| The dissertation consists of two research subjects; Giant Magnetoresistance (GMR) and Dilute Magnetic Semiconductors (DMS). Their common thread is the electronic spin. Both of these subjects are important components of the field of spintronics.; In the first GMR study, using temperature-dependent magnetoresistance and magnetization measurements on Fe/Cr multilayers, we identified hysteretic and time-dependent behavior that denotes the presence of a glassy antiferromagnetic (GAF) phase. Pronounced irreversible behavior with logarithmic time dependence manifests itself below an identified "de Almeida and Thouless" (AT) line and confirms the important role of an underlying disordered magnetic domain structure stemming from inter- and intralayer dipolar couplings. Our identification of a field-independent spin-glass temperature associated with this GAF phase sets an energy scale (140K) below which interlayer exchange coupling (IEC) dominates. At room temperature, where the GMR effect is still robust, IEC plays only a minor role in forcing the antiparallel interlayer domain orientations that give rise to the high resistance state in zero magnetic fields.; In the DMS study, by using the method of ion implantation to incorporate magnetic ions into a variety of semiconducting substrates, we investigated the nature of magnetism in magnetic semiconductor systems. The magnetic ions Mn, Fe, and Ni were implanted into each of the epitaxially grown semiconductors GaN, GaP, and SiC to achieve volume concentrations between 1 and 5 atomic %. The implanted samples were examined with both x-ray diffraction (XRD) and transmission electron microscopy (TEM) to characterize their microstructure (in most cases, no secondary phases were found) and with SQUID magnetometry to determine their magnetic properties (hysteresis, coercive fields, and differences between field-cooled (FC) and zero-field cooled (ZFC) magnetizations). We discovered room temperature ferromagnetism in p-GaP: C with high hole concentration implanted with 3 at. % Mn and its magnetism was suppressed when the Mn concentration either increased or decreased away from the 3% value. |
