Weak ferromagnetism in distorted T ′-phase cuprates

Copper weak ferromagnetic or canted-antiferromagnetic order in distorted T ′-phase (orthorhombic O′-phase) Gd 2CuO 4 and related Gd 2− x R x CuO 4 (R=rare earth, Bi, or Bi/Ce) cuprates is reported. The weak ferromagnetism is the result of the small oxygen distortion in otherwise square CuO 2 plane. Powder X-ray Rietveld refinement analysis shows that the oxygen distortion angle α(Cu–O–Cu) is directly related to the ionic size in Gd 2CuO 4-type cuprates. The correlation between the crystal symmetry, antiferromagnetism and weak ferromagnetism in T′-phase cuprates is discussed.