Study Of Ultrafast Dynamic Properties In Nanometer Cobalt Thin Films Under Femtosecond Laser Pulses

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. However, the storage density and response speed of magnetic storage devices become the bottleneck which restricts the development of information. Understanding the microscopic mechanism of thermalization dynamics and magnetization dynamics in the magnetic storage materials after excitation is very important for breaking the current storage limit. Due to the ultra-short pulse width and ultrahigh energy density, femtosecond pulse laser provides an effective method for studying ultrafast dynamics of materials. As a typical transition metal, Co is widely used in the magnetic head, magnetic recording and magnetic sensors fields. Based on this, the femtosecond laser pump-probe thermoreflectance technique was used to investigate of the ultrafast dynamic properties of nanometer Co thin films in detail.The dissertation mainly includes the following contents:Firstly, Co films with different parameters were deposited on Si and glass substrates by magnetron sputtering technique, and part of them were annealed at 300℃,400℃and 500℃separately. The surface morphology, particle size, composition and crystal structure were characterized by scanning electron microscopy (SEM), atomic force microscope (AFM) and X-ray diffraction (XRD). The results show that the Co films have smooth surface, fine particles and a continuous state when the deposited thickness is small. The size of particles increases when the film thickness increases. The microstructure of Co thin films changes after 300℃and 400℃annealing, but no significant grain is observed. The island structure increases and clear crystallization is observed after 500℃annealing. We also find that the thin films quality on glass substrate is better than on silicon substrate.Secondly, the femtosecond laser transient themoreflectance experimental system was built based on the pump-probe technique principle. Under the excitation of low laser energy, the transient reflectivity and transient transmissivity of thin films with various different parameters were measured, which focus on the different film thickness, annealing, multi-layer, substrate material, the pump power and the incidence angle of pump laser, then the process of ultrafast thermalization dynamics can be analyzed and researched. The results show that the reflectivity curve can be divided into four parts:the steady stage, up stage, decay stage and recovery stage. After the femtosecond laser heating, the reflectivity curve goes up to the peak value from the initial equipment value in 133.4fs, and then follows by a slow decay process in about a few picoseconds. As the film thickness increasing, the peak value and the equilibrium value of the reflectivity curves increase. It's due to the effect of the ballistic motion and the thermal diffusion. Annealing makes the peak value of the reflectivity curve become smaller and the period of decay process become longer, these results are attributed to the grain size of film was changed after annealing. Different composite metal layers (Cr, Cu, and Ag) join to the film makes the period of decay process become shorter and the equilibrium value decreases with the value of G increases. When the film thickness is within the optical penetration depth, the reflectivity of the samples is significantly affected by the substrate. At this time, the electronic system temperature is cooled down by both the electron-phonon coupling interaction and the energy transport between electron and substrate. The effect of substrate becomes weaker with the thickness of film increases. With the increases of pump power or the decreases of incident angle, the peak value of the reflectivity curve increases. The film thickness, annealing, different multi-layer and the pump power all affect the peak value and the equipment value of the transmissivity curves. Annealing and composite layer also affects the period of decay process.Finally, when ultra-short laser pulses heating materials, if the heating time is on the order of the relaxation time of the carriers, the non-equilibrium phenomena of electron and lattice temperature may be observed in the metals, while the demagnetization phenomena which induced by spin can also be observed in the ferromagnetic materials. At present, two temperature model and three temperature model is often used to describe the interaction between the ultra-short laser pulses and ultrafast thermalization dynamics and spin dynamics of Co films under femtosecond laser irradiation was simulated by a finite different method. The simulation results with two temperature model show that, at the early period of irradiation, there is a significantly nonequilibrium process between electron and lattice systems, the temperature increasing of electron system is faster than that of lattice systems. Electron heat capacity, laser pulse width and the pump power have great effect on the peak electron temperature. The greater electron-phonon coupling coefficient, the faster rising rate of lattice temperature, the shorter time of electron and lattice temperature is restored to equilibrium, and the equilibrium temperatures are not very different. At the same moment, with the increase of films depth, the electron temperature decreases, moreover, electron temperature grads increase with delay time firstly and then go down. The simulation results with three temperature model show that, a coefficientγs introduced to characterize the temperature dependence of the spin heat capacity, as a result, it can better describe the spin system temperature as a function of time. Lattice-spin coupling coefficient determines the delay time of spin system reached the maximum temperature to same extent, while electron-spin coupling coefficient determines the maximum temperature of spin system in quantitatively. It indicates that the demagnetization process is both results of lattice-spin interaction and electron-spin interaction. With the laser fluence increasing, the peak temperature of electron, lattice and spin system increases, and the temperature of the whole system recovered to equilibrium increases too; At the same time, when the laser fluence is higher, the maximum spin temperatures are quite different before and after modified the spin heat capacity model.