Electronic state and concentration of Fe3+ in CuAl1-xFexO2 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 CuAl1-xFexO2 (x = 0.0, 0.01, 0.05, and 0.1) samples, prepared by a solid-state reaction of Cu2O, Al2O3, and Fe2O3, at 1100 degrees 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 angstrom) is replacing the smaller Al3+ ion (r = 0.535 angstrom). The analysis of magnetization (M) vs. temperature (T, from 2 to 300 K) data in terms of the Curie-Weiss law: M = CH/(T - theta) confirms Fe3+ as the electronic state of Fe with spin S = 5/2 (magnetic moment mu = 5.9 mu(B)) as determined from the Curie constant, C; this analysis also yields a negative theta 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 - theta) 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.