Band Structures And Spin-dynamics In Ferroelectric Rashba Semiconductor GeTe

A plenty of unique phenomenon in magnetic nanometers structures are caused by Spin-orbit coupling effect.Due to the inversion symmetry broken,the Rashba effect exists at the surface of materials or interface of heterostructures.Ferroelectric semiconductors,breaking the bulk inversion symmetry,combine with the strong spin-orbit coupling,which induce a bulk Rashba spin split effect.As one of ferroelectric semiconductors,GeTe owns the largest Rashba parameters so far,which makes it possible in spintronic application.However,to integrate such a ferroelectric semiconductor film into microelectronic devices requires large bulk Rashba parameter to maintain their robust properties when the material is reduced to only several nanometers in thickness.Besides that,several related works have proved that the hybridization band structures exist between Fe and the surface states of GeTe in the heterostructure of Fe/GeTe.This hybridization suppresses the surface states of GeTe,and its influence on magnetic dynamics has remained elusive.Due to the self-doping of GeTe,only a tiny part surface states are filled below Fermi surface.To investigate the properties of surface states,the Fermi level should be adjusted.Therefore,we use molecular beam epitaxy（MBE）to grow GeTe thin films,then we use Angle-resolved Photoemission Spectroscopy（ARPES）and In-plane Vector Network Analyzer ferromagnetic resonance（VNA-FMR）to study the Rashba effect and spin dynamics with the relation of the thickness,doping concentration.It mainly includes the following parts:1.The thickness dependence band structure of GeTe thin films were characterized by ARPES.We found that the bulk Rashba is shrink with decreasing the thickness of GeTe thin film.However,as the thickness of GeTe is 5.0 nm,its Rashba parameter is about 2.12 e V（?）,which is larger enough compares with the other materials.This property is helpful to integrate such a ferroelectric semiconductor film into microelectronic devices.At the same time,the variation of Rashba parameter with thickness satisfies the scaling law.The critical exponents extracted from the fitting results by the scaling law is the identical with the result of 3D Heisenberg Model,which means the ferroelectric polarization in GeTe is a short-range correlation.Finally,we got the bulk Rashba effect can exist until the thickness of GeTe is thicker than 2.1±0.5nm and exposed the reason of the variation of Rashba parameter with thickness.2.The Fe thin film with 10 nm was deposited on GeTe（30 nm in thickness）.Via FMR experiment,we found that the damping of Fe（10 nm）/GeTe（30 nm）has a six-fold symmetry.Then we use Cu intercalated layer between Fe and GeTe to separate the interfacial coupling.The damping of Fe/Cu/GeTe has six-fold as well.The damping of Py/GeTe shows the six-fold symmetry.Finally,by excluding the extrinsic effect,the origin of anisotropic damping is from GeTe layer.Due to the thickness of GeTe is larger than its spin-diffusion length,therefore,the effective spin-mixing conductance2)^↑↓_eff of GeTe is the real part of2)^↑↓,and it shows a six-fold symmetry as well.More interestingly,the anisotropy spin-mixing conductance comes from the anisotropic band split that induces the anisotropic damping,and the anisotropic damping is universality for any kind of ferromagnetic metals.3.The different Bi concentration in Ge_1-xBi_xTe（GBT）thin films were fabricated by MBE,then their band structures were measured via ARPES.We found that the Fermi level moves to the top of valanced band with elevating Bi concentration.At the same time,more and more parts of the surface states of GeTe cross over the Fermi level and the split size of the Rashba states both the surface and the bulk are almost unchanged with increasing Bi concentration.Next,Fe thin films were deposited on the GBT thin films.By using FMR and density functional theory calculation,we found that there exist the hybridization band structures between Fe states and the surface states of GeTe and the local damping of Fe/GeTe bilayer is maximum at x=0.03.Except for the local damping,the enhancement damping caused by spin-pumping effect reaches to the maximum value as x=0.03 as well.At the same time,when x=0.03,all the bulk states are below Fermi level and the density of states of surface states is largest in all GBT samples.Combined with ARPES results and FMR,the damping caused by spin-pumping effect is related to the density of states of surface states.Finally,via the measurement of magnetic transport,we found that with increasing the concentration of Bi element,the transport behavior transforms from the bulk states to the surface states.Our works give some new methods to adjust magnetic damping and open the window for understanding the magnetic dynamics.