Magnetic Characterization And Modulation Of Two-dimensional Ferromagnetic Material CrSiTe₃

Magnetic materials constitute the cornerstone of modern data storage technologies.In particular,two-dimensional（2D）magnetic materials have recently attracted enormous attention due to their extraordinary physical properties,such as low dimensionality,flexibility,free of dangling bonds,and semiconductor properties.It does not only provide a platform to study the basic magnetic structure and ordering in the 2D limit,but also provides raw material library in the manufacture of microscale spintronic devices,flexible electronics,and high-density stacked magnetic memory devices.For practical applications,hard ferromagnetic states with strong magnetic anisotropy and higher Curie temperatures are preferred properties.However,most of the 2D magnetic materials show a soft ferromagnetic behavior with a relatively low Curie temperature and many multiple domains.In this thesis,we focus on a 2D soft ferromagnet CrSiTe₃,with a Curie temperature of 32.8 K.By modulating the interlayer and intralayer coupling,we studied the evolution of magnetic properties and explored the plausible physical mechanism.Our studies have far-reaching scientific value for exploring fundamental physics in 2D magnets and open up a new possibility for searching high-temperature magnets in the 2D limit.The main research results are summarized as follows:（1）Home-made reflection magnetic circular dichroism measurement system and Raman spectroscopy system,combined with diamond anvil technology,realized in-situ comprehensive characterization of materials under extremely low temperature,high pressure and strong field conditions.（2）CrSiTe₃ single crystals were successfully grown.The measurement results show that the CrSiTe₃ single crystal belongs to the （?）3（oo.148）space group.It exhibits ferromagnetic property,with an easy axis along the c axis direction,and a Curie temperature（T_c） of 32.8 K.Its ferromagnetism mainly comes from the trivalent Cr+3 ions.（3）We have studied the evolution of magnetism by decreasing the thickness in 2D ferromagnet CrSiTe₃.In the experiment,the magnetism of CrSiTe₃ with the thickness from 170 nm to 1.5 nm was measured using the reflected magnetic circular dichroism（RMCD）.The results show that samples with a thickness greater than 10 nm exhibit soft ferromagnetic properties with many multiple domains,and the corresponding saturation magnetic field decreases as the thickness decreases.when the thickness approaches 8 or7 nm,it transforms from a soft-ferromagnetic state to a hard-ferromagnetic state with a single magnetic domain;but below 3 nm,the ferromagnetism disappears.The appearance of hard-ferromagnetic is mainly attributed to the fact that the thickness of the sample is below 10 nm,which limits the formation of magnetic domain walls and exhibits a single magnetic domain state.Moreover,the thickness-dependent Curie temperature also shows a magnetic transition from three-dimensional to two-dimensional ferromagnetic ordering,with a critical thickness of around 4.7 nm.（4）We have studied the evolution of ferromagnetism in CrSiTe₃ by hydrostatic pressure.The ferromagnetism of CrSiTe₃ flake under pressure was studied experimentally by combining diamond counter-anvil（DAC）technology and in-situ high-pressure magnetic circular dichroism spectroscopy.The results show that when the pressure less than 4 GPa,CrSiTe₃ flake exhibits soft ferromagnetic properties which are the same as that at normal pressure.When the pressure is between 4 GPa and 8 GPa,it changes from a soft ferromagnetic state to a hard ferromagnetic state,accompanied by a significant increase of the coercive field and Curie temperature.（5）Density functional theory（DFT）calculations to explain the enhancement of ferromagnetic properties under external pressure.The Mean-field theory reveals the proportional relationship between the exchange interaction and transition temperature.Our results also show that the first,second,and third nearest-neighbor intralayer exchange interactions J₁、J₂、J₃ factors and the interlayer nearest-neighbor exchange interaction factor J₄,increase with increasing pressure,among which J₁ dominates the magnetic exchange interaction and contributes to the enhancement of ferromagnetic properties.