Experimental Investigation On Ultrafast Electron Dynamics Properties Of Ni₈₀Fe₂₀ Thin Films

Ni₈₀Fe₂₀ films are widely used in the fields of thin-film magnetic head,magnetic recording and magnetic sensors for its unique magnetic properties.With the development of the magnetic micro-devices to the nano-size and the terahertz frequency response,the ultrafast dynamics properties of materials need to be understood urgently.Unique characters of femtosecond laser such as ultra-short pulse width and ultra-high energy density have offered the way to investigate the ultrafast dynamic process of the material occurring in the picosecond and femtosecond time scales.Therefore,the ultrafast electron dynamics of NiFe films was studied by femtosecond laser pump-probe technology.Ni₈₀Fe₂₀ films and multilayered films with different parameters(thickness, substrate) were deposited by magnetron sputtering technique,and a part of them were annealed at 500℃.The composition,surface morphology and crystallinity property of NiFe films were investigated by scanning electron microscope(SEM),atom force microscope(AFM) and X-ray diffraction(XRD).The test results show that the surface of as-deposited films is uniform and smooth with poor degree of crystallinity. Crystal grain growth was observed in the surface of the films which were annealed at 500℃,and they have the higher crystallinity.The influence of multi-layers,thickness,substrate,heat treatment and pump power on the transient reflectivity and the influence of thickness and pump power on the transient transmissivity of NiFe films were studied using femtosecond laser pump-probe technology.The experimental results show that the influence of Cr,Cu and Ti of multi-layers on theâ–³R/R curves is different,and the influence of Ti onâ–³R/R curve is the greatest.After zero point,the change values ofâ–³R/R curves decrease first and then increase with the increase of thickness of NiFe films.The influence of substrates on theâ–³R/R curves in NiFe films with different thickness is different.Theâ–³R/R curves of the annealed NiFe films consist of two parts:one is the timescale＜5ps and the other is the timescale＞5ps.The change values ofâ–³R/R curves increase with the increase of the pump power,and the change of the crest value is linear.When the thickness of samples is on the order of optical penetration,the crest value of theâ–³T/T curves decrease fast,while the thickness of samples＞25nm,the crest value of theâ–³T/T curves increase slowly.The change values ofâ–³T/T curves increase with the increase of the pump power.By combining with theâ–³R/R andâ–³T/T curves,the corresponding temporal variations of the real and imaginary parts of dielectric function are analyzed.The ultrafast electron dynamics of NiFe films was simulated using two-temperature model and electron-lattice temperature coupling approximative model.The simulation results show that the electron and lattice temperature in films have different temperature gradient.The maximum of electron temperature T_e,max decreases with the increase of the electron-phonon coupling constant G under the same laser power,while the thermal equilibrium time of the electron-lattice system decreases accordingly.When the laser fluence increases,the maximum of electron temperature and the equilibration temperature of the electron-lattice system also increase.The reflectivity curves ofâ–³R_el andâ–³R_lat were simulated.