Investigations For Ultrafast Magnetic Dynamics In Ferromagnetic Thin Film And Heterojunctions

The development of ultrafast pulsed laser technology,has opened a new field for magneto-optical experiments-ultrafast magnetic dynamics,and this detection and manipulation of magnetic moments at the femtosecond and picosecond scales provide a new approach for future research and exploration of high-frequency magnetic devices.As early as 1996,E.Beaurepaire at el.observed laser-induced ultrafast demagnetization at the femtosecond scale,and later Koopmans et al.proposed and realized the possibility of using femtosecond lasers to excite nanosecond-scale spin induction and detect spin waves in magnetic thin films,which provides new ideas for spin detection and research.With more than two decades of development,there have been many studies on the rich physical mechanisms behind pulsed laser-induced ultrafast demagnetization.From the perspective of local mechanisms,the key to spin dynamics behavior at the sub-picosecond scale is the transfer of energy and angular momentum between the three thermodynamic reservoirs,spin,electron and phonon(lattice).And the coupling between the degrees of freedom is particularly critical with varying coupling strengths and time scales.For 3d ferromagnetic metals,the widely recognized mechanism is the phonon-or impurity-assisted spin-flip mechanism,where electron-phonon coupling induces spin-flip under the spin-orbit coupling effect.Throughout the process,the role of the lattice is critical and complex.According to Elliott-Yafet et al.the fast quenching of the magnetic moment is dominated by the action of electron and phonon,i.e.,the lattice,while many previous studies considered spin-lattice scattering as the main channel of angular momentum dissipation.On the other hand,femtosecond laser also induces magnetic relaxation on nanosecond time scale,i.e.,magnetic precession.The important parameter,namely damping factor,represents the spin-lattice interaction at nanosecond time scale.Therefore,the role of the lattice in the magnetic dynamics at both scales is significant,but the specific contribution especially the weight of the electron or spin coupling in the demagnetization needs to be further explored.Also,in magnetic materials,due to spin-orbit coupling and crystal field,there exists magnetocrystalline anisotropy,i.e.,the magnetic moment arrangement is spatially direction-dependent,and this lattice-induced anisotropy may be reflected in the coupling effect.Recently,a few experiments have been conducted to show the existence of anisotropic damping in ferromagnetic metals and their alloys.The study of the directional dependence at the sub-picosecond scale is still blank,and the exploration of this issue can further to deepen the understanding of the role of the lattice in ultrafast demagnetization processes.On the other hand,in 2010,M.Battaio et al.proposed the nonlocal superdiffusive spin transport leading to ultrafast demagnetization model.Subsequently in experiment,it was observed that hot electron spin current can dominate ultrafast demagnetization and can cause a short-lived increase in the magnetic moment of neighboring nonmagnetic or magnetic layers,and even excite terahertz waves through spin-charge transitions in strongly spin-orbit coupled materials.This all indicates that nonlocal ultrafast spin current does exist in ferromagnetic metals.The injection and dissipation of the spin current are closely related to the nature of the non-magnetic layer at the interface and the non-magnetic layer,which affects the lifetime and efficiency of the spin majority and minority,and may affect the formation of the ultrafast spin current,which in turn affects the demagnetization process.And in ferromagnetic/nonmagnetic heterojunctions,the properties of interface affect the damping factors,based on spin pumping effect,magnetic proximity effect or chemical reactions at the interface.For some novel quantum materials,such as topological insulators(TI)with linear dispersion relations-topological surface states(TSS),it is expected that what kind of complex interfaces will be formed to induce interesting physical phenomena when combined with ferromagnetic metals.In recent years,many peculiar phenomena such as injected quantum spin Hall have been found in FM/TI,while for magnetic dynamics it is indeed found that TI can enhance damping but the mechanism remains to be studied,and the effect of TI on sub-picosecond scale demagnetization of ferromagnetic layers has not been reported yet.Based on the above,it mainly includes the following content:Firstly,the topological surface state enhanced magnetic dynamics in Fe/Bi₂Se₃heterojunction and its mechanism is investigated using Time-Resolved Magneto-Optic Kerr effect(TRMOKE).In the Fe/Bi₂Se₃(t QL)(Quintuple layer,1QL=0.95 nm)heterojunctions,the presence of topological insulators results in the faster demagnetization,enhanced damping and large perpendicular interface magnetic anisotropy.There exists a jump at at 6 QL of Bi₂Se₃ for both the demagnetization time and damping factor.Angle-resolved photoelectron spectroscopy and band energy calculations show that Bi₂Se₃ has no topological surface states below 6 QL,while topological surface states appear at 6 QL and above.By carefully analyzing the dynamic properties of three typical samples of Fe/Cu(0,5 nm)/Bi₂Se₃(3,9 QL),it is found that TSS not only enhances the dynamic properties for Fe including the damping and ultrafast demagnetization through the spin pumping effect,but also by interfacial coupling effect.Temperature dependent experiments show that the magnetic damping factor of Fe/Bi₂Se₃(9 QL)is significantly enhanced