Magnetic Dynamics And Multilevel Memory In MgO Magnetic Tunnel Junctions

Spintronics,an emerging discipline which study the electron spin transport properties,has experienced three decades development from being born.It has showed a broad application prospects in many areas such as information storage,communication,computing,sensing and neural networks,etc.The aim of spintronics is to design and develop new functional materials and spintronic devices by using spin properties of electrons.Unlike traditional microelectronic devices,the spintronic devices are based on the generation,transport,detection and effective control of spin current.Since they have the advantages of low power consumption and high integration,the spintronic devices are considered to be the potential candidate to promote technology innovation in the post-Moore era.In recent years,for their very broad application prospects,the spintronic devices based on magnetic tunnel junctions(MTJs),such as high-sensitivity magnetic sensors,magnetic random-access memories(MRAMs),spin torque nano-oscillators(STNOs)and spin logic devices,etc.,have attracted great attentions of many research institutions in the world as well as the technology giant companies such as IBM and Samsung,etc.Some of the devices have been widely used in consumer electronics,intelligent home appliances,automotive and industrial control;and many others are being invested heavily.Great efforts were made for their development,research and application.Among the magnetic tunneljunction systems,the structure based on MgO tunnel barrier has the highest tunneling magneto-resistance(TMR)ratio so far.What's more,its technology is relatively mature.In this dissertation,we have done a series of work on the free layer magnetization dynamics and non-volatile multilevel memories around MgO MTJs.The main contents of this thesis are as follows:(1)The sputtering deposition of CoFeB/MgO/CoFeB MTJ multilayers and MTJ devices' fabrication.Using different magnetron sputtering systems,we deposited the MTJ multilayers with different structures under different experiment conditions.And then,we fabricated them to MTJ devices with micro and nanometer scales by standard micro-nano processing.such as UV photolithography,electron beam lithography(EBL)and ion etching,etc.In these MTJ devices deposited by 3 different magnetron sputtering systems,we measured 50%,190%and 109%MR ratio.More importantly,the sample with 109%MR ratio has a RA value lower than 10 Q?m2-.Besides,we gave some suggestions on the design of MTJ multilayer structure after detailly analysis on each layer's functions.(2)Geometrically enhanced magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions.When measuring the MR of MTJs by four terminal method.we found the giant TMR effect which caused by uneven distribution of current density.We established an equivalent circuit model to explain the geometrically enhanced magnetoresistance.We also found its abnormal current dependence and temperature dependence.One sample showed a 425%MR ratio at 350K.Moreover,the MR ratio is independent of the measuring current.These unique properties provide a new design thought for developing magnetic sensors with high sensitivity and high stability.(3)Study on microwave emission properties of STNOs.We designed and fabricated STNO devices based on CoFeB/MgO/CoFeB MTJs,and the microwave emission properties of STOs were investigated.Two kinds of MTJs with in-plane magnetized free layer and tilted magnetized free layer were fabricated into STNO devices,respectively.The STNO devices had two kinds of geometric design:A)the long axis of free layer has same direction with pinning field of pinned layer;B)the long axis of free layer and the direction of pinning field is orthogonal.Microwave emission signals were measured in each of them with a largest power of 11.9 nW.Moreover,we realized microwave emission under zero magnetic field in design B).(4)Numerical simulation of magnetization dynamics in a synthetic antiferromagnetic structure.Based on Bloch-Bloembergen-Slonczewski(BBS)equation which considering the short-wavelength magnon excitation,we studied the magnetization dynamics of free magnetic layer with synthetic antiferromagnetic structure by means of numerical simulation.Our simulation results showed that high frequency up to terahertz(THz)scale with no external magnetic field could be realized in STNO structures if the following requirements were fulfilled:antiferromagnetic coupling between synthetic antiferromagnetic layers is sufficiently strong;and the thickness of top(bottom)layer of synthetic antiferromagnet is sufficiently thick(thin)to achieve a wide current density window and frequency window.Additionally,the transverse relaxation time of the free magnetic layer should be sufficiently larger compared with the longitudinal relaxation time.(5)Study on nonvolatile multilevel memory.We proposed a general remnant magnetism engineering method for realizing nonvolatile multilevel memory and we demonstrated the concept in CoFeB/MgO/CoFeB MTJ devices and(Co/Pt)n multilayer samples respectively.We fabricated the CoFeB/MgO/CoFeB MTJ devices which had geometrically enhanced TMR effect and deposited the(Co/Pt),multilayer "which had perpendicular magnetic anisotropy.And then,we demonstrated the process of 10 states of nonvolatile memory by means of magnetic "write"-magnetic "read" and magnetic"write"-electronnic "read".Besides,for much better stability,we proposed a method which is manipulating the remnant domain states of the pinned layer by controlling the antiferromagnetic domain states to realize nonvolatile multilevel memory and demonstrated it in CoFeB/MgO/CoFeB MTJ device.