Magnetic ordering of Sm in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sm</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Long-range antiferromagnetic order of the Sm ions in ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ is observed via neutron diffraction at ${\mathit{T}}_{\mathit{N}}$=5.95 K. The magnetic structure consists of ferromagnetic sheets within the a-b planes, with the spins in alternate sheets aligned antiparallel. This spin structure and spin direction are completely different from those observed in any other copper-oxide superconductor system. The temperature dependence of the upper critical field ${\mathit{H}}_{\mathit{c}2}$ for the superconductor ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ (${\mathit{T}}_{\mathit{c}}$=23.5 K) shows a sudden increase in ${\mathit{H}}_{\mathit{c}2}$ at T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}0.7, but this anomaly cannot be attributed to the Sm magnetic ordering. No anomaly in ${\mathit{H}}_{\mathit{c}2}$ is observed at the ${\mathit{T}}_{\mathit{N}}$ for Sm.

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