Mössbauer studies of magnetic behavior of 3d-doped REBa2Cu3−x Fe x O7+δ (RE=Y,Er, Dy,Gd)

A Mössbauer study has been made on⁵⁷Fe ions substituted into the Cu(1) site of REBa₂Cu_3?x Fe _x O_7+δ (RE=Y, Er, Dy, Gd;x=0.15, 0.30). At low temperature, the iron atoms antiferromagnetically order with a transition temperature which is dependent on the Fe concentration. The temperature dependence of the magnetic subspectra representing Fe ions with various local oxygen environments in YBa₂Cu_3?x Fe _x O_7+δ and ErBa₂Cu_3?x Fe _x O_7+δ fit a 2D-Ising model with a ratio of the anisotropic exchange between the two directions on the order of 0.5–1.0(10^?3) for the Y-compounds and on the order of 1 for the Er-compounds. The magnitude of the local dopant magnetization is related to a short-range chemical order which determines the magnetic chain size and defines the correlation lengths. For the Y-compound, the order is quasi-1D with strong intrachain but very weak interchain coupling. For the Er-compounds, the magnetic coupling is Ising 2D. The strong fluctuation behavior expected in low dimensional systems above and belowT _N is observed via characteristic relaxation in the Mössbauer linewidth nearT _N. For both the Dy- and Gd-compounds, the magnetic order is 3D. The magnitude of the rare-earth magnetic moments appears to affect the character of the magnetic interaction in the Cu(1)-site. However, a Mössbauer effect measurement at¹⁵⁵Gd nuclei in GdBa₂Cu_2.85Fe_0.15O_7+δ (T _N(Fe)～14 K) shows paramagnetic behavior at 4.9 K.