| With the advent of the era of big data,the field of data storage has ushered in a period of rapid development.Owing to the advantages of high storage density,rapid read/write speed,low power consumption and non-volatile data storage,magnetic random access memory(MRAM)has attracted much attention.Among MRAMs,SOT-MRAM is the most promising candidate for application,which utilizes the spin-orbit torque(SOT)as the driven force for information writing.In essence,the information writing of SOT-MRAM is based on the SOT-driven magnetization switching in perpendicularly magnetized ferromagnetic metal/heavy metal heterostructure.When an electric current passes through this heterostructure,the spin current will be generated due to spin Hall effect and/or interfacial Rashba effect based on the strong spin-orbit coupling of heavy metal.Then,the spin current injects into the ferromagnetic layer and exerts spin-orbit torques to switch the magnetization.For application,it is beneficial to achieve a more efficient and faster magnetization switching.Considering that,we need to seek materials that possess large spin Hall angle?SHto achieve a high SOT efficiency.Moreover,device miniaturization is also a developing trend for MRAM,which requires a further increase in storage density.Compared with a single ferromagnetic layer,a synthetic antiferromagnetic(SAF)structure has lower stray field and higher thermal stability.The advantages of SAF will make the storage density become higher if one make SAF structure as the storage medium.Therefore,it is significant to study the SOT-driven magnetization switching in SAF system.Nowadays,with the rapid growth of global data volume,in addition to optimizing the performance of data memory,some researchers have proposed new ways of data storage and processing:multilevel storage and artificial neuron networks.The new techniques require that the basic devices realize the multilevel storage function.The memristors can meet such needs and they have been widely investigated.The memristors based on SOT effect can be divided into two categories,which utilize antiferromagnetic and ferromagnetic materials,respectively.If one combines the antiferromagnetic and ferromagnetic materials,a memristor that has more advantages can be naturally expected.According to the above-mentioned issues,we study the SOT-driven magnetization switching in Pt/Co/Ta,Ta/Pt/Co/Pt/Ru/Pt/Co/Ta and Pt/Co/Ir Mn systems with perpendicular magnetic anisotropy.The detailed research contents of this thesis are as follows:(1)To obtain a large SOT efficiency,we prepare a series of Pt/Co/Ta samples considering that Pt and Ta have opposite signs of?SH.As expected,a large effective?SHaround 0.356 is achieved.We find that?SH is a function of the thickness of Ta(t Ta).When t Ta is larger than 4 nm,the magnitude of?SH decreases with t Ta increasing,which can be explained by the varied crystallinity of Ta.Then,SOT-driven magnetization switching is investigated in Pt/Co/Ta system and a relatively small critical current density in the order of 10~6A/cm~2 is achieved due to the enhanced?SH.SOT-driven domain wall motion is also investigated,in which the interfacial Dzyaloshinskii-Moriya interaction(DMI)plays an indispensable role.DMI enables the stable existence of chiral Néel-type domain wall,so that SOT can exert an effective field on the magnetic moments in the domain wall and drive the domain wall motion.Besides,the larger DM constant is,the faster the domain wall velocity is.The DMI is also responsible for the domain wall tilting,which is harmful for practical applications.These results are helpful for us to understand the mechanisms of SOT-driven magnetization switching and domain wall motion,and they are also beneficial for designing SOT-based spintronic devices.(2)In order to obtain synthetic ferromagnetic(SF)and SAF samples,we prepare a series of Ta/Pt/Co/Pt/Ru/Pt/Co/Ta multilayer samples with the variation of the thickness of Ru.The current induced hysteresis loop shift method is adopted to measure the interlayer DMI strength in SF/SAF samples.The magnitude of interlayer DM constant is in the same order with that of interfacial DM constant,which indicates that the interlayer DMI cannot be ignored in SF/SAF systems.The interlayer DMI is anisotropic and its influence on the SOT-driven magnetization switching is investigated.The experimental results are qualitatively coincided with the results of theoretical simulation based on a macro-spin model.These results show that the interlayer DMI has a significant influence on the SOT-driven magnetization switching.(3)In order to obtain a memristor with the advantages of both ferromagnetic and antiferromagnetic materials,we prepare Pt/Co/Ir Mn multilayer samples with exchange bias.The SOT is demonstrated to be a promising approach to engineer the double-biased hysteresis loops in exchange-biased systems by magnetotransport measurements.Tailoring double-biased hysteresis loops by SOT is also investigated by magneto-optical Kerr microscope,which demonstrates that the evolution of multilevel remanence states due to SOT is based on the domain wall motion.The results have demonstrated that tailoring multilevel remanence states by SOT has potential applications in the multilevel stable storage,which is quite attractive in information industry,and in the artificial synapse device,which is indispensable for neuromorphic computing. |
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