Spin-orbit Torque Based On Ferromagnetic Perpendicular Magnetized Heterostructures

In the era of big data,it is a significant challenge for researchers to store information in a more efficient,stable,low-power,and the non-volatile way in the face of such a complex and large amount of information.Magnetic-Random-Access-Memory(MRAM)has a significant application prospect compared with traditional semiconductor data memory because of its non-volatility,high storage density,high read/write rate,and low power consumption.However,after two generations of MRAM technology,researchers have found that it still cannot meet the demand for information storage.In recent years,spin-orbit torque magnetic memory(Spin-Orbit Torque,SOT-MRAM)has become another hot area of research in magnetic storage technology,which mainly uses the charge flow-induced spin flow generated by SOT regulation control magnetism as a way to write data information,which can maintain the advantages of magnetic random memory such as high read/write speed and low energy consumption,but also to achieve complete isolation of the read-write path,thus greatly improving the device's resistance to external breakdown characteristics and device life.At present,SOT-MRAM research focuses on ferromagnetic metal/heavy metal(FM/HM)system,which takes advantage of the strong Spin-Orbit Coupling(SOC)effect of the 5d heavy metal material.The charge flow is converted into a spin flow through its Spin Hall effect(SHE)or/and interface Rashba effect,or the accumulation of spin at the interface is injected into the adjacent magnetic metal layer.The magnetic moment is switched by applying spin-orbit torque to the magnetic moment of the magnetic metal layer.In order to achieve lower power consumption in the device,it is necessary to reduce the current density(J_c)of the critical switching of the magnetic storage cell,increase the charge-spin flow conversion efficiency(i.e.,spin Hall angle),to achieve low power consumption.However,due to the weak spin Hall effect of the heavy metal system,the improvement of the spin Hall angle(about 0.1-0.3)has encountered a significant bottleneck.Therefore,there is a need to find materials with larger spin Hall angle.In this thesis,magnetron sputtering technology has successfully prepared heavy metal/ferromagnetic metal heterostructures and topological insulator/ferromagnetic metal with perpendicular magnetic anisotropy.Furthermore,the spin Hall angle,spin-orbit torque-driven magnetic switching,and magnetic domain wall motion have been investigated utilizing spin transport and magneto-optical Kerr microscopy.The main innovative research results were obtained as follows:(1)Pt/Co/W_1-xPt_x heterostructures with perpendicular magnetic anisotropy were prepared by sputtering using 5d heavy metals Pt and W with strong spin-orbit coupling and opposite sign of spin Hall angles.The relationship between Perpendicular-Magnetic-Anisotropy(PMA)and spin Hall angle with the thickness and composition of W_1-xPt_x was systematically studied.First,experimentally,we obtained a perpendicular magnetic anisotropy field of up to 10 k Oe,the maximum value obtained for the currently known Pt/Co system.Secondly,the critical switching current density for the current-induced SOT-driven magnetization switching is reduced to 10~6 A/cm~2.It is also shown that the critical switching current density can be reduced more effectively when Pt is doped,which can be a way to reduce the critical switching current density.Finally,by analyzing the test results of spin transport,the Pt/Co/W_1-xPt_x series samples obtained a maximum spin Hall angle of 0.25.Furthermore,through a series of tests at varying temperatures,we obtained the experimental results that the effective spin Hall angle of the Pt/Co/W_1-xPt_x system sample is proportional to its longitudinal resistivity,which strongly proves the spin Hall angle of the system.The Hall effect is mainly the contribution of oblique scattering(2)High-quality Bi₂Se₃ topological insulator films were successfully prepared using an ultra-high vacuum magnetron sputtering stage,and Pt/Co/Bi₂Se₃ perpendicular magnetization heterostructures were prepared with Pt/Co.The variation of the spin Hall angle with the thickness of the topological insulating layer was systematically investigated,and the source of SOT was analyzed.First,the heterostructure based on Pt/Co-based perpendicular magnetization also has a perpendicular magnetic anisotropy field of up to 12 k Oe,indicating that the devices made from samples of this structure have good stability.Second,incorporating topological insulators brings a smaller critical switching current density than the heavy metal system.Finally,a large effective spin Hall angle of 0.35 is obtained using spin transport(considering the contribution from Bi₂Se₃ alone,the spin Hall angle would be well above 1).It was found to be independent of the thickness of the Bi₂Se₃ relation.Moreover,the test results of re-temperature spin transport indicate that the spin Hall angle comes from the topologically protected state of Bi₂Se₃.The preliminary findings on heavy metal/ferromagnetic metal/topological insulator heterostructures have important implications for practical industrial applications.(3)We selected the ternary topological material(Bi_1-xSb_x)₂Te₃ as the source material of the spin-orbit torque,prepared a Pt/Co/(Bi_1-xSb_x)₂Te₃ heterostructure,and systematically studied the PMA,spin Hall angle,external magnetic field-driven domain wall motion,thermal stability,and Dzyaloshinskii-Moriya interaction(DMI)with topological insulating layer thicknesses and Sb compositions.The experimental results show that the critical switching current density of the system is as low as 10~5 A/cm~2;the effective spin Hall angle of the system increases from 0.30 to 0.75 with varying Sb composition,which indicates that the ternary topological material(Bi_1-xSb_x)₂Te₃ is more advantageous for SOT-MRAM applications.We also quantitatively analyzed the DMI of Pt/Co/(Bi_1-xSb_x)₂Te₃ heterostructure,showing that it originates from the contribution of the Pt/Co interface and is related to the composition of Sb in(Bi_1-xSb_x)₂Te₃.In addition,the results of the study on the magnetic domain wall motion driven by the magnetic field showed that the domain wall motion rate increased gradually with the increase of the external magnetic field,while the domain wall motion velocity was not uniform for different composition samples,which might be caused by the sample defects affecting the pegging field to be overcome in the domain wall motion.The results are important for further understanding the physical mechanism of the spin-orbit torque-driven magnetic domain wall motion in topological insulator groups.