A detailed magnetic study has been carried out for the air-synthesized $R\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5+\delta }$ $(R=\mathrm{La}\mathrm{Ho})$ cobaltites. $R\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5+\delta }$ compounds exhibit complex magnetic behavior depending on the rare earth, the oxygen content, and variable Co valency. Spin-state transitions have been observed in the paramagnetic region at $T\approx 350\mathrm{K}$ for $R=\mathrm{Nd}$, Sm, Eu, Gd, and Tb compounds. The spin transition was found to depend delicately on the oxygen content and can be detected at oxygen concentrations varied within 5.3–5.5. Clear effects of the rare earths on the magnetic properties of the compounds at low temperature have been observed. $R\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5+\delta }$ compounds show Co weak ferromagnetism plus rare-earth paramagnetism for the heavy rare-earth-based compounds $(\mathrm{Gd}\mathrm{Ho})$ and strongly ordered noncollinear ferromagnetic structure with high coercivity for light rare-earth-based compounds $(\mathrm{Pr}\mathrm{Eu})$, thereby indicating the existence of a competing ferromagnetic and antiferromagnetic interaction at low temperatures. We show that the spin-state transition and low-temperature magnetic behavior have two distinct sources of origin. A phenomenological model incorporating the effects of rare-earth and $\mathrm{Co}\mathrm{O}$ interactions has been proposed to explain the low-temperature magnetic behavior of the $R\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5+\delta }$ compounds.

• Received 10 September 2004

DOI:https://doi.org/10.1103/PhysRevB.71.024419