Ultrafast Demagnetization Dynamics On Magnetic Multilayer Film

In this paper,we study the static magnetic properties of several magnetic multilayer films with different structures,as well as the ultrafast dynamic properties under femtosecond laser pumping at different wavelengths.The relationship between the exchange interaction between the AFM layer and the FM layer and the magnetic precession damping was investigated.We also study the effect of pump light at different wavelengths and in different polarization states on the ultrafast demagnetization time.The main results are as the followingIn the first part,we studied the static magnetic properties of CoFeB/Pt and CoFeB/Pt/MnIr using VSM.The influence of MnIr layer on exchange bias and coercivity field was also studied.It was found that with the increase of MnIr layer thickness,the exchange bias first increased and then decreased.The dynamic properties of these samples were studied by TR-MOKE technique.We found that subpicosecond demagnetic process and subsequent precession relaxation process happened with femtosecond laser pumping.The demagnetization curves of series samples were fitted,and we got the intrinsic Gilbert damping of Co62Fe25B 13(0.63)/Pt(0.5)/Mn78Ir22(6)is 0.085,and the intrinsic damping of Co62Fe25B 13(0.63)/Pt(0.5)is 0.060,the insert of MnIr layer significantly increases the intrinsic damping.Then we made dynamic measurement and fitted the demagnetization curves,it was found that the intrinsic damping was larger than that of Co62Fe25B13(0.63)/Pt(0.5)samples.With the increase of MnIr thickness,the intrinsic damping increases first and then saturatesIn the second part,We have measured the difference of optical path between 800 nm and 400 nm beams precisely.At the same time,the demagnetization time of Tb15(Fe0.2Co0.8)85 and Tb20(Fe0.2Co0.8)so samples were detected precisely.Under the excitation of 800 nm laser,the detected FeCo signal drives the demagnetization of Tb,and the demagnetization time of FeCo is shorter than that of Tb.When the pump beam was changed to 400 nm,part of the laser energy will also excite FeCo to demagnetize directly;when the laser directly excites Tb to demagnetize,because the energy after the demagnetization of FeCo is still large,it will transfer the energy to Tb,so that Tb still demagnetized,and the demagnetization time of Tb is slower than FeCo at last.This helps us to understand the coupling relationship between Tb and FeCoIn the last part,The ultrafast dynamic properties of[Pt/Co]3/MnIr were studied at room temperature.The static properties of[Pt/Co]3/MnIr samples were measured by MOKE technique and obvious exchange bias field was found.Then,the helicity-dependent characteristics of the sample were studied,and it was found that the sample basically achieved the ultrafast demagnetization process within 1 ps time scale.When excited by a left circularly polarized(LCP)laser pulse,the demagnetization time of the sample under negative magnetic field is significantly faster than that under positive field.However,the demagnetization time under positive field is faster than that under negative field when the sample is excited by a right circularly polarized(RCP)laser pulse.The demagnetization amplitude increases with the increase of the energy of pump light.Then,the thickness of the AFM layer was changed,and it was found that the amplitude of left circularly polarized signal minus right circularly polarized signal increased with the increase of MnIr thickness.