Studies of Tetrahedral Substituted Magnetic Chromium-Based Chalcospinel Thin Films for Spintronic Applications

Abstract

Spintronics, an emerging breed of computing device, minimizes heat energy loss and leads to smaller, faster, less power-hungry electronics. It uses the spin and charge of electrons to encode and manipulate information. For the practical realization of such devices, it is important to identify materials stable at room temperature with diverse magnetic and electronic transport properties that can be easily integrated with standard semiconductors. A promising class of material for this purpose is mixed chromium-based chalcospinels such as Cd(1-x)CuxCr2S4 and ZnxCd(1-x)Cr2S4. They are predicted to exhibit wide range of magnetic exchange interactions and unusual transport properties, which can vary significantly as a function of composition. Due to strong coupling between spin, charge, orbital, and lattice degree of freedom, these systems offer exciting possibilities to tailor magnetic and electrical properties. However, despite their remarkable potential, the development of the quaternary systems remains largely unexplored. The lack of suitable metal precursors, stoichiometry control, and a delicate balance of reaction conditions to selectively synthesize the desired spinel phase makes the growth of these materials extremely challenging and, as a result, hinders their potential usage in the device applications. This dissertation addresses these critical issues and reports the first successful synthesis of polycrystalline Cd(1-x)CuxCr2S4 and ZnxCd(1-x)Cr2S4 thin films grown by a versatile chemical spray deposition (CSD) method using simple inorganic precursors. We present a detailed study of the cation substitution effect on the structural properties that drives the magnetic interaction and electrical conductivity. Depending on the substitution, we examine the existence of frustration and the origin of complex spin-glass behavior. The experimental study on these systems has been expanded to theoretical investigation using first-principles calculations within the density functional theory (DFT) framework. The calculated ground-state structural, magnetic, and electronic properties are qualitatively consistent with the experimental results and predict a wide range of half-metallic compositions in both systems. Furthermore, this work presents and analyzes the experimental Raman spectra of the frustrated ZnxCd(1-x)Cr2S4 system for the first time in close comparison with the lattice dynamics calculations within a shell model.