Magnetic properties and spin state of Co$^{3+}$ ions of layered cobaltite DyBaCo$_2$O$_{5.49}$

In cobaltites DyBaCo$_2$O$_{5.49}$, the spin state of Co$^{3+}$ ions near the metal-insulator transition was determined for the first time. In the non-conducting phase, as in other rare earth $(R)$ cobaltites $R$BaCo$_2$O$_{5.50}$, Co$^{3+}$ ions are in low (LS, $S$ = 0) and intermediate (IS, $S$ = 1) spin states. In the metal phase, Co$^{3+}$ ions are in an intermediate (IS, $S$ = 1) spin state. The metal-insulator transition occurs when the spin state of Co$^{3+}$ ions from IS/LS in octahedra changes, without changing the IS state in pyramids. Temperature hysteresis of magnetization shows that the metal-insulator transition in DyBaCo$_2$O$_{5.50}$ is a first-order phase transition. It is assumed that the metamagnetic behavior of rare earth cobaltites $R$BaCo$_2$O$_{5.50}$ is due to their layered structure and is determined by the size of the rare earth ion.