Abstract
Correlation between copper weak ferromagnetism (WF) and crystal structural symmetry of the Gd 2-x M x CuO 4 (M=Bi or Tb; 0≤x≤0.5) systems is reported. Detailed powder x-ray Rietveld refinement analysis on Gd 2-x M x CuO 4 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 O′-phase with pseudotetragonal lattice parameter a 0 ∼b 0 ∼5.508 Å. Weak ferromagnetic or canted antiferromagnetic order is the direct result of this oxygen distortion which causes a σ-transfer Cu(3d x2-y2 )-O(2p σ )-Cu(3d x2-y2 ) superexchange interaction in the CuO 2 plane with a non-180° coupling angle. The small WF saturation moment m s of ∼2-6×10 -2 μ B /Cu 2+ can be deduced from the copper moment μ(Cu 2+ ) canting angle θ=(π-α)/2 ∼2-7°. Magnetic data and internal exchange field B int estimation indicate that Cu 2+ WF saturation moment m s (Cu 2+ ) decreases with larger Bi 3+ doping and increases with smaller Tb 3+ doping. ©1999 The American Physical Society.