Electronic State and Concentration of Fe in CuAl_1-xFe_xO₂ Determined by Magnetic Investigations

CuAlO₂ is among several ternary delafossites in which the electronic bandgap (2.68 eV) is less than the optical bandgap (3.5 eV), due to Laporte selection rules. Because alloying is expected to provide band engineering in delafossites, we are investigating Fe-doped CuAlO₂. Here, magnetic characterization of the CuAl_1-xFe_xO₂ system (x = 0, 1, 5, and 10%) is reported. The samples were prepared by a solid-state reaction at 1100 °C. X-ray diffraction of the powder samples showed an expansion of the rhombohedral unit cell with increasing x, in accordance with Vegard’s Law (Fe³⁺ has a larger ionic radius than Al³⁺). Analysis of the magnetization (M) vs. temperature data (T = 5 to 300 K) and magnetic field (up to H = 90 kOe) verifies Fe³⁺ as the electronic state of Fe. M vs. T is fitted to the Curie-Weiss Law: M ∝ H/(T-θ) and, using the isothermal data of M vs. H, the variation of M vs. H/(T-θ) is fitted to a modified Brillouin function to determine x. The CuAlO₂ (x=0) sample also shows significant paramagnetism probably due to defects or Fe³⁺ impurities. The Curie-Weiss fits require a negative θ, which signifies antiferromagnetic Fe³⁺-Fe³⁺ exchange coupling. Optical investigations are now in progress.