| This Ph.D.dissertation involves the electrical transport properties of three-dimensional topological insulator(TI)thin films and the spin-orbit torque(SOT)of CoFeB/MgO multilayer films.In the previous part of this paper,we study the low-temperature electrical transport properties of Bi2Te3,(Bi,Sb)2Te3,and Cr-doped topological insulator films prepared by magnetron sputtering and use the back gate to effectively tune their properties.In the latter part of this paper,we report the perpendicular magnetic anisotropy(PMA)and spin-orbit torque-induced magnetization switching of Zr/CoFeB/MgO thin films and W/CoFeB/Zr/MgO thin films.The main contents are as follows:(1)We use magnetron sputtering to prepare large-area,high-quality Bi2Te3 family topological insulator thin films on the amorphous SiO2.The ARPES data of Bi2Te3 thin films and the bipolar effect of the(Bi,Sb)2Te3 samples indicate that there are topological surface states in these magnetron sputtered polycrystalline films.By gate tuning,the anomalous Hall resistance of Cr-doped(Bi,Sb)2Te3 films reaches 16.4 kΩ,which is about 60%of quantum anomalous Hall resistance(h/e2≈25.8 kΩ).This research is of great significance for the large-scale preparation of spintronic devices with complex multilayer films.(2)The Bi2Te3 thin film sputtered at room temperature and then post-annealed can overcome the shortcoming of the large surface roughness of the Bi2Te3 thin films sputtered on the heated substrate,have a very uniform and flat interface,and have a better c-axis preferred orientation.For electrical transport properties,the post-annealed Bi2Te3 films have lower carrier concentration and larger phase coherence length than the films sputtered at high temperatures,and hence better two-dimensional topological surface state characteristics.(3)The negative magnetoresistance(MR)behavior of Cr-doped(Bi1-xSbx)2Te3 films at low temperatures has been studied.At the temperature T=2 K and magnetic field H=2 T,a large negative MR of-61%is observed near the charge neutral point of Cr-doped(Bi1-xSbx)2Te3 films.By tuning the Fermi level of Cr-(Bi1-xSbx)2Te3 thin films we find that the large negative magnetoresistance of the film strongly depends on the position of the Fermi level EF:when the Fermi level is far away from the charge neutral point,the negative MR is relatively small,and when the Fermi level is tuned to near the charge neutral point,the negative MR reaches its maximum.(4)The SOT effect in the perpendicularly magnetized Zr/CoFeB/MgO films is studied.It is shown that the current-induced magnetization switching can be realized in the 4d light metal Zr-based CoFeB/MgO system,and current-induced torque effective fields exhibit a significant dependence on the Zr layer thickness.The spin Hall angle of Zr is about-0.03,which is much larger than that of the Mo(-0.003)in the literature.The spin Hall angle and SOT efficiency may be further enhanced by engineering the interface between the ferromagnet and the light metal and injecting more spin/orbital current.(5)We report that insertion of an ultrathin Zr layer(0.4 nm)between the CoFeB and MgO in W/CoFeB/MgO films effectively induces perpendicular magnetic anisotropy.The perpendicular magnetic anisotropy field Hk of the film increases with the increase of annealing temperature and reaches its maximum value at 540℃.It is shown that when the annealing temperature is increased from 270℃ to 600℃,the structure of W changes from a high-resistivity β phase to a low-resistivity amorphous phase.We perform current-induced magnetization switching on W(4.5)/CoFeB(1.2)/Zr(0.4)/MgO(3)samples with different annealing temperatures and measure the damping-like and field-like SOT efficiencies by harmonic Hall voltage.We find that as the annealing temperature increases,the resistivity of W gradually decreases,and the damping-like efficiency almost keeps a high constant value(about-0.2),and the normalized power consumption can be reduced by more than 60%. |
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