Exploration of spin state and exchange integral of cobalt ions in stoichiometric ZnCo_2O_4 spinel oxides

摘要

This work reports on spin state and exchange integral of cobalt ions in stoichiometric ZnCo_2O_4 nanoparticles with varying particle size from about 24 to 105 nm. Cobalt ions in ZnCo_2O_4 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 ZnCo_2O_4 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 ZnCo_2O_4 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.