Dynamics of domain wall (DW) nucleation, relaxation and switching is studied in a series of GaMnAsP films with anisotropy perpendicular to the film plane using <i>ac </i>susceptibility measurements. Measurements of <i>dc </i>and <i>ac </i>magnetization along the  crystal axis (the easy axis of GaMnAsP film) were performed by a superconducting quantum interference device (SQUID). The <i>ac </i>driving field was applied at various frequencies, from 0.1 Hz to 1000 Hz at an amplitude of 3.77 Oe. Characteristic dynamic modes in the <i>ac </i>spectra were mapped in all GaMnAsP films near the Curie temperature <i>T<sub>C</sub></i>, where the coercive field is comparable to the <i>ac </i>driving field. More significantly, we were able to identify DW relaxation, sliding, and switching from Cole-Cole plots. We found that DW segmental relaxation becomes dominant in GaMnAsP films with higher P concentration, where the coercive fields are enlarged as P concentration increases.
Four-wave mixing techniques are applied to Ga<sub>1-x</sub>Mn<sub>x</sub>As, providing new insight into the fundamental properties of the III-Mn-V semiconductors. Clear signatures of (s,p)-d hybridization associated with the Mn<sub>Ga</sub> substitutional impurity are observed in the spectrum of the four-wave mixing signal. Experiments on a ferromagnetic sample show no evidence of hole occupation in the valence band for x = 5%. A photon echo is observed on the interband transition, despite the strong exchange coupling and large defect density in this system. Rapid dephasing associated with Mn-hole spin-flip scattering is observed, with a measured upper bound on the spin-flip scattering time of 40 fs. An excitonic signature tied to many-body effects is observed in LT-GaAs.
The electroluminescent properties of various porous silicon pn junction devices have been investigated. Devices were fabricated by constant current anodization method as well as a novel method developed for anodizing heavily doped pn junctions. The constant current anodized devices show electroluminescence only under reverse bias condition. The light emission mechanism in these devices is believed to be similar to the hot electron relaxation mechanism observed in a surface-treated crystalline silicon pn junction diode at breakdown. The pn junctions fabricated by the novel anodization technique show electroluminescence under forward bias condition. The light emission mechanism in these devices is believed to be due to electron-hole injection in the silicon quantum wires.