below 150 K and has the same trend as the perpendicular interfacial magnetic anisotropy,while the damping and interfacial anisotropy constants do not change with temperature in Fe/Bi₂Se₃(3 QL)without TTS and in samples with Cu-isolated Fe in contact with TSS.This suggests that the temperature dependence of the dynamic enhancement effect is due to the interfacial coupling effect between TSS and Fe.Most importantly,the damping of Fe/Bi₂Se₃(9 QL)is linearly related to the perpendicular interfacial magnetic anisotropy constant,while both are closely linked to the spin-orbit coupling.Theoretical calculations show that strong energy band hybridization of TSS with Fe occurs at the Fermi plane in 6 MLFe-Bi₂Se₃(9 QL).Therefore,we consider that the enhanced ultrafast spin dynamics by TSS originates from the femtosecond laser-induced super-diffusive spin current,and the strong energy band hybridization of Fe and Bi₂Se₃ at the Fermi plane induced the enhanced interfacial spin-orbit coupling.Our work not only reveals the mechanism of multiple spin current dissipation channels in Fe/Bi₂Se₃,providing new insights into the mechanism of TSS enhancement dynamics and enriching the study of nonlocal ultrafast demagnetization in Fe thin films.And it provides a new perspective for the research of magnetic dynamics based on topological materials.Secondly,the Fe(110)/Mg O(111)single-crystal thin film was chosen to study the magnetic damping factor and ultrafast demagnetization under the magnetic field along the different in-plane orientations using TRMOKE,which was found to exhibit six-fold symmetry in demagnetization time and damping factor.Compared with the instantaneous reflectivity experiments,the electron-phonon coupling is isotropic,excluding its directional variability,indicating that the anisotropic ultrafast demagnetization is related to the directional dependent spin-lattice coupling.This directional-dependent spin-lattice interaction is still reflected in the nanosecond scale,where anisotropic damping appears.Theoretically,both ultrafast demagnetization and damping factor are related to the band structure and spin-orbit coupling.We speculate that the anisotropy of the magnetic dynamics originates from the energy band structure of the six horns at the Fermi plane of Fe(110).The experimental results confirm the anisotropy of demagnetization and damping in 3d metal,clarifying the key role of spin-lattice coupling,and filling the blank in the field of direction-dependent ultrafast demagnetization research.Thirdly,we have successfully probed the room-temperature antiferromagnetic spin dynamics of GFO without spin reorientation transition in Fe/GFO(Gd Fe O₃)(100)heterojunction using all-optical pumping probe technique.Through the interfacial exchange coupling between the net magnetic moments of GFO and Fe thin film,multiple dynamic modes of GFO,including phonon mode,quasi-ferromagnetic mode,and impurity mode,were detected at low magnetic fields.In addition,we have studied the excitation efficiency of the antiferromagnetic dynamics.First,the relative alignment of the Fe magnetic moment and the net GFO magnetic moment(antiparallel or parallel)at the interface can be simply modulated by the magnitude of the external magnetic field,but unfortunately the interfacial magnetic configurations do not affect the excitation amplitude of each mode,because the strong antiferromagnetic interaction in the GFO and the stable antiferromagnetic exchange coupling at the interface make the spins at the interface insensitive to the external field.However,the excitation amplitude of the antiferromagnetic dynamic mode can be modulated by the pump laser fluence.The excitation efficiency of the photon modes becomes larger with higher influence,which is attributed to the strong lattice vibrations due to the large optical absorption.The excitation efficiency of the quasi-ferromagnetic mode becomes larger and then decreases,which is attributed to the influence of the large light absorption on the anisotropy of the ferromagnetic moment,and then the induction of the ferromagnetic moment itself becomes more intense,which regulates the interfacial exchange coupling and finally makes the induction of the net magnetic moment of the GFO larger.However,the further strengthening of the fluence may cause the disorder of the magnetic moment or the redistribution of the charge at the interface,which finally affects the induction of the GFO layer.In conclusion,we can simply use the laser power to modulate the excitation efficiency of antiferromagnetic dynamics.Besides,the temperature-dependent magnetization flip behavior of the Fe/B-LSMO(La_2-2xSr_1+2xMn₂O₇,x=0.35)heterojunction is investigated using the magneto-optical Kerr effect.When the B-LSMO is in the ferromagnetic(FM)state,there is a great enhancement of the coercivity for the Fe film and exists a jump at the PM to FM transition point of B-LSMO.It is also interesting to note that the small Kerr loop shows a positive exchange bias phenomenon,revealing an antiferromagnetic exchange interface exchange coupling between Fe and B-LSMO,which is attributed to the superexchange interaction between the Fe and Mn O₂ layers.Based on the competition between the antiferromagnetic interface exchange coupling and anisotropic fields,the distinct magnetization reversal behavior in the easy and hard magnetization directions is discussed in an phenomenological method.We believe that the discovery of the antiferromagnetic interface exchange coupling between 3d ferromagnetic metals and bilayers of Mn O₂ will provide new ideas for materials with artificial magnetic coupling and provide insight for the design of new spintronic devices.