Design And Commissioning Of Aberration Corrected Spin Polarized Low Energy Electron Microscope

With the rapid development of information technology,people have put forward new demands and challenges for data storage density as well as reading/writing speed.As one of the most important storage media,magnetic storage plays an irreplaceable role.The development and innovation of magnetic storage technology is always accompanied by the breakthroughs in basic physical problems.Taking magnetic hard disk as an example,the magnetic heads based on the giant magnetoresistance greatly increases the sensitivity of data reading,and thus,increases the storage density to Gbit/in~2.The perpendicular recording media further improves the storage density,but it is still subject to the superparamagnetic effect and thus difficult to break through Tbit/in~2.To further increase the storage density,it is necessary to deeply understand the magnetic physics behind it.At the same time,magnetic topological structures at nanoscale,including magnetic vortices,skyrmions and magnetic domain walls are considered to be next generation storage candidates,such as the reported Racetrack Memory.Among them,especially skyrmions attract much attentions due to its small physical size,topological protection and low manipulation current.But how to stably obtain and manipulate skyrmions with size less than 30 nm at room temperature and zero-field environments is still an open issue in this field.Therefore,an in-depth understanding of the physical mechanism of nano-magnetic topologies is an important foundation for magnetics applications,which requires advanced high-resolution imaging methods for material analysis and device characterization.Spin-polarized low-energy electron microscope(SPLEEM)is a unique and powerful tool with monoatomic magnetic sensitivity and nanometer magnetic resolution,which are widely used in surface science and low-dimensional magnetism,including spin reorientation transition,anti-ferromagnetic interlayer coupling and magnetic chiral domain wall.The magnetic resolution of the spin-polarized low-energy electron microscope was limited in the past mainly due to the aberration of the objective lens,the brightness and spin polarization of the electron source,which is on the order of 10nm.Further improving its magnetic spatial resolution to 3 nm will provide an important insight for the in-depth understanding of nanoscale magnetic topologies such as skyrmions.An aberration-corrected low-energy electron microscope system based on mirror corrector can stably obtain a resolution of better than 1.8 nm,and a back-illumination transmission superlattice spin-polarized photocathode can achieve a spin polarization up to 90%and a laser focusing spot down to 1.5?m.As high brightness spin-polarized electron sources and aberration correction are now available,it is possible to design an aberration corrected SPLEEM which a resolution around 3 nm at low landing energies is expected.This thesis mainly introduces the design and commissioning process of aberration-corrected spin-polarized low-energy electron microscope(AC-SPLEEM).By combining high-brightness spin-polarized electron gun and aberration corrector,we first realized a magnetic resolution of 3.3 nm.We have completed the design and construction of a high-brightness spin-polarized electron gun,a spin polarization of92%and a sharp optical focus with FWHM below 1.5?m have been achieved in this system.These parameters are important guarantees for obtaining high-brightness and high-spin polarized electron sources.To ensure the quality of the beam during the spin manipulation,the machining and alignment error of the octupole spin manipulator needs to be less than 30?m,and the electron beam needs to be focused into the center of the octupole spin manipulator.The coupling between spin polarized electron gun and low energy electron microscope system is accomplished by three magnetic lenses.The optical design needs to meet the key requirements are the axial and field rays from the cathode are symmetric and anti-symmetric in MPA1(and MPA2),respectively.It is hard to achieve achromatic transport based on the present setup.However,this is not an issue due to the fact that the energy spread of the Ga As-Ga As P photocathode is small enough(<0.24 e V)so that the performance of AC-SPLEEM is not degraded.During the alignment procedure,we installed a fluorescent screen on the image plane above the first magnetic prism array to monitor the focus status of the electron beam,and to optimize the excitation ratio of three magnetic lenses for the correct size and opening angle.Finally,we successfully completed the mechanical and optical coupling of the two systems,and realized the aberration-corrected spin polarization low energy electron microscope for the first time.The system can deliver a spatial resolution of 2.85 nm in Graphene/Si C(0001)system.The magnetic performance was tested on Fe/W(110)system,with a spin asymmetry up to 17%and magnetic spatial resolution of 3.3 nm.We analyzed the two-dimensional magnetization vector of Fe nano-islands and observed the magnetic vortex core indirectly by using SPLEEM for the first time.We observed the metastable state with double vortices in same circularity in one nanoscale Fe island,and the micromagnetic simulation indicates the island was formed as two isolated islands merging during annealing,while keeping their original magnetic states.Meanwhile,we have initially completed the construction and test of the time resolved setup based on pump-probe technique,with a magnetic spatial resolution of about 7 nm.Finally,we have discussed the possibility for using slow electrons to image ultrafast laser field in our system.The system will be the unique state of the art ultrafast spin polarized low energy electron microscope,with is a five-dimensional microscope integrating three-dimensional space,time and spin.It will open a new direction in the field of in-situ and high-resolution surface physics and chemistry.