Correlation between copper weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order and structural symmetry of the Gd2-xRxCuO4 (R = Eu and Tb) systems is reported. With smaller rare earth ions, lattice mismatch between (Gd, R)2 O2 and CuO2 layers results in a small oxygen distortion in square CuO2 plane. The distortion lowers the crystal symmetry from tetragonal T′- to an orthorhombic O′-phase. The weak ferromagnetism is a direct result of such a non-180° α(Cu-O-Cu) coupling with CAF alignment and a WF saturation moment ms(Cu2+) = μ(Cu2+) sin θ, with canting angle θ = (π - α)/2. Magnetic and crystallographic data indicate that the magnetic moment m(T) and the canting angle θ decrease (increase) with larger (smaller) ion doping.
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