This thesis describes experimental studies of magnetization dynamics in both spin valves (SVs) and magnetic tunnel junctions (MTJs) subject to spin-polarized currents. A spin-polarized electrical current can transfer its angular momentum to a ferromagnet through a spin-transfer torque (STT), resulting in intriguing magnetization dynamics such as the reversal of the magnetization direction, precession and relaxation.
The ferromagnetic systems investigated were nanopillars, tens to hundreds of nanometers in cross section and a few nanometers in thickness, which were further integrated into SV or MTJ structures.
The magnetization switching and relaxation studies were performed on all-perpendicularly magnetized SVs. The switching probabilities were investigated for different pulse conditions at room temperature, where thermal fluctuations can play an important role. The pulse duration was varied over 10 orders of magnitude, from the fundamental timescales of magnetization precessional dynamics, 50 ps, to 1 s. Three switching regimes were found at different timescales. In the short-time regime, the switching probability was mainly determined by the angular momentum transfer between the current and the magnetization. In the long-time regime, the switching becomes thermal activation over an effective energy barrier modified by the STT. In the crossover regime, both spin-transfer and thermal effects are important.
The magnetization relaxation was studied by a two-pulse correlation method, where the relaxation time is measured by the interval between the two pulses. The thermal effects were shown to be important even at nanosecond time scales.
The switching and precession of magnetization were also studied in structures where a perpendicular spin polarizing layer is employed with an in-plane magnetized MTJ. When subject to pulses, the initial STT from the polarizer to the free layer is perpendicular to the free layer plane. For a large enough STT, this tilts the free layer magnetization out of the plane to create a large demagnetization field, typically at tens or hundreds of millitesla. This demagnetization field then becomes the dominant magnetic field acting on the free layer, leading to the precession of its magnetization. This magnetization precession was observed through real-time device resistance measurements, where precessions with hundreds of picoseconds are found from single current pulse stimuli.
|Advisor:||Kent, Andrew D.|
|Commitee:||Grier, David G., Mincer, Allen, Mitra, Aditi, Stein, Daniel L.|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-B 74/07(E), Dissertation Abstracts International|
|Subjects:||Physics, Condensed matter physics|
|Keywords:||Magnetic tunnel junctions, Magnetization, Spin transfer, Spin valves, Spin-polarized currents|
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