Correlation between weak ferromagnetism and crystal symmetry in (formula presented)-type cuprates

Correlation between copper weak ferromagnetism (WF) and crystal structural symmetry of the (Formula presented) (Formula presented) or Tb; (Formula presented) systems is reported. Detailed powder x-ray Rietveld refinement analysis on (Formula presented) shows a systematic variation of oxygen distortion angle α(Cu-O-Cu) with ionic size where the lattice layer mismatch lowers the crystal symmetry to an orthorhombic (Formula presented)-phase with pseudotetragonal lattice parameter (Formula presented) Weak ferromagnetic or canted antiferromagnetic order is the direct result of this oxygen distortion which causes a σ-transfer (Formula presented) superexchange interaction in the (Formula presented) plane with a non-180° coupling angle. The small WF saturation moment (Formula presented) of (Formula presented) can be deduced from the copper moment (Formula presented) canting angle (Formula presented) Magnetic data and internal exchange field (Formula presented) estimation indicate that (Formula presented) WF saturation moment (Formula presented) decreases with larger (Formula presented) doping and increases with smaller (Formula presented) doping.