Electronic state and concentration of Fe3+ in CuAl1− x Fe x O2 determined by magnetic measurements

CuAlO₂ is among several ternary delafossites in which the electronic bandgap is less than the optical bandgap due to Laporte selection rules. Because alloying is expected to provide band engineering in delafossites, we are investigating Fe-doped CuAlO₂. Here, results from the detailed magnetic characterization of the CuAl_1−xFe_xO₂ (x = 0.0, 0.01, 0.05, and 0.1) samples, prepared by a solid-state reaction of Cu₂O, Al₂O₃, and Fe₂O_3, at 1100 °C, are reported. X-ray diffraction (XRD) of the powder samples showed an expansion of the rhombohedral unit cell with increasing x showing that the larger Fe³⁺ (r = 0.645 Å) is replacing the smaller Al³⁺ion (r = 0.535 Å). The analysis of magnetization (M) vs. temperature (T, from 2 to 300 K) data in terms of the Curie-Weiss law: M = CH/(T − θ) confirms Fe³⁺ as the electronic state of Fe with spin S = 5/2 (magnetic moment μ = 5.9 μ_B) as determined from the Curie constant, C; 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 in terms of modified Brillouin function involving H/(T − θ) support the results obtained from the M vs. T analysis. The small paramagnetism observed in undoped CuAlO₂ is related to the presence of a few percent of CuAl₂O₄ impurity observed in XRD of the sample.