Ultrafast Spin Dynamics In Magnetic Thin Films

Extensive experimental and theoretical studies have been carried out in photo-induced ultrafast spin dynamics because of its importance in both science and technology.However,the mechanism of the ultrafast demagnetization process has not been clarified yet.An optical pump probe with MOKE(magnetic optical Kerr effect) capability is used to investigate the ultra-fast demagnetization phenomenon.In the research work here,several magnetic systems are chosen to study ultrafast spin dynamics.The dissertation mainly includes the following contents.Firstly,ultrafast spin dynamics in TbFeCo and TbFe alloy films is studied as a function of the Tb content and pumping fluence.For specific TbFeCo film with low fluences,the ultrafast demagnetization process only consists of one fast stage with the demagnetization time of about 500 fs.As the pumping fluence is enhanced,a slow stage occurs,in addition to the fast stage,and the demagnetization time is increased. At a fixed pumping fluence of 1.0 mJ/cm~2,only the fast stage occurs and the demagnetization time is as short as about 500 fs for low Tb contents.At 21%Tb,the fast and slow stages occur and the demagnetization time reaches a maximum of 3.0 ps. Similar results are also observed for TbFe film.The long demagnetization time is suggested to arise from the giant magnetostriction effect in Tb-based fills.After post-annealing,the demagnetization time is reduced due to a reduction of the magnetostriction coefficient.Secondly,for FeCo alloy thin films with Ag,Cu,Pt,Ta,and Cr as heat sink layers,ultrafast demagnetization and recovery processes of transient magnetization have been studied by time-resolved magneto-optical Kerr effect.For all samples,the ultrafast demagnetization process is accomplished within almost the same time interval of 500 fs,which is independent of materials of heat sink layers and pump fluences.With heat sink layers,the recovery rate is enhanced,compared with that of FeCo grown on Si(100) substrate.It is also found that the recovery rate is independent of heat sink layer thickness and decreases with increasing pump fluence.Among all heat sink layers,the sample with Cr layer achieves the highest recovery rate because of the same BCC structure as FeCo layer and small lattice mismatch.For sample with Ta layer,it has the largest damage threshold of pump fluence because of the highest melting point.Thirdly,photo-induced magnetization dynamics in FePt/CoFe bilayer is investigated by optical pump-probe measurements.The transient Kerr loops exhibit an instantaneous separated switching behavior due to exchange decoupling between FePt and CoFe layers induced by photoexcitation,which subsequently recover to the original uniform switching at a certain delay time of 10-20 picoseconds.By analyzing the temporal dependence of magnetization ratio of FePt to CoFe and coercivity of FePt phase,we infer that the decoupling and slow magnetization recovery phenomena are ascribed to the laser-created spin disordering mainly in FePt layer,while the following onset of interfacial coupling results from spin/lattice cooling process.In the study,we also found the proper thickness of CoFe layer to investigate transient decoupling phenomena.