MICROWAVE STUDIES OF SEMIMAGNETIC SEMICONDUCTORS
Abstract
The topic of this thesis is the study of microwave transmission in ternary semiconductors which contain magnetic ions randomly substituted in the lattice. Examples of such materials are HgMnTe and HgMnSe. These "semimagnetic" semiconductors exhibit some unique physical properties, particularly when an external magnetic field is applied. Microwave helicon transmission and related solid state plasma experiments provide measurements of these materials electrical and magnetic properties, e.g., electron paramagnetic resonance (EPR). The propagation of electromagnetic waves in a semimagnetic semiconductor in the presence of an external magnetic field is examined for a plasma of high mobility electrons and low mobility holes in the Faraday geometry. Particular attention is given to the effects introduced by the presence of the holes. The effective permittivity is formulated in terms of the Drude model and approximate expressions are obtained for the dielectric constant and the phase and attenuation of the wave for various ratios of the electron to hole concentrations. Three distinctive features are observed in the microwave transmission spectrum. Approximate expressions are obtained for relating the magnetic fields at which these features occur to the electrical parameters of the material. Comparison is made with the positions of these features as observed in two hole-dominated samples. The problem of EPR of localized magnetic ions in semimagnetic semiconductors is examined. These materials exhibit a strong dependence of their electrical properties on the magnetic field, which in turn affects the EPR lineshape. For samples thinner than both the skin depth and the internal wavelength, the EPR lineshape is Lorentzian, determined by the dissipative part of the dynamic magnetic susceptibility. When the samples are thicker than the skin depth, a variety of lineshape is predicted, determined not by the value of the conductivity, but rather by the loss tangent, i.e., the ratio of the dissipative-to-dispersive components of the conductivity. This ratio is dependent on the carrier concentrations and mobilities. Specific ranges of the loss tangent, characteristic of these materials, are discussed analytically and illustrated by numerical calculations and EPR data where available. Attempts to extend the class of materials known as semimagnetic semiconductors to include HgSe alloyed with Mn and both HgTe and HgSe alloyed with other magnetic elements are discussed. Results of the microwave helicon transmission studies in a HgMnSe sample are used for a quantitative study of the magnetic susceptibility. Specifically a deviation from a Curie-Weiss magnetic susceptibility and a broadening of the magnetic resonance linewidth with decreasing temperature are observed. The susceptibility data is explained using an antiferromagnetic exchange interaction between spins in isolated clusters of two and three nearest neighbors. The best fit to the data is achieved by using a number of single isolated ions less than the statistically probable number. Preliminary results for HgFeTe alloys are presented. Microwave powder transmission experiments show Fe('++) magnetic resonance with a g-factor of approximately 2.25. The value of g is observed to decrease with increasing temperature. The value of the static magnetic susceptibility obtained from this data is independent of temperature.
Degree
Ph.D.
Subject Area
Condensation
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.