Preparation And Physical Properties Of Two-dimensional And Quasi-two-dimensional Layered Magnetic Materials

Two-dimensional materials are leading a new technological revolution,among which two-dimensional magnetic materials are considered to be one of the most potential solutions in next-generation semiconductor technology.The emergence of two-dimensional magnetic materials has promoted the development of semiconductor technology towards higher performance and lower power consumption,bringing new possibilities for the development of future electronic devices.Therefore,research and development work on these materials are gradually accelerating,attracting the attention and input of more and more scientists and engineers.Growing and studying 2D and quasi-2D magnetic material pairs provides an ideal platform for understanding the fundamental spin of matter under strong quantum confinement.Two-dimensional magnetic materials have rich physical properties,including spin-density wave,topological properties,etc.In this thesis,centering on crystal growth and physical properties studies of two-dimensional and quasi-two-dimensional magnetic materials with layered structure,single crystals of CrOCl,Fe₆Ge₅ and CrNb₃X₆（X=Se,Te）were prepared and studied,including their magnetic properties,electrical transport properties,thermodynamic properties.Discovering their peculiar physical properties provides a basis for the development of two-dimensional magnetic materials and spintronic devices in the future.The main research content and results of this thesis are as following:1.Through chemical vapor transport,single crystal sample of CrOCl was grown,and its basic physical properties were measured and analyzed,including its magnetic and thermodynamic properties.Through the measurement of magnetism,it is found that the magnetic state of CrOCl in low field is collinear antiferromagnetism.Spin-flip is observed at high fields.The magnetic susceptibility curves measured by temperature rise with field and temperature drop with field.The results show that CrOCl undergoes a first-order phase transition.The magnetic entropy changeΔS_M of CrOCl was calculated by Maxwell’s equation.A positiveΔS_M was observed at low field with a maximum value of2.2399 J kg^-1 K^-1 and a negativeΔS_M was observed at high field with a maximum value of It is 2.2738 J kg^-1 K^-1.By fitting the RCP,the critical exponentis calculated.Through the measurement of the specific heat,it is fitted to obtain the Debye temperature,and the magnetic entropy change of 3-5 T is calculated,which is close to the calculation result of Maxwell’s equation.2.Through improved chemical vapor transport,a high-quality single crystal sample of Fe₆Ge₅ was grown,and its basic physical properties were measured and analyzed,including magnetic properties,thermodynamic properties and electrical properties.Magnetic studies show that there are two phase transition points in the sample,one is antiferromagnetic-antiferromagnetic transition,and the other is antiferromagnetic-paramagnetic transition.The sample exhibits a metal-semiconductor-metal transition.E_g=2.39～3.61 meV is found in the temperature range of 120-340 K.Negative magnetoresistance is observered,that is caused by weakened electron scattering.Meanwhile,the anomalous hall effect is observed below the phase transition temperature.3.Through chemical vapor transport,high-quality single crystal samples of CrNb₃Se₆ and CrNb₃Te₆ were grown for the first time,and their physical properties were measured and analyzed,including magnetic properties and electrical properties.Through the magnetic measurement of CrNb₃Se₆ single crystal,it is observed that single crystal of CrNb₃Se₆ undergoes a phase transition which is not observed in polycrystal crystal of CrNb₃Se₆,and obvious anisotropy is observed.Through the electrical transport measurement of single crystal of CrNb₃Se₆,anomalous magnetoresistance was found in the temperature range between two phase transition points,and anomalous Hall effect and topological Hall effect were observed in this range.Through the magnetic measurement of CrNb₃Te₆,a low-temperature spin glass state was observed,and it was proved to fit the Heisenberg model by fitting.