Study On Magneto-optical Properties Two-dimensional Ferromagnetic Material CrI₃ And Magneto-electric Coupling Of Its Heterostructure

Magnetic materials represent a crucial constituent of spin electronic devices.In comparison with conventional thin films,two-dimensional magnetic materials have emerged as a promising alternative in sensing,storage,electronics,and medical applications,owing to their unique advantages.At the atomic level,two-dimensional magnetic materials exhibit novel magnetic,electrical,mechanical,optical,and chemical properties and have diverse development prospects in domains such as magnetic-optical storage,magnetic-optical sensors,and spin valves.In addition,the weak van der Waals interactions between the layers of two-dimensional magnetic materials enable the combination of atomic layers with different degrees of matching,which creates various types of van der Waals heterostructures,overcoming the limitations of lattice matching and compatibility.This opens new avenues for realizing magnetic sensors and non-volatile random access memories that exhibit mechanical flexibility,high-density three-dimensional stacking,fast response rate,and high switching ratio performance.Magneto-optical properties and magneto-electric coupling characteristics are the physical basis for the design of magnetic memory.However,the above effects are rarely studied in the field of two-dimensional magnetic materials.Therefore,this dissertation focuses on the two-dimensional magnetic material CrI₃,and employs magnetic circular dichroism（RMCD）and magneto-optical Raman spectroscopy to comprehensively study its structural characteristics,magneto-optical properties,and magneto-electric coupling properties.The primary research content and conclusions of this dissertation are presented below:1.In this work,we reveal the intrinsic monoclinic structure of few-layer CrI₃ and discover the spring damping effect of hBN on the structural phase transition,which solves the key scientific problem that it is difficult to determine the lattice structure of bare CrI₃.3-5 layers of bare and hBN-encapsulated CrI₃ samples were prepared by mechanical exfoliation and two-dimensional transfer technique,respectively,and tested by RMCD and magneto-optical Raman spectroscopy.It is found that there is a quadruple angular dependence both in bare and hBN-encapsulated CrI₃ samples by polarized Raman spectroscopy at room temperature and low temperature,indicating that the intrinsic structure of few-layer CrI₃ is monoclinic.In addition,a half-bare and half-encapsulated three-layer CrI₃ sample is designed.By comparing the slopes of the two-channel Raman mode energy difference with temperature in the two regions,it is confirmed that hBN layer has a certain spring damping effect on the lattice phase transition,but can not determine the occurrence of the lattice phase transition,which overturns the hypothesis that the hBN-encapsulation effect hinders the lattice phase transition.The results reveal that the magnetic order of CrI₃ is closely related to the stacking order,and the change of stacking order will cause the reversal of the magnetic state.For this reason,we have studied the regulation effect of magnetic field on the crystal structure of few-layer CrI₃.The results show that the external magnetic field can not cause the change of lattice phase,indicating that there is a unidirectional regulation behavior between the magnetic order and stacking order of CrI₃.This work can help to understand the relationship between the intrinsic structure and magnetism of CrI₃ and lay an important foundation for future research.2.In this work,we found a novel non-reciprocal strong magneto-optical scattering effect of two-dimensional ferromagnetic material,which is two orders of magnitude stronger than the Kerr effect and breaks through the key scientific problem of"weak"magneto-optical effect.3-layer,5-layer and bulk CrI₃ were prepared by mechanical exfoliation,and the changes of polarization planes under different magnetic fields were measured by magneto-optical Raman spectroscopy.It is found that the magneto-optic phonon scattering polarization plane rotates with the magnetic field,and the magneto-optical rotation angle is as high as 40°,while the magneto-optical rotation angle of the conventional Faraday effect and Kerr effect is only 0.1°.In addition,the magneto-optical rotation angle of the positive and negative magnetic field shows non-reciprocity,and the magneto-optical rotation angle of the positive magnetic field is 3 times larger than that of the negative magnetic field.Finally,based on the Lorentz electron kinematics model,the physical mechanism of"Lorentz force-parity symmetry-non-reciprocity"is revealed,and the mechanism of non-reciprocal magneto-optical scattering effect caused by Lorentz force is clarified.This work provides a new idea for the preparation of new non-reciprocal magneto-optical devices.3.In this work,we designed a multiferroic heterostructure of two-dimensional ferromagnetic material and ferroelectric thin film.The modulation range of the exchange bias field is as high as～0.48 T,breaking through the key scientific problem of“weak”magnetoelectric coupling effect at the interface of multiferroic heterostructure.Using two-dimensional transfer technique,the multiferroic heterostructure of two-dimensional magnetic material CrI₃ and novel ferroelectric thin film Y:Hf O₂ was constructed.It is found that the CrI₃ homostructure exhibits an obvious exchange-bias effect without applied voltage,and the exchange-bias field is up to～-0.15T.By applying gate voltage,the exchange-bias field of CrI₃ homostructure is greatly tuned from～-0.15T to～+0.33T,and the modulation range is 0.48T.In addition,the positive and negative voltages show the same regulation behavior to the exchange-bias field.Through the first-principle calculation,it is revealed that the exchange-bias effect originates from the electrostatic doping effect at the interface.By applying different gate voltages,the type and concentration of electrostatic doping at the interface can be changed,which leads to the change of exchange interaction between CrI₃ layers,which reasonably explains the physical origin of exchange-bias effect and electrical regulation behavior.Moreover,by tracking the magnetic state of the sample after removing the voltage,it is found that the exchange-bias effect is reversible and can basically return to the initial state after a period of time.This work provides a broad prospect and direction for the development of novel magnetic memory.The aforementioned study employed advanced techniques,namely RMCD and magneto-optical Raman spectroscopy,to investigate the intrinsic structural ground-state,magneto-optical effect,and exchange-bias effect based on multiferroic heterojunctions of a novel two-dimensional magnetic material,namely CrI₃.The findings of this investigation have established a fundamental basis for further exploration of two-dimensional magnetic materials,demonstrating their noteworthy potential in innovative areas such as magneto-optical storage and spin electronics.