The Investigations For Microscopic Mechanism Underlying Femtosecond Laser-induced Ultrafast Demagnetization In Magnetic Heterostructures

Since the pioneering work on ultrafast demagnetization of Ni thin film after femtosecond laser irradiation was demonstrated in 1996 by Beaurepaire et al,the quest for ultrafast modification of the magnetic moments has triggered a new field of research:Femtomagnetism.Later,the all-optical switching of GdFeCo thin films taking place in less than a few picoseconds was observed in experiment.On the other hand,the demonstration of laser induced ultrafast spin current can be used to induce magnetic domain switching as well as the high frequency spintronic devices.Nevertheless,the microscopic mechanism underlying ultrafast quenching of magnetization remains elusive.Some pronounced contradictions make the mechanisms of ultrafast demagnetization further complicated.For instance,Zhang et al proposed that the spin-orbit coupling plays a major role in determining the ultrafast demagnetization rate.Also,the experimental evidence has been demonstrated by comparing the ultrafast demagnetization time in Co and Co/Pt bilayers by Koopmans et al.In contrast to the results in these reports,S.Iihama et al found that the demagnetization time in Ni was less than that in FePt and FePd.Moreover,Eschenlohr et al demonstrated the local spin flip mechanism in Ni thin films,which is in contract to the theoretical prediction by M.Battiato et al in Ni.In addition,both the local spin flip scattering mechanism and non-local spin transport mechanism were proposed respectively to account for the non-equilibrium hot electrons induced ultrafast demagnetization.It is harmful for clarifying the underlying ultrafast demagnetization mechanism in the metallic heterostructures.Therefore,an effective method to distinguish the two dominant contributions to ultrafast demagnetization in metallic heterostructures is highly desirable.Here,we propose that investigating both the ultrafast demagnetization time and Gilbert damping simultaneously is a candidate method,although the relationship between the two parameters has never been unified successfully so far between the experiments and theoretical predictions.An inverse relation between ?_M and ?intr was first derived by Koopmans et al.from a quantum-mechanical calculation on the basis of the Elliott-Yalfet(EY)spin-flip scattering model.Later,the attempted experiments have ever been carried out todemonstrate the prediction in rare-earth-doped permalloys and amorphous TbFeCo films.In this case,the localized 4f electrons rather than itinerant 5d6s electrons domain most of the large magnetic moment in rare-earth elements.Because the 4f electrons are far from the Fermi level,their ultrafast demagnetization processes are medicated by 5d6s electrons after laser pulse excitation.The indirect excitation leads to the so called type_? ultrafast demagnetization behavior in rare-earth elements,which is much slower than that of itinerant electrons.Therefore,it is not unexpected that the ultrafast demagnetization time ?_M of permalloys increases with the doping contents of rare-earth elements increasing.Meanwhile,it happened that the Gilbert damping constant of permalloys is also increased by doping 4f elements,which mainly comes from the so called "slow relaxing impurities mechanism".Therefore,by introducing the extra mechanism unavoidablely,a trivial consequence was obtained that the ultrafast demagnetization time ?_M increases as the Gilbert damping a increases in rare-earth-doped permalloys.In hindsight,from this experiment,one can not confirm the relation between ultrafast demagnetization time ?_M and Gilbert damping ? due to the defects of the experimental design.A genuine relation between ultrafast demagnetization time and Gilbert damping should be explored in a clean system without extra demagnetization mechanism.So far,the explicit relationship between the two parameters has never been unified successfully between the experiments and theoretical predictions.More importantly,the Gilbert damping dominant spin precession is in the timescale of nanosecond timescale,in which the spin-lattice coupling determines the relaxation time.For a long time,the scientists think this as the time limit to tailor the magnetization.However,the femotosecond laser-induced ultrafast demagnetization challenges the general knowledge.In this regard,it is necessary to investigate the relationship between the spin dynamics in these two time scales.Therefore,in this thesis I will mainly focus on the relationship between ultrafast demagnetization time ?