Dynamics of Laser-excited SrRuO3 and Spin Current-pumped FeCoB Films

Author:Wang Kang

Supervisor:wu xiao shan


Degree Year:2019





Transient changes in electronic states,lattice and magnetic structures of functional materials,can be revealed by well-developed pump-probe techniques.The time resolution of pump-probe techniques can reach picosecond,and even faster.This leads it to be possible to gain microscopic understandings on macroscopic properties that cannot be fully understood through static studies,and to explore potential applications of functional materials in devices.In this thesis,we study dynamics of electron and phonon in SrRuO3,and evolutions of chiral magnetic structures in heavy metal/ferromagnetic multilayer stacks,through different pump-probe techniques.The main contents of the thesis are summarized as follows:(1)SrRuO3 attracts a lot of attention because of its typical properties including itinerant ferromagnetism and non-Fermi liquid behavior at high temperature,etc.Photo-induced strain above 1%has been observed in SrRuO3 films with the optical excitation intensity above 60 W cm-2,suggesting the great potential of SrRuO3 in applications of optomechanical and optoelectronic devices.Photoexcitation and energy transfer via electron-phonon scattering have been studied,but recent studies inferred that more processes that remain to be identified contribute to the light-induced deformation.We image excitation and relaxation characteristics of SrRuO3,through two complementary methods that are ultrafast X-ray diffraction and ultrafast optical reflectivity measurements.Upon photoexcitation,photo-carriers redistribute both in space and energy,and then relaxation with fast direct recombination,phonon-assisted relaxation and phonon-phonon scattering dominating on a longer time scale.The phonon-assisted relaxation process,together with the thermal effect resulting from the electron-phonon scattering,contributes to the photo-induced strain in SrRuO3.The bottleneck effects of phonons on relaxations are proposed and demonstrated by temperature dependent measurements.The fast carrier relaxatio supports a scaled behavior depending on the electron correlation.The scaled relationship for SrRuO3 shows a divergence from that derived from the Fermi-liquid model,giving a hint that phonon effect occurs in the bad-metal region of SrRuO3.(2)The heavy metal/ferromagnetic multilayer system with perpendicular magnetic anisotropy(PMA)has been attracting much attention in past few years.In the vicinity of the spin reorientation transition from the perpendicular magnetization to in-plane magnetization,labyrinthine phases appear,wherein multiple topological defects and magnetic skyrmions are present.Because of the non-zero topological charge densities of topological defects,we expect their typical responses to the spin current,therefore causing transitions of magnetic domain phases.Using the polar magneto optical Kerr effect microscope,we observe in a prototypical multilayer system of Ta/CoFeB/MgO a spin current-driven transformation of an initial labyrinthine domain pattern.The labyrinthine domain phase can either transform into a dense array of skyrmions,or simply has a change in the orientational order,depending on the strengths of the applied spin current pulse and the external magnetic field.We conclude both from experimental studies and micromagnetic simulations that the applied spin current leads to the generation/deletion of topological defects through modifying the motion of domain walls and the magnetic configurations within the domain walls,leading to the observed transformations.(3)A train of skyrmions information with codes "1"/"0" defined by existence/absence of a single skyrmion has been proposed to be digital information carriers in spintronic devices.However,correcting or revising skyrmions information in a controlled manner has not been studied so far as we know,which is of great significance and high-demand for future skyrmion-based data storage and memory systems.In our work,we propose and demonstrate through micromagnetic simulations studies an effective and experimentally feasible method to modify a train of skyrmions information by a low spin polarized current.We use a "generator" to write skyrmions information,a "collector"to detect it,and a "controller" to correct a single skyrmion.In studying the skyrmion manipulation processes,we have observed and further described the skyrmion’s gyrating motion(mode)and its breathing mode.The dynamics is analysed via a Thiele equation derived from the micromagnetic model.From the equation,the gyration mode of a skyrmion is suggested to be coupled to the breathing mode dynamics.