MICROWAVE STUDIES OF ELECTRON PARAMAGNETIC RESONANCE IN DILUTED MAGNETIC SEMICONDUCTORS (MERCURY-MANGANESE - SELENIDE, CADMIUM, TELLURIDE, ZINC)
Abstract
Electron paramagnetic resonance (EPR) studies can give valuable information on the dynamic magnetic properties of solids. We have used EPR as a tool to investigate the family of materials known as diluted magnetic semiconductors (DMS). Our method of observing EPR is based on microwave transmission and is thus sensitive to both dispersion effects associated with the real part of the magnetic susceptibility and absorption effects due to the imaginary part. We have used helicons (circularly polarized microwaves which travel through a medium with relatively little attenuation) to excite EPR in conducting (zero-gap and narrow-gap) Hg(,1-x)Mn(,x)Se for values of x up to 0.1. We have established that the helicon technique becomes impractical for x > 0.10 in narrow-gap DMS. Cadmium- and zinc-based DMS are transparent to microwaves. We measured EPR in these wide-gap materials by observing the Faraday rotation and ellipticity associated with the resonance. We studied CD(,1-x)Mn(,x)Te over the entire range of possible manganese concentrations (x < 0.7). For x < 0.15, the observed Faraday effect could be explained using the standard Bloch model of the dynamic magnetic susceptibility. Above x (DBLTURN) 0.15, however, the increasingly large EPR linewidths prevented the Bloch model from adequately explaining the data. Modification of the model to allow for very short relaxation times, which lead to broad lines, allowed us to fit the data in this range. At the highest concentrations a new type of behavior emerged at low temperatures. Small, sharp features appeared in the Faraday effect spectra that even the modified form of the Bloch model failed to explain. We also studied other wide-gap DMS: CdMnSe, CdMnS, ZnMnTe, ZnMnSe and ZnMnS, all containing about 10% manganese. Although the results we observed were qualitatively the same, several trends became evident. The strength of the absorption and dispersion due to EPR were strongest in cadmium-based DMS. Also the effects were strongest for sulfides, followed by selenides and then tellurides. Finally, this thesis reports on several brief experiments which gave information on EPR in DMS. We discuss fine and hyperfine structure in the EPR spectra of very dilute DMS, microwave heating effects, magnetic hysteresis, microwave "photoconductivity," and EPR in DMS other than II-VI-Mn compounds.
Degree
Ph.D.
Subject Area
Condensation
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