Complex magnetic properties of the rare-earth copper oxides,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>R</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>, observed via measurements of the dc and ac magnetization, EPR, microwave magnetoabsorption, and specific heat

We report the results of an extensive investigation of the magnetic properties of a large series of undoped ${R}_{2}$${\mathrm{CuO}}_{4}$ single crystals with R==Pr, Nd, Sm, Eu, and Gd (which are the host compounds for the newly discovered series of electron cuprate superconductors) and mixture versions of the form ${A}_{2\mathrm{\ensuremath{-}}x}$${B}_{x}$${\mathrm{CuO}}_{4}$, with A==Pr, Nd, Sm, Eu, or Gd, and B==Gd, Tb, or Dy. We have measured dc and ac magnetization, microwave magnetoabsorption, EPR, and specific heat. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low-field dc magnetization, an unusual anisotropy in the EPR resonance field for R=Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temeperature at \ensuremath{\sim}270 K is associated with antiferromagnetic ordering of the Cu moments which are strongly coupled within the ${\mathrm{CuO}}_{2}$ layers. The lower, at \ensuremath{\le}20 K, cannot be attributed to antiferromagnetic ordering of the R moments and is tentatively attributed to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high-temperature superconductivity.