Study On Ultrafast Spin Dynamics Of DyFe Alloys

In the ultrafast spin dynamics process induced by femtosecond（fs）laser,the magnetization reversal time breaks through the subpicosecond scale,and the manipulation of spins is also realized in the femtosecond scale.Therefore,the ultrafast spin dynamics is of great significance for potential applications such as ultrafast reading and writting of modern magnetic storage industry and logic computing devices.In magneto-storage media,rare-earth transition ferrimagnetic alloy（RE-TM）has been widely studied,because it has special magnetic behavior and can achieve low power consumption and high density of memory.The ultrafast spin dynamics process induced by femtosecond laser in RE-TM alloy is different from that of ferromagnetic metals,since it always shows a slow two-step demagnetization phenomenon,which is attributed to the different response time of 4f localized electrons of RE and 3d conduction electrons of TM to the excitation of femtosecond laser,the influence of spin orbit coupling and inter atomic exchange interaction.Although domestic and foreign research teams have carried out a large number of experiments and theoretical calculations on the fs-laser induced ultrafast spin dynamics process,the microphysical mechanism of the ultrafast demagnetization process is not clear.Therefore,the ultrafast demagnetization processes in DyFe alloy films induced by fs-laser are systematically studied in this dissertation.By changing the composition of Dy and annealing temperature,the modulation of both the spin lattice coupling of 4f electrons and exchange coupling of3d-4f electrons on ultrafast demagnetization are studied.The details of the study are as follows:First,the spin-orbit coupling and the exchange coupling of 3d-4f electrons of the DyFe alloy systems can be modulated by varying the Dy composition x.The ultrafast demagnetization process of Dy_xFe_1-xalloy films with various x were studied by time-resolved magneto-optical Kerr effect（TR-MOKE）.For the small x,the ultrafast demagnetization is completed within sub-picosecond time scale,followed by a recovery of magnetization within a few picoseconds,which could be ascribed to the heat transferring among the reservoirs,such as electrons,3d spins and phonons.For the large x,a two-step demagnetization phenomenon appears in the ultrafast demagnetization process,including a fast and a slow demagnetization process,respectively.The fast demagnetization process is still dominated by 3d spins,and the fast demagnetization time shows a nonmonotonic variation with increasing x.However,the slow demagnetization time decreases with increasing x.The 4f electrons in the deep shell of Dy atoms participate in the demagnetization process through inter-sublattice coupling,and then reheat the 3d spins,resulting in the two-step demagnetization phenomenon;as the value of x increases,the spin-orbit coupling is enhanced,which increases the heat transfer efficiency,so the slow demagnetization time decreases.The 4-temperature model is introduced for calculation and the calculated results are in good agreement with the experimental results.Secondly,in order to further explore the mechanism of the two-step demagnetization phenomenon,three samples with high Dy contents were selected for annealing.As both x=0.26 and 0.30,the two-step demagnetization is transformed into one-step demagnetization for a high annealed temperature,since Fe grains grow up and locally crystallize under high annealing temperature.As a result,3d spins dominates the demagnetization process.For the samples of x=0.38,the dynamics process shows a two-step demagnetization for all the annealing temperatures,while the slow demagnetization time increases with increasing the annealing temperature.On the one hand,the exchange coupling between Fe atoms is greatly weakened by the high Dy doping.On the other hand,further annealing in high temperature leads to the decrease of the spin-lattice coupling,which slows down the heat transfer from the spins to the lattices.Finally,the experimental data are well agreed with the calculated one by applying for the 4-Temperature model.We have systematically studied the ultrafast spin dynamics of both post-prepared and annealed DyFe alloy films by the time-resolved magneto-optical Kerr effect,and have found that the demagnetization process is dominated by the competition between the spin-lattice coupling and interatomic exchange interactions.It has enriched our knowledge and understanding for the ultrafast demagnetization mechanism...