Magnetization Dynamics In Magnetic Films And Spin Transport In Topological Materials

The study of magnetic dynamics in magnetic nanomaterials has always been an important direction in magnetism research.In spintronics devices based on the magnetic moment,magnetic relaxation process determines the magnetic moment switching speed and the critical driven current density.At the same time,the development of spintronics makes it possible for information processing and storage device based on electron’s spin.One of the key technologies is the generation and detection of spin current.The key technology in the materials science involves the transformation between the spin and charge current.Understanding the interconversion between the spin and charge flows is very important for exploring new applications of low-power devices based on pure spin current.In this thesis,the well-known ferromagnetic resonance technology is used to characterize the dynamics of magnetic nano-films,including adjustment of damping constants and the anisotropic behavior research.At the same time,topological materials——topological superconductor and ferroelectric Rashba semiconductor have been successfully prepared by using the molecular beam epitaxy technology,and their spin transport properties have been characterized in detail.It mainly includes the following contents:1.The magnetization dynamic properties of the obliquely grown Fe Ga films were characterized by ferromagnetic resonance technique.For the Fe Ga samples with oblique deposition,the damping factors show anisotropic behavior,which is contrary to the first-principles calculation considering lattice stretching.Based on the dependence of the resonance linewidth on the applied magnetic field directions,we find that the two-magnon scattering caused by the film defects also plays an important role in the magnetic relaxation process.The uniaxial anisotropy generated by oblique growth mode induces a two-fold twomagnon scattering channels,and the intensity of which increases with the increase of oblique angle.At the same time,the magnetocrystalline anisotropy determines the area including degenerate magnon modes,as well as the intensity of the four-fold two-magnon scattering corresponding to the crystal axis.2.The spin dynamics of epitaxial Fe and polycrystalline Py thin films were studied by angle-resolved and frequency-swept ferromagnetic resonance.Due to the existence of strong anisotropic field,the magnetic drag effect is caused by the field-magnetization misalignment.However,in ferromagnetic resonance measurements,this can lead to the nonlinear dependence of resonance linewidth on frequency,which leads to the false anisotropy of the intrinsic damping.After considering the contribution from magnetic drag effect,the nonlinear behavior of resonance linewidth vs.frequency can be fully fitted.3.The topological superconductor β-Pd Bi2 and Fe/β-Pd Bi2 heterojunctions were successfully prepared by molecular beam epitaxy.The detailed spin transport parameters of β-Pd Bi2 at room temperature were characterized by the spin pumping excited by ferromagnetic resonance and the inverse spin Hall effect.The spin Hall angle and the spin diffusion length are 0.037 and 1.76 nm,respectively.The spin-to-charge conversion efficiency is an order of magnitude greater than reported values of conventional superconductors,and is comparable to those of the best heavy metals and topological insulators.In addition,we found that the spin-polarized surface states of β-Pd Bi2 can be coupled with the adjacent ferromagnetic layer by using spin pumping measurements at variable temperature,which can induce another interfacial anisotropy in the ferromagnetic layer.The coupling effect was greatly enhanced at low temperature,and the effective spin to charge conversion efficiency of β-Pd Bi2 was greatly reduced.4.Ferroelectric Rashba semiconductor α-Ge Te film was successfully prepared by molecular beam epitaxy.Angle resolved photoemission spectroscopy(ARPES)shows that α-Ge Te has both bulk and surface Rashba states.We find a nonreciprocal charge transport behavior up to room temperature in α-Ge Te.We combine the band structure measurements and theoretical calculations showing that the nonreciprocal response is due to the giant bulk Rashba spin splitting rather than the Rashba surface states.Notably,we find that the size of the nonreciprocal response shows an unexpected non-monotonic dependence on temperature.Based on the extended theoretical model of second-order spincharge conversion,we can establish the relationship between the nonreciprocal magnetoresistance and the spin structures in the Rashba systems.