WEAK FERROMAGNETISM INDUCED BY THE EXTERNAL-FIELD ABOVE TN IN GDZCUO4

A detailed study of the temperature and field dependence of the dc mag netization near the antiferromagnetic (AF) transition in Gd2CuO4 shows the presence of field-induced weak ferromagnetism (WF) well above the Neel temperature T(N). ac susceptibility measurements indicate T(N)=2 66(3) and 282(3) K, for single crystals and ceramic samples, respectiv ely. The shift of the electron-spin-resonance line of the Gd3+ ions as a function of temperature T, deltaH(r)(T), is compared with the stati c internal field H(i)(T), which describes the coupling between the par amagnetic Gd lattice and the WF component of the Cu moments. Both delt aH(i)(T) and H(i)(T) show that the WF persists up to almost-equal-to 5 0 K above T(N). The initial susceptibility (for H < 100 G) presents a contribution in excess of the paramagnetism of Gd ions. For T greater- than-or-equal-to T(N), it decays as K(T-T(N))-1, with K=0.20(3) emu K mol. The relationship between this result and the strong two-dimension al AF correlations in these cuprates is discussed.