Ultrafast Manipulation Of Magnetism And The Dynamics Of Quasi-particles

With the development of modern information technology,the size of the electronic devices composed of conventional semiconductors has reached the nanometer scale,closing to the limits of Moore's Law.Further reduction in device size and power consumption will be limited by quantum effects.To overcome these,spintronics is proposed to store and process information by utilizing the spin degree of freedom,which is expected to be the next-generation information storage technology.In the emerging field of spintronics,high speed of information manipulation and storage is also required as in a typical example,magnetic random access memory devices.The emergency of femtosecond pulsed laser provided the possibilities of fast manipulation of magnetism due to its ultrafast time-resolution of hundreds of femtoseconds.In 1996,E.Bearurepaire et al.reported for the first time that the magnetization can be reduced within a few picoseconds by femtosecond pulsed lasers.This finding opened up a new area of femtosecond magnetism.A large number of studies have been reported so far on the photo-controlled magnetic phenomena as well as the corresponding physical mechanisms in various materials.However,there still exist controversies in the proposed physical mechanisms,and some are even contradictory.In addition,there are still many technical problems for the commercial applications of the spintronics devices.Therefore,the study of the fundamental and practical limits on the speed of manipulation of the magnetization direction is obviously of great importance for magnetic recording and information processing technologies.In this thesis,the ultrafast pump-probe technique is employed to mainly study the spin dynamic behavior within nanoseconds in different types of ferromagnetic materials,such as the manganite perovskite La0.7Sr0.3MnO3 and the 3d-transition metal CoFeB thin films.In addition,considering the correlation of carriers and quasi-particles in materials,studying the spin behavior in different material systems will inevitably involve other quasi-particles,such as polarons,phonons,and excitons.Therefore,we also investigated the dynamics of different quasi-particles in this thesis,which is helpful to indirectly understand the underlying physics of spin manipulation.Specifically,the contents of this thesis can be summarized as the following aspects:(1)The high-quality magnetic complex oxide La0.7Sr0.3MnO3 prepared by laser pulse deposition method is studied.By using the time-resolved magneto-optical Kerr effect method,the ultrafast demagnetization process is revealed to be strongly dependent on the orbital orientation of La0.7Sr0.3MnO3 thin film.The demagnetization process along different spatial direction can be accessed via tuning the polarization angle of probe beam and rotating the sample.By doing so,the transient magnetization is controlled to be increased or decreased within sub-picoseconds.Taking the anisotropic spin-orbital interaction into consideration,we provided a clear physical picture to explain the ultrafast orbit-oriented manipulation of magnetization.(2)The transition 3d ferromagnetic metal CoFeB thin films prepared by magnetron sputtering method were studied.We studied the effects of ultrafast lasers heating on the magnetization dynamics of both the perpendicular and in-plane magnetic anisotropy films.The results show that for the in-plane magnetic anisotropy CoFeB film,the intrinsic damping factor increases significantly with the increase of transient thermal effect,while the effective demagnetization field is hardly affected by the laser thermal effect;for the CoFeB film with perpendicular magnetic anisotropy,the intrinsic damping factor remains stable with the fluence of the laser.On the contrary,the effective magnetic anisotropy field decreases with the increase of the laser fluence.These results indicate that the transient thermal effects of pulsed lasers on thin films with different thicknesses are rnot the same and we need to treat them differently.(3)This part mainly studies the dynamic processes of several quasi-particles other than spin.The involved materials are magnetic oxide La0.7Sr0.3MnO3 and monolayer-layer transition metal sulfide MoSe2.In La0.7Sr0.3MnO3 thin films with different thicknesses,we found that the polarons caused by the Jahn-Teller effect have a coupling behavior with the acoustic phonons generated by intensive optical excitations.In specific,as the absorption of polarons increases,the amplitude of acoustic phonons is reduced.In monolayer MoSe2,We used time-resolved reflectivity spectroscopy to study the exciton dynamics in detail.The results show that the relaxation processes corresponding to different time scales are mutually coupled.For example,in the process of exciton-exciton annihilation,it is also necessary to consider the contributions of trapped excitons by the defect states.We also found that,by changing pump fluence,the coupling strength between the two relaxation channels can be adjusted.When the excited exciton density is high enough,the occupied defect states will be saturated.So that defects states cannot facilitate the exciton-exciton annihilation and the correlation between them disappears.