Complex magnetic properties of the rare-earth copper oxides,R2CuO4, 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 $A_{2\mathrm{-}x}$ $B_x$ ${\mathrm{CuO}}_4$, 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 ∼270 K is associated with antiferromagnetic ordering of the Cu moments which are strongly coupled within the ${\mathrm{CuO}}_2$ layers. The lower, at ≤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.