Spin-Orbit Torque Efficiency Of Topological Dirac Semimetals

MRAM with low writing power,high reading-writing speed,long service life,has become a research topic in recent years.In 2019,semiconductor companies such as Samsung and TSMC realized a large-scale integration process of 22nm～28nm STT-MRAM,which injected unprecedented hope for the research and development of MRAM series products.However,STT-MRAM uses the same path for reading and writing.Although high writing current density can improve writing speed and writing efficiency,high current density will not only increase the writing power,but also reduce the life of magnetic tunnel junctions(MTJ).In addition,the lower spin polarization efficiency of STT-MRAM also limits the reduction in writing power.SOT-MRAM reads and writes in different ways.Although the reading path is exactly same as STT-MRAM,the write current does not pass through the MTJ,which will avoid the problem that the writing current is to damage the MTJ.In addition,high-efficiency spin polarization efficiency and fast magnetization switching speed will greatly improve the development prospects of MRAM.The MTJ and the spin-orbit moment(SOT)write path are the two core components of the SOT-MRAM memory cell.Among them,the MTJ is the storage carrier and the data reading carrier.The MTJ with high perpendicular magnetic anisotropy can not only increase the unit storage density but also increase the thermal stability factor to ensure the stability of data storage;SOT writing material and interface conditions will affect the writing speed,energy consumption and manufacturing cost.Based on the above two points,this article conducts researches from the following four aspects:(1)The study of spin-orbit coupling on the perpendicular magnetic anisotropy of the heavy metal/ferromagnetic heterojunction(Ta/Co Fe B)interface.This research mainly explores how the film growth sequence will affect the performance of the perpendicular magnetic tunnel junction.The conclusion shows that growing a ferromagnetic layer on heavy metals will help to improve the perpendicular magnetic anisotropy performance.(2)Study the influence of the magnetic proximity effect of the heavy metal/ferromagnetic heterojunction(Pt/Co Fe B)on the spin-orbit torque efficiency.The study found a new method of regulating the magnetic proximity effect by interface stress,and the interface stress only changes the proximity magnetic moment of Pt in the heterojunction without changing the magnetic moment of the ferromagnetic layer Co Fe B.In the end,it was realized that the spin-orbit torque efficiency was adjusted by the interface stress.(3)Preparation of high-quality epitaxial single crystal topological Dirac semi-metallicα-Sn thin film.Topological materials are well-known for their high-efficiency spin polarization.In this study,the topological Dirac material α-Sn,which still maintains Spinmomentum-lock at room temperature,was achieved,and the spin-texture observed by the second-harmonic signal measurements.It has a perfect three-axis symmetric structure.This conclusion proves that the prepared α-Sn has a perfect topological surface state.(4)The writing module of SOT-MRAM made of topological Dirac semi-metal α-Sn film is realized.In this research,we not only realized the magnetization reversal writing of the ferromagnetic layer with the topological Dirac semimetal α-Sn surface state,but also carefully studied the performances of α-Sn under different conditions.Subsequently,in order to further determine and quantify the spin-orbit efficiency of topological Dirac semimetal α-Sn at room temperature,I built an ST-FMR test system.Through comparative studies of different thicknesses,I found that The SOT efficiency of 5nm can reach 4.3±0.41,and the SOT efficiency is 20～30 times larger than that of ordinary Hall material Pt.In summary,this thiese first optimizes the preparation process of the PMA-MTJ,hoping to provide a reference for the preparation of higher quality MTJs;Secondly,it is found that the interfacial stress in the Hall material system can control the magnetic proximity effect(MPE),which will have great opportunities for the application of the MPE;Finally,the expansion and development of a new material system-topological Dirac semimetal α-Sn,this research not only visually demonstrates the existence of topological surface states at room temperature,but also demonstrates the absolute advantage of SOT efficiency and ultra-low magnetization switching current.It is very expected to become a candidate material for commercial SOT devices in the future.