Research On Optical Properties Of Magnetic Two-dimensional Material CrI₃

According to the Mermin–Wagner theorem:long-range magnetic ordering is not observed in two-dimensional systems at non-zero temperatures,but the recent discovery of long-range ferromagnetic orders in two-dimensional intrinsic monolayer such as CrI₃,Cr₂Ge₂Te₆ and Fe₃GeTe₂ has broken this point of view.The emergence of two-dimensional magnetic materials provides new opportunities for studying the magneto-optical effect and spin manipulation in two-dimensional van der Waals materials.The bulk CrI₃ is a typical ferromagnet with a Curie temperature（T_c）of approximately 61 K.The long-range ferromagnetism in CrI₃ monolayer still persists.The magnetism of the monolayer CrI₃ arise from the extremely strong magnetic anisotropy of Cr³⁺.The magnetic moments in the same layer are aligned out of the plane,and the adjacent layers of CrI₃ are antiferromagnetically coupled.The oppositely aligned magnetic moments in the two adjacent layers make the bilayer CrI₃ exhibit antiferromagnetic properties.Previous studies on the two-dimensional magnetic material CrI₃ have mainly focused on its magnetic and electrical properties,and there is rare research on its optical properties.We prepared few-layer CrI₃ samples by mechanical exfoliation method and in-situ transferred them to 300 nm SiO₂/Si substrates,and further study their optical properties particularly the Raman features.The main contents are as follows:1.We recorded the photoluminescence,Raman and reflective magnetic circular dichroism spectra of 2-5 layers and bulk CrI₃,and studied the relationship between CrI₃photoluminescence,lattice vibration,magnetic states and layer number.2.We studied the linear and circular polarization dependence of the 2-5 layer and bulk CrI₃ Raman spectra,and found that the A_g modes have two-fold anisotropy pattern,but the E_g modes show isotropy,and the A_g and E_g modes have opposite circular dependence.Based on Raman selection rule,we can validate that of the few-layer CrI₃adopts rhombohedral stacking at low temperature.3.The temperature dependence Raman spectra show that,the line width of the A_g and E_g model phonon deviates from the phonon-phonon mode below 60 K arising from spin-phonon coupling.4.The magnetic-field dependence of the Raman spectra of CrI₃ flakes show that the magnetic field has no obvious effect on the line width,frequency and total scattering intensity of the phonon,but the magnetic field non-reciprocally rotated the polarization axis of Raman scattering light.As the magnetic field increases from-2 T to+2 T,the polarization angle increases from-15°to+40°,probably indicating a new magneto-optical scattering effect in CrI₃.