic susceptibility and specific heat under ultralow temperatures and in high magnetic fields.It was found that there is an antiferromagnetic order at very low temperature(TN～148 mK).A non-zero residual thermal conductivity κ0/T(～0.0062 W/K2m)was extrapolated from the thermal conductivity data above TN,indicating the QSL state with itinerant spinon excitations.Moreover,this material exhibits a series of quantum spin state transitions with applying magnetic field along the c axis(perpendicular to the triangular plane)at ultralow temperatures.These results demonstrate that Na2BaCo(PO4)2 possibly behaves as a gapless QSL with itinerant spinon excitations above TN and its strong quantum spin fluctuations persist below TN.The low temperature physical properties of Na2BaNi(PO4)2,which is a new spin1 triangular lattice antiferromagnet by replacing Co in Na2BaCo(PO4)2 with Ni,were studied by using thermal conductivity,magnetic susceptibility,and specific heat measurements under ultralow temperatures and in high magnetic fields.Na2BaNi(PO4)2 exhibits an antiferomagnetic order at 430 mK with a weak ferromagnetic moment along the c axis(perpendicular to the triangular plane).With increasing magnetic field along the c axis,a 1/3Ms quantum magnetization plateau(corresponding to the up-up-down spin structure)was observed,which means an easy-axis spin anisotropy in this system.The ac magnetic susceptibility data display a series of quantum spin state transitions with applying magnetic field along the c or the a axis;correspondingly,the κ(B)curves exhibit minima at the transition fields.The magnetic phase diagram was constructed based on the above measurements,and exhibits various field-induced spin state transitions besides the up-up-down phase.The magnetic and transport properties of the natural kagome spin ice compound HoAgGe were studied at low temperatures.The low-temperature magnetizations display a series of field-induced magnetic transitions and magnetization plateaus for magnetic field along the kagome plane,while they only display a weak metamagnetic transition for magnetic field perpendicular to the kagome plane.These magnetic transitions induce maxima in ρ(B)and minima in κ(B)curves,which demonstrate the strong coupling among the spins,electrons and phonons in HoAgGe.In addition,the transverse magnetoresistance(MR)of HoAgGe displays an overall field dependence between the B2 and B-linear behavior for magnetic field along the kagome plane.This cannot be explained by the traditional MR theory and needs to be confirmed by the further researches.The thermal conductivity and thermal Hall effect of Kitaev QSL candidate Na2Co2TeO6 were studied in antiferromagnetic state below TN(～26.5 K),with magnetic field perpendicular to the honeycomb plane.The longitudinal thermal conductivity κxx shows a roughly T1.2 behavior at subkelvin temperature,indicating the existence of magnon scattering of phonons.The high-field valley of κxx(B)curves around 10 T should be related to the gap closure and reopening where κxx usually reaches a minimum.More importantly,a large thermal Hall effect was observed under the magnetic field perpendicular to the honeycomb plane,and the thermal Hall conductivity κxy can reach 10-3 W/Km.Moreover,both the temperature(3～4 K)and the magnetic field(～10 T)induce sign reversal in κxy,and the Hall angle κxy/κxx reaches high values of 2%.These experimental results cannot be explained by the pure phonon or magnon conduction,but are probably originated from the magnon-phonon hybridization.In summary,this thesis aims at the ultralow-temperature heat transport behavior of several typical two-dimensional spin frustrated materials.Using ultralowtemperature heat transport measurements,combined with the other low-temperature experimental method,the conduction of itinerant spinons,quantum spin state transitions and large thermal Hall effect were observed.Therefore,experimental studies of ultralow-temperature heat transport is critical for quantum magnetic materials with strong frustration effect,and there are many related experiments that can be carried out further. |
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