First-Principles Studies of Perpendicular Magnetic Anisotropy and Tunneling Magnetoresistance of Capped Fe/MgO/Fe Tunnel Junctions

• Main Authors: Shu-Wei Lin, 林書緯
• Other Authors: Guang-Yu Guo
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/45795979064262810097

碩士 === 國立臺灣大學 === 物理研究所 === 102 === Recent studies have reported giant magnetoresistance (GMR) in Fe/Cr/Fe trilayer systems. This phenomenon usually occurs in systems consisting of two magnetic electrodes separated by one nonmagnetic metallic layer. Due to the dependence of the electric resistance on the relative magnetization directions in the two magnetic electrodes, the antiparallel magnetization can produce an electric resistance that is stronger than the parallel magnetization. Thus, applying a weak magnetic field can cause a large variation of electric resistance. This dependence of electric resistance on magnetization directions originates from the electronic energy band that splits near the Fermi level in magnetic materials, which results in the different spin density of states at Fermi level. Therefore, the discovery of GMR brought about the spintronics, which exploits the spin-degree of freedom, to be an important branch of physics. We have performed the first principle calculations to study the physical properties of the Fe/MgO/Fe trilayer system, which has been shown to have the large perpendicular magnetic anisotropy (PMA) and tunneling magnetoresistance (TMR). In the present work, these properties are due to the hybridization between the iron and the oxygen at the Fe/MgO interface, which leads to a surface state with a large spin-polarization. Furthermore, due to the broken symmetry at that interface, if the spin-orbit coupling is taken into account, the energy band of the interfacial Fe would split, leading to an energy dependence of the magnetization direction. In the second part of this thesis, the first principle calculation is performed to study the influences on PMA and TMR when the Fe/MgO/Fe is capped with some materials. Here the MgO, Ta and Ru capping layers are considered, and they reveals nonmagnetic, antiferromagnetic and ferromagnetic coupling to the Fe layer, respectively. In the Fe/MgO/Fe trilayer systems, TMR is usually largest in the Ta-cap systems whereas PMA is usually largest in the MgO-cap systems.