Laser-induced Ultrafast Demagnetization And Spin Wave Dynamics In Ferromagnetic Alloy Thin Films

In the new era that nowadays the amount of information develops rapidly,how to satisfy people's gradually increased requirement of information storage and processing has become a focus issue that scientific and technological development requires.As a resu lt of that ultrafast demagnetization induced by a femtosecond laser pulse in ferromagnetic material was firstly put forward,people begin to focus on the ultrafast interaction between photon and electronic spin.On the other hand,the size of magnonic spin tronics device is expected to break through the limit of quantum effect,which takes enormous advantage in energy assumption as well as heat dissipation.Consequently,the ultrafast magnetization(spin)dynamics and analysis of the microscopic physical mechanism in ferromagnetic materials have gradually become a vital research subject towards developing high speed magnonics device and ultrafast spintronics device as well as achieving information storge,transmission and processing.It is well-known that time resolved magneto-optical Kerr spectrum(TR-MOKE)is a powerful tool for measuring and investigating transient and ultrafast magnetism or spin-related process.After introducing double pump pulse technology,the optically coherent control of ultrafast magnetization dynamics in different timescales can be measured effectively.Based on background described above,in this thesis the ultrafast magnetization(spin)dynamics in full Heusler alloy film Co₂Fe Al,ternary ferromagnetic alloy film FePd_1-xPt_x and perpendicular exchange coupled bilayer film Tb Fe Co/Gd Fe Co has been investigated by self-designed double pump pulse TR-MOKE spectrum,the major work includes 4 aspects as follow.First,in order to investigate coherent control of spin wave dynamics as well as selective excitation of spin wave modes,we design and build up a double pump pulse TR-MOKE spectrum,which is adopted to study the effect of the second pump pulse towards the control of spin wave dynamics under different excitation conditions of spin wave modes.Besides,based on the controllable polar magnetic field,the other controllable transverse magnetic field is introduced,which can be used for investigating the relation between dispersion character of spin wave mode and the angle of external field.By comparing the measurement results tested before and after the reformed spectrum setup,the feasibility and reliability of this reformed setup used for subsequent experiment is ensured.Second,the magnetic field dependence and laser fluence dependence of magnetization dynamics for different spin wave modes as well as the relation between excited spin wave mode and the angle that magnetic field deviates from the normal of sample plane in Co₂Fe Al film induced by ultrafast laser have been studied.The precession frequency of surface spin wave mode(DE mode)displays unusual performance that it decreases with increased magnetic field when the angle described above is extremely small.Under the condition of specific pump excitation fluence and magnetic field orientation,the dispersion character of spin wave mode deviates from normal trend,which may be attributed to different mode-mode coupling.Third,double pump pulse TR-MOKE spectrum is adopted to investigate the coherent amplitude and phase control of spin wave dynamics under the condition that only a kind of spin wave mode is excited and the selective excitation when more than a spin wave mode is effectively excited.The spin wave amplitude variation modulated by two pump pulse with different time interval presents a periodic and sinusoidal trend,while the phase changes in an incompletely linear way,which may originate from phase saltation appearing when the process of magnetic precession happens amplitude suppression.Also,if a suitable experimental condition is satisfied,the selective excitation between DE mode and PSSW mode can be realized on the premise of multiple mode excitation.Fourth,femtosecond laser induces ultrafast magnetization dynamics in FePd_1-xPt_xfilm and Tb Fe Co/Gd Fe Co bilayer has been studied.The time of demagnetization and magnetization recovery in FePd_1-xPt_x shows component dependence,which means spin-orbital coupling modulates the process of demagnetization and spin-lattice relaxation.The type of demagnetization dynamics will be transformed if the laser fluence is increased to a degree.In addition,the ultrafast modulation of local magnetization orientation in Tb Fe Co/Gd Fe Co bilayer can be realized by femtosecond laser excitation.The magnetization reorientation can be driven by exchange bias field and the process will speed up as the laser fluence increases.