| Currently,with the rapid development of big data and cloud computing technology,the total amount of data in human society will show explosive growth.In addition,the application of information processing and transmission systems is gradually spreading in harsh environments such as deep space,deep sea,and underground,which puts forward higher requirements for data storage devices.Thus,it is necessary to develop a high-speed,low-power consumption,nonvolatile and highly reliable storage technology.Magnetic random access memory based on spin-orbit torque（SOT-MRAM）is a storage device that can meet the above requirements and is currently the most competitive new generation of memory.The core of the SOT-MRAM writing unit is a heavy metal/magnetic layer spin heterojunction,where the magnetic layer is generally a ferromagnetic conductor layer with perpendicular magnetic anisotropy.The conductivity of the ferromagnetic layer leads to the transport of spin currents in the ferromagnetic layer,accompanied by carrier transport,and also produces energy losses and signal interference.Therefore,the idea of replacing ferromagnetic layers with magnetic insulators to form heavy metal/magnetic insulator heterojunctions has attracted researchers’attention.The biggest characteristic of magnetic insulators is that spin currents are transported in the form of spin waves or magnons,without accompanying the transmission of carriers,eliminating energy loss and signal interference.Among all magnetic insulators,ferrimagnetic insulators represented by yttrium iron garnet（YIG）have become potential spintronic materials due to their low loss characteristics.The research in this dissertation focuses on the heavy metal/ferrimagnetic insulators spin heterojunction and its perpendicular magnetization switching.Based on density functional theory,the physical mechanism of perpendicular magnetic anisotropy in stress induced ferrimagnetic insulators YIG and Bi doped YIG（Bi YIG）materials has been revealed.Under the guidance of this mechanism,Bi YIG thin film materials with both large perpendicular magnetic anisotropy and ultra-low Gilbert damping have been successfully prepared.The Pt/Bi YIG spin heterojunction exhibits good spin transport properties.Based on the macrospin simulation method,the process of SOT driving perpendicular magnetization switching in various spin heterojunctions such as HM/Bi YIG is described in detail,and the relationship between the parameters such as current density required for perpendicular magnetization switching,as well as the perpendicular magnetic anisotropy field Hk and Gilbert damping coefficientαof ferrimagnetic insulators are given.The specific research results are as follows:（1）Based on density functional theory,the physical mechanism of stress induced perpendicular magnetic anisotropy in ferrimagnetic insulators YIG and Bi YIG is presented.By considering spin-orbit coupling effect into density functional theory,the magnetic anisotropy energy under in-plane and out-of-plane tensile/compressive stresses in YIG and Bi YIG was calculated,and the variation of magnetic anisotropy energy with different types of strain was given.The partial density of states diagrams of the tetrahedral FeTetrand octahedral FeOct atoms in YIG and Bi YIG under strain indicate that the energy shift generated by the split states of the d orbitals near the Fermi level with strain is the main factor affecting the magnitude of magnetic anisotropy energy in YIG and Bi YIG.This work has helped deepen the understanding of magnetic anisotropy in ferrimagnetic insulators such as YIG,and has certain guiding significance for the experimental preparation of ferrimagnetic insulators thin film with perpendicular magnetic anisotropy.（2）Bi YIG thin films with ultra-thin thickness,high perpendicular magnetic anisotropy,and ultra-low Gilbert damping were prepared using pulsed laser deposition technology.The crystal structure characterization results of Bi YIG films with different thicknesses show that the prepared Bi YIG films have good surface and high crystal quality.The magnetic property characterization results show that the perpendicular magnetic anisotropy field Hk of the 8.2 nm Bi YIG thin film is about 2900 Oe,and the perpendicular magnetic anisotropy field Hk of the 17.8 nm Bi YIG film is about 2200 Oe.When the thickness of Bi YIG thin film increases to 25.1 nm,its perpendicular magnetic anisotropy disappears.The ferromagnetic resonance test results show that the ferromagnetic resonance linewidthΔH of Bi YIG in the out-of-plane direction.is only 20Oe,and the Gilbert damping coefficient is of the order of 10-4.The magnitude of the anomalous Hall resistance and resistivity of the Pt/Bi YIG spin heterojunction was measured to be-6 mΩand-1.5×10-3μΩ/cm,the value of imaginary part of spin mixing conductance Gi is 2.06×1013Ω-1m-2,which is an order of magnitude larger than that of Pt/Tm IG heterojunctions,reflecting that Bi YIG is a promising spintronic material.（3）Based on the magneto dynamics theory of SOT,the process of SOT-driven perpendicular magnetization switching in four kinds of spin heterojunctions,HM/Bi YIG,HM/Tm IG,HM/Eu IG1,and HM/Eu IG2（The main difference between Eu IG1 and Eu IG2is the size of perpendicular magnetic anisotropy field Hk）was studied using macroscopic simulation methods.The distribution of the magnetization switching phase diagrams of the four heterojunctions strongly depends on the the size of perpendicular magnetic anisotropy field Hk and the Gilbert damping coefficientαof the ferrimagnetic insulators.The larger the perpendicular magnetic anisotropy field Hk of a ferrimagnetic insulators,the better the stability of magnetization switching in the heterojunction,but the larger the damping-like effective field HDL（proportional to the switching current density）required to achieve magnetization switching in the heterojunction.The existence of a field-like effective field HFL can suppress the oscillation phenomenon of magnetization switching in spin heterojunction with ultra-low Gilbert damping coefficientsαsuch as HM/Bi YIG,and improve the reliability of magnetization switching.The smaller the Gilbert Damping Coefficientαof Ferrimagnetic insulators,the smaller the critical switching current density of the magnetization in the heterojunction needed.The reduction of the magnetization switching current density is of great significance for the practical application of SOT devices. |
PDF Full Text Request