Copper magnetic centers in oxygen deficient RBa(2)Cu(3)O(6+x) (R=Nd, Sm): An EPR and magnetic study

EPR and magnetic results are reported for oxygen deficient, nonsuperconducting RBa(2)Cu(3)O(6+x) (R = Nd, Sm) compounds. The magnetic-susceptibility chi(T) and isothermal M(H) data are analyzed as the superposition of the rare-earth R(3+) contribution with another strongly ferromagnetic (FM) contribution arising from FM copper clusters with large total spin S. The rare-earth paramagnetic contribution in chi(T) and M(H) are calculated using the results of consistent crystal-field analysis (intermediate coupling wave functions, J-mixing effects) of Nd3+ and Sm3+ ions. The corresponding EPR spectra comprise an intense, almost isotropic EPR line whose intensity I(T) exhibits a ferromagnetic behavior, while g(eff) and the linewidth Delta H-pp diverge al T < 10 K indicating the presence of slowly fluctuating ''internal'' fields. The origin of the FM clusters is related to spin-polarized copper clusters through oxygen holes in the Cu(1) or Cu(2) layers, while the ferromagnetic interaction of the Cu2+(1) with the Cu2+(2) moments may be involved in the low-temperature (Te 10 K) behavior of the EPR parameters. On increasing the oxygen deficiency, the ferromagnetic contribution is drastically reduced and more isolated Cu2+ centers appear as shown by the corresponding EPR data. Exact simulation of the latter anisotropic EPR spectra, shows that the anisotropic linewidths Delta H-i (i = x,y,z) gradually broaden at low temperatures, while the intensity I(T) shows antiferromagnetic behavior. EPR measurements on an ''aged'' Nd0.5Y0.5Ba2Cu3O6+x sample revealed that the Cu2+ EPR spectrum intensifies with time, a behavior probably related to oxygen ordering processes or to surface degradation effects. Analysis of the EPR resonance of Nd3+ and Sm3+ ions in combination with the absence of the corresponding EPR spectra indicate the presence of very fast spin-lattice relaxation of the rare-earth ions.