Magnetic interactions of Gd inGdBa2(Cu1−xNix)3Oycompounds

Electron-spin resonance (ESR) and dc-susceptibility are measured for ${\mathrm{GdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ compounds. From the Curie-Weiss susceptibility and the frequency dependence of the linewidth, we determine Gd dipolar fields of about 3 kOe and exchange fields of about 9 kOe in this system. In order to make the origin of the exchange fields clear, ESR measurements are also performed for Ni-doped compounds. The Gd linewidths and the Néel temperature depend on the Ni concentrations. The origin of the exchange field in ${\mathrm{GdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ compounds is discussed on the basis of the results obtained.