Antiferromagnetism and heat capacity of NaCo2−xCuxO4 ceramics

Polycrystalline samples of NaCo2−xCuxO4 (x=0, 0.01, 0.03, and 0.05) were synthesized in two different ways: 1) by a mechanochemically assisted solid-state reaction method (MASSR) and 2) by a citric acid complex method (CAC). In this work, we examined the influence of these synthesis routes and small Cu concentrations on magnetic properties and the heat capacity of sintered samples. The magnetic susceptibility (χ) of all samples followed the Curie-Weiss law in the temperature range between 50 K and 300 K, while a negative Weiss constant (θ) implied an antiferromagnetic interaction. According to the magnetic susceptibility data, a peak around 30 K indicating the presence of Co3O4 as a secondary phase appeared for all MASSR samples and CAC samples with Cu content above 1%. The effective magnetic moment (μeff) of CAC samples was lower than the theoretical, spin only value obtained for the Co4+ ion in the low spin state indicating the presence of low spin Co3+(S =0). These values were also lower compared to the values obtained for MASSR samples. The highest μeff of 1.75 μB/atom Co was obtained for the undoped MASSR sample. The heat capacity of CAC samples at 2 K decreased with Cu concentration due to the lowering of the electronic specific heat coefficient (γ). The highest γ of 63.9 mJ/molK2 was obtained for the undoped CAC sample. This reduction in γ values was the result of the decrease in the density of state and/or mass enhancement factor.