Effect of the oxygen content on the metal–insulator transition and on the spin state of Co$^{3+}$ ions in the layered NdBaCo$_2$O$_{5+\delta}$ cobaltite ($0.37\leq\delta\leq0.65$)

The effect of the oxygen content $\delta$ in layered NdBaCo$_2$O$_{5+\delta}$ cobaltite, where $0.37 \leqslant\delta\leqslant 0.65$, on the metal–insulator transition, as well as on the magnetic and spin states of Co$^{3+}$, is studied for the first time. An increase in $\delta$ reduces the metal–insulator transition temperature $T_{\text{MI}}$, the antiferromagnetic ordering temperature $T_{N}$, and the Curie temperature $T_C$ by about 100–150 K. For all values of $\delta$, the metal–insulator transition occurs when the spin state of Co$^{3+}$ ions changes from the HS/LS state in the metallic phase to the IS/LS state in the semiconducting phase, whereas with an increase in $\delta$, the spin state of Co$^{3+}$ ions changes from the IS/LS to HS/LS state. At $\delta\sim 0.65$, a heavily doped semiconductor–bad metal transition occurs without any change in the spin state of Co$^{3+}$ ions. The ferromagnetic behavior of NdBaCo$_2$O$_{5+\delta}$ in the antiferromagnetic phase below $T_N$ is interpreted in terms of the metamagnetic model as the effect of the size of the rare earth Nd$^{3+}$ ion on the antiferromagnetic state in layered cobaltites.