Spin Polarized Current Phenomena in Magnetic Tunnel Junctions a Dissertation Submitted to the Department of Applied Physics and the Committee on Graduate Studies of Stanford University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

Spin polarized current is of significant importance both scientifically and technologically. Recent advances in film growth and device fabrication in spintronics make possible an entirely new class of spin-based devices. An indispensable element in all these devices is the magnetic tunnel junction (MTJ) which has two ferromagnetic electrodes separated by an insulator barrier of atomic scale. When electrons flow through a MTJ, they become spin-polarized by the first magnetic electrode. Thereafter, the interplay between the spin polarized current and the second magnetic layer manifests itself via two phenomena: i.) Tunneling magnetoresistance (TMR) effect. The relative alignment of the electrode moments determines the resistance and its change. This TMR effect is largely determined by the spin polarized density states of the electrodes, interface states, tunneling matrix, and so on. However, despite extensive experimental and theoretical efforts, many aspects of TMR remain poorly understood. In my research, it is shown that thin CoFe alloy can be made amorphous by sandwiching the normally crystalline CoFe electrode between two amorphous layers. Incorporating amorphous CoFe with Al O to form MTJs, both the TMR and the tunneling spin polarization are significantly enhanced when the alloy is amorphous. The tunneling anisotropic magnetoresistance effect in both MgO and Al O based MTJs is also investigated. 2 3