Investigation On The Magneto-optical Properties Of Two-dimensional Metallic Cr_1-x Te And Ta₃FeS₆ Ferromagnets

Two-dimensional(2D)materials have rich optoelectronic and physical properties,which have been widely used in many fields.Among them,2D ferromagnetic materials have potential applications in magneto-optical storage,spin valves and magnetic tunnel junctions for the large magneto-optical Kerr effect(MOKE).Cr I₃,Cr₂Ge₂Te₆ and Fe₃Ge Te₂are intrinsic van der Waals(vd W)2D ferromagnetic materials which were discovered first and most widely studied.Their heterojunction devices show good electrical properties under the control of electrical,magnetical and strain.However,they cannot meet the the application requirements of room temperature ferromagnetic devices for the low Curie temperature and the poor air stability.Therefore,one of the research priorities is exploring2D ferromagnetic materials with high Curie temperature and good air stability.MOKE is an effective method to investigate the magneto-optical properties of 2D materials.Chemical vapor deposition(CVD)method can be used to synthesize 2D non-vd W materials with atomic thickness.In this paper,chromium tellurides and Ta₃Fe S₆ are used as research objects.The nanosheets are prepared by CVD and Au-assisted mechanical exfoliation methods and the magneto-optical properties are investigated by MOKE measurement.The main works are listed as follows:1.Investagation on the magneto-optical properties of 2D Cr Te nanosheets and obtained the thickness-temperature phase diagram.Cr Te nanosheets were successfully synthesized on Si O₂/Si substrate by CVD method.The thinnest thickness is 2.4 nm.Wavelength and thickness-dependent MOKE signals of Cr Te nanosheets were investigated.The magneto-optical Kerr rotation of 11 nm Cr Te nanosheets can reach 19 mrad.Wavelength dependent MOKE and RMCD signals meet Kramers–Kronig relationship.Thin nanosheets show rectangular hysteresis loops,revealing that Cr Te is a hard magnet.The hysteresis loops of thick nanosheets are more complicated with two ferromagnetic transition processes and the thickness-temperature phase diagram has been obtained.From the thickness-temperature phase diagram,the three-dimensional(3D)to 2D evolutions of the ferromagnetism in Cr Te were explored,which improves the understanding of Cr Te and enables spintronic applications.2.Investagation on the magneto-optical properties of 2D Cr₂Te₃ nanosheets and realization of Curie temperature modulation by strain.Cr₂Te₃ nanosheets were successfully synthesized on mica substrate by CVD method.The basic physical properties were characterized by XRD,TEM,EDS and Raman spectra.The MOKE results show that Cr₂Te₃ nanosheets are ferromagnetic ordering with out-of-plane spin polarization.The Curie temperature of 12.5 nm Cr₂Te₃ nanosheet is 140 K.Magneto-optical Kerr rotation is4 mrad with the laser wavelength is 760 nm and temperature is 30 K.Curie temperature and coercive field can be effectively controlled by applying strain.Curie temperature increases under tensile strain and decreases under compression strain.This work not only enriches the members of 2D magnetic material family,but also provides a new method for controlling the magnetic properties of 2D magnetic material.3.Investagation on the magneto-optical properties of 2D Ta₃Fe S₆ nanosheets and discovered high air stability.2D Ta₃Fe S₆nanosheets with different thickness were successfully prepared by Au-assisted mechanical exfoliation method.The thinnest thickness is 2 layers.The maximum magneto-optical Kerr rotation of the 9-layers nanosheets is 48mrad,which is much larger than the maximum magneto-optical Kerr rotation of single crystal.Fe content-dependent Curie temperature was investigated by MOKE.Curie temperature increases with the decrease of Fe content.The magnetic properties of Ta₃Fe S₆nanosheets remains unchanged after 4 months in the air.The large Kerr rotation and high stability lay the foundation for the application in magneto-optical storage devices.In summary,we investigate the magnetic and magneto-optical properties of 2D Cr Te,Cr₂Te₃ and Ta₃Fe S₆ ferromagnetic materials by MOKE and clarify the temperature,wavelength and thickness-dependent Curie temperature and magneto-optical Kerr rotation.Stability and strain control characteristics are investigated.These results lay the foundation for the application in magneto-optical storage and other spintronic devices.