Investigation Of The Antisymmetric Magnetoresistance In Co_1-xTb_x Thin Films

With the advent of the 5G network era,the amount of data created each day on the Internet is proliferating exponentially.According to the IDC’s“Data Age 2025”whitepaper,the global digital data had reached 33 ZB in 2018 and could grow to 175ZB by 2025.The generation of massive data has posed an unprecedented challenge to the speed,density and reliability of data processing and storage.Because of the ther-mal effect and the quantum effect,the traditional microelectronic devices which only utilize the charge degree of freedom have approached the quantum limit.As an alter-native,the spintronics structures combined with the charge current and the spin cur-rent exhibit distinct advantages,such as low power consumption and non-volatility,which offer a new approach for high-speed and mass information processing.In spintronics structures based on magnetic materials,the different saturation magnetization states of magnetic units are usually used to represent the"0"and"1"logic states,respectively.However,if the magnetic domain walls exist in the magnetic structure,the additional electron scattering processes will cause domain wall magne-toresistance,resulting in various"intermediate states"and chaotic logical states.By clarifying the magnetoresistance phenomenon caused by domain wall structures and combining with the control of domain walls by spin polarization current,new types of sensors,magnetic memory devices and logic devices based on domain walls can be developed.On the other hand,the topological insulator based spintronic devices,which use the spin-momentum locked surface states to improve the conversion effi-ciency between charge and spin current,has attracted tremendous attention recently for their possibilities of application as well as fundamental interest.This dissertation presents our investigation of the anomalous electron transport phenomena induced by magnetic domain walls.1.The experimental realization of single domain wall is a key factor for study-ing domain wall magnetoresistance.By delicate controlling of the magnetic field gra-dients,we investigate the antisymmetric magnetoresistance induced by the tiled single domain wall in Co_1-xTb_x thin film by simultaneous transport measurement and domain imaging using the magneto-optical Kerr microscope.Our experiments clearly demon-strate that inhomogeneous magnetization driven by a magnetic field gradient gives rise to highly tunable antisymmetric magnetoresistance in Co_1-xTb_x films.We find that the antisymmetric magnetoresistance obviously arises from the nonuniform current in the vicinity of tilting domain walls.Our theoretical calculations show that the induced nonequilibrium current is basically determined by the geometry factor of the tilting-wall texture.Delicate control of magnetoresistance by properly modulating the geo-metric parameter demonstrated here provides an insight into the single-domain-wall-based spin-transport phenomenon.2.The antisymmetric magnetoresistance induced by domain-wall tilting texture is closely related to anomalous Hall effect.In this part,we investigate the correlation between anomalous antisymmetric magnetoresistance and anomalous Hall effect by varying Tb concentration of Co_1-xTb_x alloy films.We show that the Co_1-xTb_x films exhibit anomalous antisymmetric magnetoresistance due to the tilting DW distribution.The polarity reversal of antisymmetric magnetoresistance is directly dependent on the film composition,and synchronous with anomalous Hall effect.Besides,the variation of anomalous Hall coefficient R_s directly leads to the remarkable change of antisymmetric magnetoresistance.Furthermore,our numerical calculation results show that the value of antisymmetric magnetoresistance exhibits obviously linear re-lationship with anomalous Hall coefficient,and the changes of magnetic structure and electron scattering caused by varying Tb concentration are complex multi-factor pro-cesses.This study hence offers new opportunities for designing DW based spintronics devices.3.In addition to the generation of antisymmetric magnetoresistance,the magnet-ic domain wall can give rise to anomalous thermoelectric effect.We explore the effect of domain wall tilting in Co_1-xTb_x on the thermopower behaviors driven by a gradient magnetic field.We observe a novel antisymmetric thermopotential,which is a ther-moelectric analogue of the antisymmetric magnetoresistance.We find that the anti-symmetric thermopotential leads from the tilting domain wall in Co_1-xTb_x and is pro-portional to the temperature gradient,which can be further enhanced by structure op-timizations.4.We fabricate high quality topological insulators Bi₂Se₃,Bi₂Te₃ and Sb₂Te₃single-crystalline films by molecular beam epitaxy.In the spin pumping experiment of topological insulator/ferromagnetic heterojunction,we observed spin pumping signal arised from the inverse Edelstein effect of the topological insulator surface states.The dependence of the spin pumping voltage on the magnetic field direction is similar to that in the inverse spin Hall effect.We suggest this similarity results from the charge current generated by the inverse Edelstein effect of topological insulators is quite sim-ilar to that generated by the inverse Hall effect in heavy metals,which is overall pro-portional to the spin current and perpendicular to the spin polarization.