Exploration of spin state and exchange integral of cobalt ions in stoichiometric ZnCo₂O₄ spinel oxides

This work reports on spin state and exchange integral of cobalt ions in stoichiometric ZnCo₂O₄ nanoparticles with varying particle size from about 24 to 105 nm. Cobalt ions in ZnCo₂O₄ nanoparticles are present as trivalence in mixed spin state. The effective magnetic moment is distributed in the range of 2.1 ∼ 1.31 μ_B at room temperature with coarsening of nanoparticles. Further, it is demonstrated that stoichiometric ZnCo₂O₄ undergoes a magnetic transition from paramagnetism to antiferromagnetism with decrease of temperature, showing a transition temperature of about 5 K. The standard molar entropy and enthalpy for 24 nm ZnCo₂O₄ are 170.6 ± 1.7 J K⁻¹ mol⁻¹ and 28.2 ± 0.3 kJ mol⁻¹ at 298.15 K, respectively. Based on the heat capacity data, the exchange integral is determined to be 4.16 × 10⁻²² J. The results report here are really important for further understanding the magnetic and electronic properties of spinel oxides.