Experimental and Computational Exploration of the Dilute Magnetic Delafossite CuAl1-xFexO2 Alloys

Abstract

CuAlO2 is among several ternary delafossites, which is a rare p-type semiconductor with potential applications as a transparent conductive oxide, photocatalyst, and spintronics when doped with transition metal ions. Reported in this thesis are results from our investigations of CuAl1-xFexO2 (x = 0 to1) with a focus on the x-dependence of structural, magnetic, vibrational, optical properties, and the role of defects and impurities. Samples are prepared by solid-state reactions. We performed a complete study of magnetic properties to investigate the possibility of room temperature ferromagnetic alloys, which are used in transparent ferromagnet applications, suggested by a computational study. Analysis of magnetization (M) vs. temperature (T, from 2 to 300 K) data by Curie-Weiss law confirms Fe3+ as the electronic state of Fe; this analysis also yields a negative θ characteristic of an antiferromagnetic Fe3+-Fe3+ exchange coupling and magnitudes of x in good agreement with the nominal values. The isothermal M vs. H (up to H= 90 kOe) data analyzed by the modified Brillouin function support the results obtained from the M vs. T analysis. High-resolution M-H loop measurements at 300 K and 10 K show negligible coercivity (HC)at 10 K but HC ~ 100 Oe at 300K. The results suggest that the room temperature ferromagnetism can originate from hematite impurity, but not for CuAl1-xFexO2 alloys. The understanding of the phonon dynamics of alloys is crucial because they have a fundamental indirect bandgap. I introduce a new approach, which is applicable to anisotropic, dilute alloys with allowance for a large variety of alloying elements. This approach has significant advantages over previously reported methods, especially for the lattice dynamics of such complex alloys. We use this approach to model the effects of Fe-doping on the vibrational modes in dilute alloys of CuAl1-xFexO2 (x = 0-0.10) delafossite powders. Raman and FTIR spectroscopies are performed to measure optical phonon frequencies. For the phonon calculations, an approach using a disordered supercell is not feasible because it is computationally expensive. Instead, we developed our weighted dynamical matrix (WDM) approach that uses a straightforward ordered supercell for force-constant calculations of the CuAlO2 and CuFeO2 parent endpoints and combines them using a WDM approach. Computationally, when Fe is substituted for Al (increasing x), an increase in the bond length is observed, leading to a redshift in the peak positions in all the phonon modes vs. x, in agreement with the experimentally observed trend. CuAlO2, with an indirect bandgap