Effect of Cu substitution on magnetic properties of layered NaCo2O4

Abstract

Polycrystalline samples of NaCo2-xCuxO4 (x = 0, 0.01, 0.03, 0.05) were obtained by two syntheses procedures: the solid-state reaction method of previously mechanically activated precursor powders (MASSR) and the citric acid complex method (CAC). The influence of small concentrations of Cu as a dopant on the magnetic properties of the sintered samples obtained by two different syntheses procedures was observed. In the temperature range from 50 K to 300 K the magnetic susceptibility, c, of all samples obeys Curie-Weiss low. It was found that in the MASSR samples c decreased with increasing Cu concentration in whole temperature range, unlike the CAC samples, where c decreased in the temperature range from 2 K to 15 K. Negative Weiss constant, q, indicated an antiferromagnetic interaction in all the MASSR and the CAC samples. A broad peak in the magnetic susceptibility was observed near 30 K for all MASSR samples. Since the magnetic measurements are highly sensitive this transition occurred due to the antiferromagnetic order in the unreacted Co3O4, which wasn’t detected by X-ray diffraction. In the CAC samples this transition appeared for x ≥ 0.03. The values of the effective magnetic moment, meff, were higher for MASSR (where some Co ions are in the intermediate spin state), compared with the CAC samples, where meff was lower than the theoretical spin-only value of Co4+ in the low spin state suggesting the presence of low spin Co3+ (S = 0).