Microwave magnetic resonances in solids: Part I. Electron paramagnetic resonance in diluted magnetic semiconductors (DMS). Part II. Magnetic dimensional resonances in magnetite spheres

Sridevi Rajagopalan, Purdue University

Abstract

In part I, we present the results of electron paramagnetic resonance (EPR) studies in two systems of diluted magnetic semiconductors--$\rm Zn\sb{1-x}Mn\sb{x}Te$, $\rm Zn\sb{1-x}Mn\sb{x}Se$, $\rm Zn\sb{1-x}Mn\sb{x}S$, and in $\rm Cd\sb{1-x-y}Mn\sb{x}Fe\sb{y}Se$. In the case of the Zn-based DMS alloys, we have analyzed the high temperature EPR linewidth as a function of the Mn concentration x, temperature T, and anion (Te, Se or S) and the cation (Cd or Zn), using the exchange narrowing theory. Our results are in agreement with those of previous investigators. As in the case of the Cd based DMS, our results confirm that the Dzyaloshinskii-Moriya (D-M) anisotropic exchange interactions are the major source of broadening in $\rm Zn\sb{1-x}Mn\sb{x}Te$ and in $\rm Zn\sb{1-x}Mn\sb{x}Se$. In the sulphides, the contribution from the dipolar and D-M interactions are nearly equal. Taking into account the corrections due to single ion anisotropies and Mn-associated D-M interactions, a better quantitative agreement is obtained between theory and experiment, especially in the case of the sulphides. From the extrapolated value of the infinite temperature linewidth in $\rm Zn\sb{1-x}Mn\sb{x}Te$, we can extract the nearest neighbour D-M exchange constant, which is in good agreement with the theoretically predicted values of Larson et al. More importantly, the anion dependence of the linewidth is in reasonable agreement with the theory. Our results also show that the EPR lines are broader in the Zn-based DMS alloys than in the Cd alloys for equal values of x, T and the same anion. The agreement with the theoretical predictions is best for the tellurides, and worsens for the selenides and sulphides. In almost all the samples investigated, the linewidth varies linearly as $\vert\Theta\vert$/T, where $\Theta$ is the Curie-Weiss temperature. This behavior is true even when the condition for the high temperature limit (T $\gg \vert\Theta\vert$) is not satisfied, indicating that the theory was satisfied for a broader range of conditions, which is encouraging. Our results for EPR in $\rm Cd\sb{1-x-y}Mn\sb{x}Fe\sb{y}Se$ show that the lines are dramatically broadened as y is increased for nearly equal values of x, even for very small values of y(y $<$ 0.01). We assume that this broadening is due to the rapid spin-lattice relaxation of the Fe$\sp{+2}$ ions. In Part II, we describe a series of new magnetic resonances observed in magnetite ($\rm Fe\sb3O\sb4$) spheres. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Furdyna, Purdue University.

Subject Area

Condensation

COinS