_M and Gilbert damping ?,based on which to explore the microscopic mechanism of ultrafast demagnetization.Our works were undertaken within several magnetic heterostructures such as Co/Ni bilayers with perpendicular anisotropy,FeGa/IrMn exchange bias thin films as well as Co/Pt multilayers with nanoscale magnetic domains.Firstly,the fast and ultrafast dynamic properties of Ta(3 nm)/Pt(2 nm)/Co(0.8 nm)/Ni(dNi nm)/Pt(1 nm)/Ta(3 nm)bilayers with the electrons relaxing near the Fermi surface have been investigated by using TRMOKE pump-probe technique.An genuine proportional relationship,contrast to previous trivial consequence induced by impurities mechanism,between ultrafast demagnetization time and Gilbert damping constant is confirmed from experimental results.The estimated value of spin-mixing parameter on the basis of breathing Fermi-surface model is far larger than that of Co or Ni,which is originated from the strong spin-orbital coupling at the interface.More importantly,distinguishing the dominant mechanism underlying ultrafast demagnetization in metallic heterstructures has been a tough task for a long time.Here,an effective method by unification of the ultrafast demagnetization time and Gilbert damping is proposed to solve this task,namely that,a proportional relation between the two parameters indicates the local spin flip scattering mechanism domains,otherwise the non local spin current effect domains.Secondly,both ultrafast demagnetization time ?_M and Gilbert damping a for Fe81Ga19/Ir20Mn80(FM/AFM)exchange bias heterostructure have been achieved using time-resolved magneto-optical Kerr effect(TRMOKE)technique.We found that in contrast to FM/NM/AFM system,where the damping factor increases monotonically and approaches to saturation with the thickness of AFM layer due to the conventional spin pumping effect,the Gilbert damping factor increases firstly to a peak value at tIr20Mn80=2nm,and then decreases with further increasing the thickness of AFM layer.We proposed a model that the precessional motion of the AFM due to the direct magnetic coupling pumps a spin current back into the FM layer to cancel partly the spin pumping by the FM layer to explain this unconventional behavior.On the basis of this model,the calculated damping factors of Fa8iGai9/Ir20Mn80 by the first-principle electronic calculation are consistent well with the experimental results.Furthermore,an explicit linearly proportional relationship,namely ?1/?_M=?s/h?? was derived,where ?1/?_M,?? represent the enhancement of ultrafast demagnetization rate and Gilbert damping induced by the transverse spin current,respectively,and the bridge between the seemingly unrelated parameters ?_M and ? is the spin chemical potential ?s.We concluded that the transport of transverse spin current between FM and AFM layers in FM/AFM plays an additional dissipation channel for accelerating the ultrafast demagnetization.It confirms that,apart from the spin flip scattering and superdiffusive spin transport mechanism which dominates in the bulk systems,the transverse spin current contributes significantly to ultrafast demagnetization in the systems where the spin pumping results in the spin splitting at the interface.Thirdly,in this study,laser induced ultrafast demagnetization dynamics in[Co/Pt]20 multilayers with magnetic domain configurations has been studied using both TRMOKE experiment and atomistic spin dynamics simulations.It is found experimentally that the demagnetization time ?_M keeps a constant value of 150 fs with various magnetic domain structures,justifying that the spin dependent hot electron transport between neighboring domains plays a minor role in ultrafast demagnetization in our samples.Moreover,the experimental evidence for a local spin-flip scattering mechanism,namely,the demagnetization time increases with the laser fluence increasing,is reproduced exactly by an atomistic spin dynamics simulation based on the model of local spin angular momentum dissipation.Via atomistic spin dynamics model,the transversal spin fluctuations mechanism has been demonstrated to be responsible for the ultrafast demagnetization in the case of Co/Pt multilayers with inhomogeneous magnetic structures.This is a significant progress in clarifying the microscopic mechanism underlying the ultrafast demagnetization in the inhomogeneous magnetic structures.