Spin Dynamics And Spin Manipulation In Confined Quantum Structures

The rise of spintronics in the late 20th century provided new solid-state hardware implementations for information storage and processing.With the help of "spin",the intrinsic property of electrons which is parallel to "charge",it is hoped that the energy consumption will be greatly reduced on the basis of improving the computing power,while maintaining the character of non-volatility.Another fascinating prospect of spin-based devices is that it may be promising for quantum computing.With the development of ultrafast micro-/nano-optics,one holds an opportunity to understand the key aspects of spin dynamics in real space and time domain.Namely,the efficient generation of spins,the effective transmission and manipulation of spin states,and the precise measurement of spins,could be intuitively imaged with high time-and spatial resolution.In addition to relying on advanced detection techniques,the foundations that limit the performance of spintronic devices are the physical properties of the semiconductors and the structures as well as processing technology of the devices.When reducing the thickness of semiconductor materials with quantum confinement in the z-axis direction,not only the symmetry and energy band structure of the system which induce the generation and transport properties of spin states are inherently changed,but also the three-dimensional electrostatic shielding effect is weakened,that the spin state and related interface coupling effects are more easily to be tuned and controlled by the external field.Constructing such a new type of spin-dependent information processing and sensing heterostructures by quantum confined semiconductor systems is an essential step for quantum technology towards quantum engineering.The key point of this thesis is to develop advanced optical spectroscopic techniques to investigate the spin dynamics and spin manipulation via electrostatic field in both non?magnetic semiconductor GaAs-based heterostructure and the emerging few-layered van der Waals(vdW)intrinsic ferromagnetic semiconductor Cr2Ge2Te6 heterostructures,which provides a reference for spin logic,sensing and storage devices based on confined quantum structures.The main works of the thesis are as follows:1.The home-made time-and spatially resolved two-color magneto-optical Kerr rotation technique is utilized to study the spin diffusion of n-GaAs heterostructure samples.Nonequilibrium spin polarization ensemble is generated by optically oriented spin injection through non-resonant inter-band excitation,and the dynamical evolution of the spin packet is detected by the magneto-optical Kerr effect.It is found that the spins diffuse sub-linearly at low temperature,and gradually linearly with the rise of temperature.In order to explore the effect of dynamical hot electrons on spin diffusion,we develop one kind of ultrafast electron thermometer by using magneto-optical Kerr spectrum.After obtaining the relationship between the intrinsic electron temperature and the characteristic linewidth of the magneto-optical Kerr spectrum under weak excitation,we measure the evolution of the temperature of hot electrons in the same condition with studying spin diffusion.The spatially resolved electron temperature shows that the spatial distribution of hot electrons is quite localized,and its characteristic width ?Te is similar to the optical resolution ?0.Then we confirm that the spin diffusion coefficient of the n-GaAs sample has a linear dependence on the electron temperature Ds?Te.With the help of Te(At)in different temperature and excitation power conditions,we replicate the spin packet diffusion process,and the replications are consistent with the experimental data.2.The home-made time-and spatially resolved two-color Faraday rotation technique is applied to study the laser-induced magnetization dynamics in a few-layered vdW ferromagnetic Cr2Ge2Te6 heterostructure.We measure the hysteresis loops of the few-layered Cr2Ge2Te6 ferromagnets by using static micro-area magneto-optical Kerr rotation and Faraday rotation at different angles between sample and the direction of external magnetic field,and observe the uniaxial perpendicular anisotropy of Cr2Ge2Te6 nanoflakes.With the help of the two-color time-resolved Faraday rotation system,we measure the magnetization dynamics at different angles between sample and the direction of external magnetic field and different external magnetic fields.Analyzing by the linearized Landau-Lifshitz-Gilbert equation,we find that the anisotropic magnetic field of the few-layered Cr2Ge2Te6 nanoflake is ?0Hk=125±8 mT,and the intrinsic Gilbert damping coefficient is ?0=0.006±0.002.In the condition of weak external magnetic field,the effective damping coefficient is dominated by the extrinsic damping.With the enhancement of external magnetic field,the effective damping tends to the intrinsic damping coefficient.We attribute the extrinsic damping to the spatial deviation broadening induced by perpendicular anisotropy.For an estimation,the standard deviation of the perpendicular anisotropic magnetic field is calculated to ?0?Hk=8.0±2.0 mT.3.The prototype device of planar field effect transistor with a few layers of Cr2Ge2Te6 is constructed by dry transfer method.The silicon-based solid gate is fabricated to dope carriers to Cr2Ge2Te6 active layer via electrostatic field.Below the Curie temperature,the spin polarization(net magnetization)is sensitively monitored by using the static micro-area magneto-optical Kerr measurement.In the spin degree of freedom,the saturation magnetization and saturation magnetic field of Cr2Ge2Te6 are manipulated with a bipolar manner respectively.While the magnetism is controlled,the few-layered Cr2Ge2Te6 field effect transistor remains the bipolar switching behavior in the charge degree of freedom.First-principle calculations and micromagnetic simulations show that the spin majority bands of the few-layered Cr2Ge2Te6 is mainly contributed by the d-orbital of Cr atoms in the vicinity of conduction band minimum,and the spin minority is contributed by the p-orbital of Te atoms near the valence band maximum.When doping with either electrons or holes via electrostatic field,whether the Fermi energy moves to the conduction band or to the valence band from band gap,the ratio of spin minority and spin majority is decreased.The special spin-polarized energy band of Cr2Ge2Te6 is an important reason for the bipolar manipulation of spin and charge simultaneously.