Temperature dependence of Fe hyperfine field in RBa₂Cu₃O_z

Following the discovery that the Cu(2) sublattice in some RBa₂Cu₃O₇ (R=rare earth) systems orders magnetically a series of ⁵⁷Fe-probe Mossbauer studies have been performed aiming at the Cu(2) magnetisation and its Neel temperature. It is shown here that the observed temperature dependences of the Fe hyperfine field are identical for all systems studied to date, if the normalized field H_eff(T)/H_eff(OK) is plotted versus the reduced temperature (T/T_N). This universal experimental curve can be reproduced within a molecular-field model calculation for a spin- 1 2 system, if it is assumed that the iron is trivalent (S = 5 2) and the Cu(2)-Fe exchange is only 0.26(1) of the Cu(2)-Cu(2) exchange in all cases.