Magnetic and transport properties of the layered perovskite system<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Sr</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>y</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Y</mml:mi><mml:mi>y</mml:mi></mml:msub><mml:mi mathvariant="normal">Co</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:mn>0</mml:mn><mml:mo>⩽</mml:mo><mml:mi>y</mml:mi><mml:mo>⩽</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math>

X. L. Wang, E. Takayama-Muromachi

doi:10.1103/physrevb.72.064401

Layered perovskite cobalt oxides ${\mathrm{Sr}}_{2\ensuremath{-}y}{\mathrm{Y}}_{y}\mathrm{Co}{\mathrm{O}}_{4}$ ($y=0$, 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 ${T}_{c}=255\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The ${T}_{c}$ decreases with increasing $y$ to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $y=0.5$, and ferromagnetism was not observed for $y\ensuremath{\geqslant}0.67$. Assessment of spin states for ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions suggested strongly that both are present as intermediate spin states when $y\ensuremath{\leqslant}0.67$ 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.

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