Dynamic Properties And Thermal Stability Of Spin Transfer Torque Effect In Magnetic Tunnel Junctions

With the evolution of the storage technology requirements, the spin transfer torque based magnetic random access memory(STT-MRAM) has attract huge research interest due to its various technological advantages. As the core component of random access memory, the magnetic tunnel junction(MTJ) characteristic needs to be studied and understood thoroughly in order to achieve extremely high storage density and scalability. In recent years, the CoFeB-MgO based magnetic tunnel junction has become a promising choice for magnetoelectronic devices as its superior performace and provides a bright future for the commercialization of the STT-MRAM. However, nano-sized magnetic tunnel junctions are more susceptible to thermal fluctuation, and the high current density may cause the breakdown of the insulating layer and hence hindering the commercial realization of spin-transfer torque magnetic random access memory. It is therefore of compelling urgency to solve the problem and to improve dynamic characteristics of the magnetic moment precession while maintaining the high thermal stability.In this dissertation, both the macro spin model and the micromagnetic model are developed based on the Landau-Lifshitz-Gilbert-Slonczewski(LLGS) equation, to investigate the dynamic characteristics of the magnetic tunnel junction and thermal stability of spin transfer torque effect. The main research objectives are given as follows:1. First, the background and significance of the magnetic storage technology, the discovery and understanding of the magnetic resistance effect, as well as applications of magnetic tunnel junction in magnetoelectronic devices are introduced, and achievements and challenges of magnetic memory technologies are summarized. The analytical calculation of dynamic characteristic equations and presentation of energies and their interactions serve to provide theoretical basis of researches for dynamic characteristics on the magnetic moment.2. The shape anisotropy of the in-plane magnetic tunnel junction and perpendicular anisotropy of perpendicular magnetic tunnel junction are discussed. The aspect ratio of the structure is seen to have a significant influence on the dynamic characteristics of in-plane magnetic tunnel junction. An increase of the aspect ratio and the thickness of the free layer will lead to an increase of shape anisotropy, and the switching time and threshold current density increases. In the perpendicular magnetic tunnel junction, with the increase of the perpendicular anisotropy coefficient, it is more difficult for the magnetic moment to deviate from the easy axis, and the threshold current density and the switching time increase.3. The effect of material parameters on the thermal stability factor and the spin-transfer efficiency are studied, mainly considering the influence of saturation magnetization, perpendicular anisotropy coefficients, free layer thickness and temperature. Spin transfer torque efficiency of perpendicular magnetic tunnel junction increases with the decreasing of the free layer thickness and saturation magnetization, and decreases with the perpendicular anisotropy coefficient. STT efficiency increase with the same magnitude decrease of the saturation magnetization and perpendicular anisotropy coefficient, the enhancement of the saturation magnetization on the dynamic characteristics is greater than the weakening effect of the perpendicular anisotropy coefficient.4.We also investigated the influence of the field-like spin transfer torque and titled polarization magnetization on dynamic characteristics in particular, on the threshold current density and the switching time. Using micromagnetic analysis, we investigated the impact of both Slonczewski and field-like torques, and discuss the spin transfer torque(STT) effect in such non-collinear magnetic multilayer structure. Results show that the threshold current density decrease dramatically, as much as 37%, with a relatively small tilt angle of the polarization layer magnetization. Furthermore, Slonczewski and field-like torques can have a distinctive impact on the threshold current density and the switching time. It is found that an improvement in the switching propertie can be achieved in magnetic tunnel junction under certain circumstances, and the field-like spin transfer torque may be effectively helpful to the switching process,and it can reduce the threshold current and the switching time in this condition.