Study On Laser-Induced Ultrafast Spin Dynamics Of GdCoFe Alloys

In modern magnetic storage industry,how to improve the density of magnetic recording medium and to obtain fast reading and writing speed are two important issues.Up to date,the writing speed limit is about 100 picoseconds.Femtosecond(fs)laser-induced spin dynamics provides an effective method to manipulate the spins in the sub-picosecond time domain,which has great potentiality for applications in improving the writing speed of magnetic records.In recent years,the rare earth(RE)-transition metal(TM)ferromagnetic alloys have been considered as one of the potential candidate materials for high-density and low-power magnetic memory devices.RE-TM alloys always exhibit special laser-induced ultrafast spin dynamics since their magnetism originates from both 4f localized electrons and 3d itinerant electrons,which have been attracted extensive attentions of the researchers.So far,the mechanism of magnetization dynamics is still in debatable.Exploring the mechanism of laser-induced energy transferring is a key issue to modulate the ultrafast spin dynamics of the RE-TM alloys,which is also very important to design and develop the ultra-high density and high-speed magnetic memory devices,Therefore,in this paper,the time-resolved magneto-optical Kerr effect(TR-MOKE)system is used to systematically study the laser-induced ultrafast demagnetization and magnetization precession processes with various Gd compositions.The effects of 3d-5d6s-4f exchange coupling between the sublattices on the demagnetization time,precession frequency and Gilbert damping factor are further explored.The specific contents and conclusions are as follows:(i)The fs laser induced energy transferring in antiferromagnetic coupled sublattices of Gd Co Fe alloy is determined by both the intra-atomic and intersublattice exchange coupling.Exploring the effect of the sublattices exchange coupling on ultrafast spin dynamic is a key issue to maniplate the efficiency of energy transferring from 3d to 4f spins.The ultrafast spin dynamics of Gd Co Fe alloy films were studied by TR-MOKE measurements.According to the different components of Gd,there are three modes of laser-induced ultrafast demagnetization:one-step demagnetization,continuous two-step demagnetization and discontinuous two-step demagnetization.The one-step demagnetization time is within several ps,which shows a nonmonotonic variation with x increasing,and reaches the maximum around the magnetization compensation point.The two-step demagnetization only occurs for the samples around the magnetization compensation point,and the demagnetization time of the second step is up to tens of picoseconds.We attribute the different demagnetization phenomena to the establishment of the angular momentum transferring channel between 3d and 4f spins,which could be well explained by combining the spin-flip scattering theory of Elloitt-Yafe type and 4-Temperature model with specific 3d-4f coupling constants.Furthermore,the demagnetization time was observed to be external magnetic field dependent in the two-stage decay case,arising from the magnetic cooling effect.(ii)The laser-induced magnetization precession process of Gd Co Fe alloy films is systematically studied,and the dependence of the precession frequency f,the effective Landes factor g_eff,and the Gilbert damping constant?_s on Gd component is obtained.We observe a rapid attenuation of the precession peaks near the angular momentum compensation point of the Gd Fe Co alloy film,which is related to the strong 3d-5d6s-4f exchange coupling near the angular momentum compensation point leading to a rapid dissipation of energy.The experimental results show that the?_s and g_effincreases significantly near the angular momentum compensation point,which is consistent with the calculation results by Kittel uniform precession theory model.However,the trend of f with increasing x is opposite to those of the previous works,which may be related to the shape effect caused by the columnar growth of Co Fe as Gd increases.In this dissertation,the influences of 3d-5d6s-4f exchange coupling on the laser-induced ultrafast demagnetization and magnetization precession processes were systematically studied,which provides an effective method to manipulate the ultrafast spin dynamics for practical application.