Ultrafast Thermelization Dynamics Of Nickel Film And Its Dependence On Nanostructure

With the rapid development of the information science and technology,it is necessary to storage a huge number of data and to read and write these data rapidly. Therefore,the key point is to increase the storage density and to probe the ultrafast response of the storage device.Nickel,as a typical ferromagnetic material,its ultrafast dynamics is helpful for improving the read/write speed of the data and the response speed of the micro-devices.The ultrafast dynamics of the nickel film and the film with micro-and nano-structure is investigated by the femtosecond pump-probe measurement.Based on magnetron sputtering technology,three classes nickel film with different structures were prepared.One is the uniform film on Si substrate by direct magnetron sputtering method.Another is the etched film with different particle size,which is prepared by ammonia etching under different temperature for nickel film.The third one is three classes antidotes structures nickel film on the porous aluminas,which is prepared by anodic oxidation.The surface topographies of these nickel films are investigated by scanning electronic microscope(SEM) and atomic force microscope (AFM).Based on the former prepared nickel film,the following works were carried out by femtosecond pump-probe equipment.The transient reflectivity changes of the Cu film,the Au film and the Ni film with the same thickness were measured by femtosecond pump-probe device.The peculiar transient reflectivity changes of the Ni film were studied and the result showed that it is different between the Ni film and the other two films on the ultrafast dynamics.The transient reflectivity of Ni film went up to the maximum and holding for longer time.This phenomenon is speculated to be induced by the complicated band structure near the Femi level of the Nickel.The strong electron-lattice coupling of nickel retains much of the absorbed energy near the surface and the temperature changes of electron are small. Also,the influence of the difference between the electron-phonon scattering time and the electron-electron scattering time on the trend of the transient temperature curve is investigated by simulating the transient temperature curve as a function of the electron-phonon scattering time based on the approximate TTM.It found that the same trend between the curve in short necessary electron-phonon scattering time and the transient reflectivity curve for the Ni film.This indicates that the electron-phonon scattering time is close to the electron-electron scattering time.The transient refiectivity changes for Nickel films with different thickness showed up that no difference occurs in thermalization time and no influence of the thickness on the amplitude of the transient reflectivity changes when the thickness is thicker than the optical penetration depth.The difference of the thermalization time between four classes of samples(the thicker film,the thinner film,the film with nanoparticle and the film on the porous alumina) were compared.It found that the dimension of the structure is simpler,the longer the thermalization time is.This is attributed to the quantum size effect of the structure.Also,through modulating the size of these structures,this quantum size effect was validated.