A Study Of Perpendicular Magnetic Anisotropy And Interfacial Magnetoelectric Effect In Magnetic Tunnel Junctions

Spin-Torque-Transfer Magnetic Random Access Memory (STT-MRAM) has been considered as a candidate for the "universal memory" that could be applied in the high performance and low power computer storage system because of its fast speed, non-volatility, and long endurance. As demands for the performance of the storage system have increased, the development of STT-MRAM with high storage capacity and ultralow-power consumption is still challenging. STT-MRAM uses magnetic tunnel junctions (MTJs) to store data, and its performance largely depends on the quality of the MTJs. How to improve the storage density, decrease the power consumption of STT-MRAM and increase the tunnel magnetoresistance ratio of MTJs are the key obstacles to overcome. In order to increase the storage density, the magnetic anisotropy of magnetic materials should be enhanced to maintain the thermal stability while the size of MTJs scales down; the most promising mothed to decrease the power consumption is to use electric-field (EF) to modulate magnetic anisotropy which could be combined with spin torque switching or field induced switching; to obtain a larger TMR needs us to have a deeper insight into the spin transport properties of MTJs.As the size of MTJs scaling down, either the magnetic anisotropy or the magnetoelectric effect in MTJs is more dependent on the properties of interfaces. This dissertation systematically studied the effect of interface structure on the perpendicular magnetic anisotropy (PMA), the electric-field controlled magnetic anisotropy and the spin transport properties of MgO-based MTJs. This thesis is roughly divided into three parts:(1) In order to fulfill the requirements of MTJs for high perpendicular magnetic anisotropy (PMA) materials, this study mainly focused on the next generation PMA material FePt. To optimize heat treatment temperature for CMOS process, the influence of the rapid thermal annealing and the Ag seed layer on the order-disorder transformation of FePt thin films has been studied. We demonstrated that crystallographic ordering of the Llo-FePt phase was significantly promoted and the coercivity increases when the Ag seed layer was added. Then a novel stack structure CoFe/Al-FePt with strong PMA was proposed to match the materials of MgO-MTJs, and its interface structure and the mechanism of PMA were analyzed. The results indicated that the PMA originated form the CoFe/Al-FePt interface, and an enhanced PMA would be obtained by introducing proper strain at the CoFe/Al-FePt interface region.(2) In the case of electric-field controlled magnetic anisotropy, the effect of interfacial oxidation, metal-metal interface and strain on this magnetoelectric effect were elucidated. Based on first-principle calculations, the stable interface structures of MgO/Fe under ideal, over-or under-oxidized state were obtained. It is found that the MgO/Fe interface oxidation states have great impact on electric-field modified magnetic anisotropy in MgO/Fe. In order to enhance the magnetoelectric effect in MTJs, the effects of ferromagnetic metal (FM) and non-magnetic metal (NM) interface states on the electric-field modified PMA were elucidated. It is demonstrated that the contribution from a metal-to-metal interface can be strong enough to dominate the electric-field effect on magnetic anisotropy of MgO/Fe-based films. Next, due to the inevitable lattice mismatch in stack structures, the strain modulation of magnetoelectric effect was studied. The results suggest that the strain could also effectively tune the magnetoelectric effect.(3) To have an insight in to the tunnel mangntoresistance effect, the spin transport properties of MgO-MTJs were simulated based on Non-Equilibrium Green Function. Through combining the analyses of the spin transportation and the local density of states, it is found that the MgO interfacial oxidation and the interface structure of MgO/Ferromagnetic-binary-alloys, as well as the bulk materials, heavily affect the spin transportation and the tunnel magnetoresistance effect in MgO-based MTJs. Optimising the interface structures of MTJs is an extremely effective method for improving the tunnel magnetoresistance ratio.