Correlation between weak ferromagnetism and crystal symmetry in Gd2CuO4-type cuprates

Correlation between copper weak ferromagnetism (WF) and crystal structural symmetry of the Gd2-xMxCuO4 (M = Bi or Tb; 0 less than or equal to x less t han or equal to 0.5) systems is reported. Detailed powder x-ray Rietveld re finement analysis on Gd2-xMxCuO4 shows a systematic variation of oxygen dis tortion angle alpha(Cu-O-Cu) with ionic size where the lattice layer mismat ch lowers the crystal symmetry to an orthorhombic O'-phase with pseudotetra gonal lattice parameter a(0 similar to)b(0 similar to)5.508 Angstrom. Weak ferromagnetic or canted antiferromagnetic order is the direct result of thi s oxygen distortion which causes a sigma-transfer Cu(3d(x2-y2))-O(2p(sigma) )-Cu(3d(x2-y2)) superex-change interaction in the CuO2 plane with a non-180 degrees coupling angle. The small WF saturation moment rn, of similar to 2 -6X10(-2)mu(B)/Cu2+ can be deduced from the copper moment mu(Cu2+) canting angle theta =(pi-alpha)/2 similar to 2-7 degrees. Magnetic data and interna l exchange field B-int estimation indicate that Cu2+ WF saturation moment m (s)(Cu2+) decreases with larger Bi3+ doping and increases with smaller Tb3 doping. [S0163-1829(99)03341-X].