Layered perovskite cobalt oxides ${\mathrm{Sr}}_{2-y}{\mathrm{Y}}_y\mathrm{Co}{\mathrm{O}}_4$ ($y=0$, 0.1, 0.3, 0.5, 0.67, 0.83, and 1) were synthesized under high pressure and high temperature conditions. Structure refinement revealed that these compounds crystallize in ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type structures with space group $\mathrm{I}4∕\mathrm{mmm}$. The parent compound ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_4$ undergoes a ferromagnetic transition with $T_c=255\mathrm{K}$. The $T_c$ decreases with increasing $y$ to $150\mathrm{K}$ for $y=0.5$, and ferromagnetism was not observed for $y⩾0.67$. Assessment of spin states for ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions suggested strongly that both are present as intermediate spin states when $y⩽0.67$ at least for the higher temperature range above $T_c$. Fairly large negative magnetoresistance was observed for ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_4$ in the vicinity of $T_c$ and in the lower temperature region.

• Received 20 May 2004

DOI:https://doi.org/10.1103/PhysRevB.72.064401