PhD Dissertation Defense - Li Gao @ 2:00pm June 10th
ligao at stanford.edu
Wed Jun 3 23:19:07 PDT 2009
University PhD Dissertation Defense
Spin Polarized Current Phenomena in Magnetic Tunnel Junctions
Department of Applied Physics
Research Advisors: Professor James Harris and Dr. Stuart Parkin
10 June 2009 @ 2:00p.m. in Packard Building, Room 101 (Refreshments @ 1:40p.m.)
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 an 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 usually used crystalline CoFe electrode between two amorphous layers. Incorporating amorphous Co70Fe30 with Al2O3 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 Al2O3 based MTJs was also investigated.
ii.) Spin-transfer torque (STT) effect. The spin-polarized current exerts a torque on the local moments and can thereby induce steady-state precessional excitation modes or complete switching of a nanomagnet. This effect has mostly been studied, to date, in metallic structures where the spin-valve magnetoresistance is small so that the output power is limited. However, the giant TMR in MgO base MTJs, which also have much higher resistance than spin-valves, can give rise to much higher rf power outputs. It is also found that the spectrum is very sensitive to small variations in device structures, even in those devices which exhibit similarly high TMR (~120%) and have similar resistance-area products (~4-10 Wm m2).
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