Research On The Synthesis And Magnetic Properties Of Van Der Waals Two-dimensional Magnetic Materials

Two-dimensional van der Waals material benefits from the natural layered structure and rich element composition,exhibiting various mechanical,thermal,optical,electrical,and magnetic properties.It not only provides an ideal research platform for exploring novel properties in low dimensions,but also provides new construction materials for building various miniaturized functional devices.In the context of the post-Moore’s Law era,the research frontier of two-dimensional materials has already touched the quantum limit of the dimensional effect,which has also brought a series of problems and challenges.This thesis focuses on the following core issues in two-dimensional materials:（1）The intrinsic physical properties of the two-dimensional system are more easily affected by the quality of the sample,so higher requirements are put forward for the controllable synthesis of high-quality two-dimensional material.We take Nb Se₂ as an example.Based on the in-depth analysis of the thermodynamics and kinetics in chemical vapor transport,we propose a new process of oscillating temperature gradient,which can achieve highly supersaturated growth at a lower temperature.If the performance of electrical transport is used as an indicator to compare the quality of single crystals,the superconducting transition temperature of our prepared Nb Se₂ single crystal is the highest among the literature reports.（2）Two-dimensional magnetism is particularly susceptible to fluctuations,which seriously affects the stability of the two-dimensional magnetoelectric device.In Cr XTe₃（X=Ge,Si）,we found that the g factor measured by ferromagnetic resonance has an abnormal shift near the Curie temperature.Starting from the anisotropic Heisenberg model,we constructed a quantum model of spin fluctuations and two-dimensional correlations in ferromagnetic resonance.We not only double-checked the relationship between temperature-dependent,angle-dependent,and magnetic-field-dependent g-shifts by experiment and theory,but also further found that the anisotropic g-factor induced anomalously parallel pumped excitations in ferromagnetic resonance.This phenomenon has universality in two-dimensional magnetism and can be developed as an experimental tool for testing the stability of two-dimensional magnetism.（3）Two-dimensional magnetic materials form similarly with the magnetic elements in a honeycomb lattice,but their macroscopic properties are quite different.Therefore,the regulation of non-magnetic atoms is of great significance for the exploration of new states in two-dimensional magnetic materials.We successfully prepared a new quaternary Cr Ge_δSi_1-δTe₃ single crystal by substitution of non-magnetic atoms of Ge and Si.Further analysis of magnetometry found that the increasement of Ge substitution can increase the Curie temperature,reduce the magnetocrystalline anisotropy,and induce a new ferrimagnetic phase under low field.We found that the critical behavior of the magnetic phase transition in this material is closest to the two-dimensional Ising model.The substitution of non-magnetic atoms can tune the exchange interaction,and provides a new idea for the study of novel properties in two-dimensional magnetic materials.