Two-dimensional VSe₂ Magnetism And Its Strain Modulation

Atomically thin two-dimensional（2D）van der Waals materials have novel physical properties,flexible characteristics,and the potential for high-density integration,arousing great interest in basic research and technical applications.Among them,the magnetic order of 2D magnetic materials can be controlled by the number of layers and the interlayer exchange coupling,which does not exist in bulk.Therefore,2D magnetic materials are an excellent system for studying low-dimensional spintronics.However,most 2D materials do not have intrinsic ferromagnetism.For example,early 2D ferromagnetism was realized by magnetic elements doping into 2D semiconductors.In recent years,the successful preparation of Cr I₃,Cr Ge Te₃,Fe₃Ge Te₂ with high-quality and atomic thickness has set off a boom in intrinsic 2D magnetic materials.However,most of these materials have low throughput and poor stability,making their direct synthesis and post-processing much more difficult.Besides,the lower Curie temperature also limits their practical applications.In addition,the magnetism of 2D layered ferromagnetic materials can be controlled by electric field and strain engineering,such as inducing electrostatic doping via gating voltage increase the Curie temperature,pressure through changing the layer stacking configuration drive magnetic phase transition,tuning the magnetic anisotropy by tensile strain achieve magnetic reversal and so on.This provides vast opportunities for the application of 2D magnetic materials in spintronics devices and strain sensors,indicating that the manipulation of two-dimensional magnetism is very meaningful.As a layered ferromagnetic material,2D VSe₂ has higher Curie temperature than room temperature.And theoretical calculations have predicted that strain can modulate magnetic properties in the monolayer VSe₂,so 2D VSe₂ is a material worth studying.At present,the research on the magnetism of 2D VSe₂ is still in its infancy,especially the experiment about magnetic tuning by strain has not been reported.Therefore,we mainly conducted the following research:（1）2D VSe₂ was achieved high-throughput preparation by chemical vapor deposition（CVD）,and studied the change laws of its morphology and thickness with growth conditions（temperature,source-substrate distance,raw material,gas flow,etc.）.The results show that the growth temperature is 540～550°C,and the source-substrate distance is 16～20 cm are the best growth window.Uniform size（～50 um）,ultra-thin thickness（～4 nm）and flat 2D VSe₂ single crystal can be obtained,which provide material guarantee for subsequent magnetic research.（2）The variation of the in-plane and out-of-plane magnetism with temperature,and the dependence of microscopic magnetic domains on thickness of 2D VSe₂ are systematically explored.The macroscopic magnetic results show that the Curie temperature（T_c）of VSe₂ is about 320 K.The saturation magnetization decreases and coercive field shows an overall decreasing trend with the increase of temperature.There is an abnormal increase around 70 K for coercive field owing to the Charge Density Wave（CDW）transition.The micro-magnetic characterization shows that the magnetic domain signal is strongest when the thickness is close to a single layer and dropping sharply for bilayers and disappearing when the thickness reaches more than ten layers.The results confirmed that the magnetism of VSe₂ changed from ferromagnetism to paramagnetism as the thickness increased.（3）Explored the magnetic manipulation of VSe₂by strain.It is found that VSe₂ maintains ferromagnetic order regardless of appling tensile or compressive strain,which is caused by the through-bond interaction that determinates the ferromagnetic coupling always dominates in strain.Furthermore,the magnetic moment of VSe₂ increases and the coercive field decreases as appling tensile strain,the magnetic moment decreases slightly,and the coercive field basically does not change when compressive strain is applied,which is because that the distance between V and Se increases when the tensile strain is applied,resulting in V-Se ion interaction is stronger than the covalent interaction,increasing the number of unpaired electrons,leading to an increase in the magnetic moment,while appling compressive strain,the distance between V and Se decreases leading to the covalent interaction increases and the number of unpaired electrons decreases,causing the magnetic moment to decrease.