Controllable Preparation And Physical Properties Study Of Two-dimensional Layered Ferromagnetic Transition Metal Tellurides

Magnetism control is one of the most important foundations of future electronics,spintronics,topological science and quantum technology.Based on the integration convenience of two-dimensional materials,the introduction of long-range ferromagnetic order in two-dimensional crystals is beneficial to the realization of optical,electrical and magnetic coupling multifunctional devices.However,all two-dimensional van der Waals intrinsic ferromagnets are limited by low Curie temperatures and therefore cannot operate stably at room temperature.Moreover,the electrical and magnetic characteristic parameters,such as magnetic moment,direction of easy magnetization axis,electrical conductivity,mobility,etc.cannot be effectively regulated,and thus cannot be applied to various application scenarios.Therefore,this paper studies the electrical and magnetic properties of ferromagnetic transition metal tellurides with different ferromagnetic induction mechanisms,such as structure defect,magnetic element doping and infinitely soluble alloy induced ferromagnetism.For the first time,the combination of type-II Weyl semimetallicity and ferromagnetism has been realized in WTe₂.The ferromagnetic infinite solid solution with precisely adjustable composition and continuously tunable ferromagnetic properties has also been realized for the first time.Firstly,through controlling the defect densities in crystals,highly adjustable room-temperature ferromagnetism was achieved in two-dimensional semiconducting Mo Te₂nanosheets.Initially,it was found that the crystal defects in the two-dimensional semiconducting Mo Te₂ nanosheets make them ferromagnetic at room temperature and the Curie temperature is higher than 400 K.By summarizing the reported ferromagnetism in two-dimensional materials,we found that the ferromagnetism caused by defects has a wide range of universal applicability to two-dimensional materials.Then,a higher concentration of crystal defects was introduced into the two-dimensional semiconducting Mo Te₂ nanosheets via Ta doping,and it was found that the saturation magnetization of the Ta doped Mo Te₂ nanosheets was enhanced by more than four times.The interference of magnetic impurity on the observed crystal defect magnetism was eliminated by the analysis of microelements in the samples.Although the crystal defect ferromagnetism has the advantage of high Curie temperature,its saturation magnetization is relatively small,which makes it difficult in practical applications.Secondly,based on the problem of small saturation magnetization of crystal defect magnetism,by introducing magnetic Cr atoms into type-II Weyl semimetal WTe₂,long-range ferromagnetism was induced in WTe₂ for the first time.Meanwhile,the Curie temperature of Cr-WTe₂ can be regulated to near room temperature by changing the Cr doping concentration.Through HRTEM and EELS tests,it was found that Cr atoms were uniformly dispersed in the WTe₂ lattice by replacing W atoms,and the valence of Cr was highly correlated with the Curie temperature of Cr-WTe₂.Then,the long-range ferromagnetism of Cr-WTe₂ was confirmed by anomalous Hall effect tests.And the Weyl semimetallic feature of the Cr-WTe₂ was confirmed by ARPES,Sd H oscillations and MR tests.Although magnetic element doping combines ferromagnetism with the unique properties of the host material,the amount of magnetic elements that can be incorporated is very limited.Thirdly,in view of the limitation of doping concentration in magnetic doped van der Waals materials,the layered/quasi-layered infinite solid solution alloys with precise and controllable composition were synthesized,in which highly and continuously adjustable ferromagnetic and electrical transport properties were realized.Initially,by analyzing the crystal structures and ion radii of different tellurides,we selected Cr-V-Te as a possible infinite solid solution alloy system.Then,we successfully synthesized 10 different Cr concentrations Cr VTe(Cr_xV_1-3x/4Te₂)alloys by Te flux methods.The infinite solid solution characteristic was confirmed by the characterizations of the composition and structure changes of Cr VTe.Through magnetic and electrical tests,it was found that the Curie temperature,saturation magnetization,easy axis of magnetization,carrier density,conductivity,mobility and other characteristic parameters of infinitely soluble Cr VTe single crystal can be effectively controlled by adjusting the Cr concentration.By calculating the DOS and exchange interactions of Cr VTe,the mechanism of the long-range ferromagnetism was revealed.