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Imaging and controlling spins in semiconductors and ferromagnets

Nowakowski, Mark
Degree Grantor:
University of California, Santa Barbara.Physics
Degree Supervisor:
Awschalom David D
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Condensed matter physics and Physics

Spins possess robust coherent and exchange-driven properties in semiconductors and ferromagnets. In this work, we investigate three experiments that incorporate and exploit these spin properties to demonstrate innovated quantum information processing, magnetic detection and control techniques. In the first experiment we spatially confine an effective magnetic field to control the coherent state of moving electron spins. Optically-injected electron spin ensembles are transported through a gate-controlled, spatially-isolated region with a large effective magnetic field created by locally polarized nuclear spins within a GaAs channel at T = 8 K. By tuning the localized effective field strength and drift velocity we detect, using time-resolved Kerr rotation (TRKR), induced spin rotations of up to 5π radians in 6 ns over a 30 μm distance.

In the second experiment, we develop a sensitive electrical technique derived from the anomalous Hall effect (AHE) to measure domain wall (DW) motion with nanometer precision. We then use this system to study the elastic properties of single ferromagnetic DWs in (Ga,Mn)As. Full understanding of the electrical signal is only possible after accurately determining the DW location with respect to the electrical contacts. Therefore, we image the DWs using a custom-built, diffraction-limited video magneto-optical Kerr effect (MOKE) microscopy system while simultaneously measuring the AHE. By combining these detection schemes we are able to precisely measure temperature-dependent elastic DW dynamics and kinetics below T<sub>C</sub>.

Finally, the third experiment relates our progress toward understanding the coupling between the multiferroic oxide BiFeO<sub>3</sub> (BFO) and a CoFe magnetic layer. The exchange-bias mediated coupling between ferroelectric domains of the BFO and ferromagnetic domains in the CoFe layer suggest a pathway to realize electrical control of the magnetization properties. We investigate and model the ferroelectric influence on magnetocrystalline anisotropies in the CoFe thin film by measuring the static and dynamical magnetic properties using anisotropic magnetoresistance (AMR) and transport-based ferromagnetic resonance (FMR) measurements.

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