Copper magnetic centers in oxygen deficient RBa2Cu3O6+x (R=Nd, Sm): An EPR and magnetic study.

Abstract

EPR and magnetic results are reported for oxygen deficient, nonsuperconducting R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (R=Nd, Sm) compounds. The magnetic-susceptibility \ensuremath{\chi}(T) and isothermal M(H) data are analyzed as the superposition of the rare-earth ${\mathit{R}}^{3+}$ contribution with another strongly ferromagnetic (FM) contribution arising from FM copper clusters with large total spin S. The rare-earth paramagnetic contribution in \ensuremath{\chi}(T) and M(H) are calculated using the results of consistent crystal-field analysis (intermediate coupling wave functions, J-mixing effects) of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ ions. The corresponding EPR spectra comprise an intense, almost isotropic EPR line whose intensity I(T) exhibits a ferromagnetic behavior, while ${\mathit{g}}_{\mathrm{eff}}$ and the linewidth \ensuremath{\Delta}${\mathit{H}}_{\mathrm{pp}}$ diverge at T10 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 ${\mathrm{Cu}}^{2+}$(1) with the ${\mathrm{Cu}}^{2+}$(2) moments may be involved in the low-temperature (T10 K) behavior of the EPR parameters. On increasing the oxygen deficiency, the ferromagnetic contribution is drastically reduced and more isolated ${\mathrm{Cu}}^{2+}$ centers appear as shown by the corresponding EPR data. Exact simulation of the latter anisotropic EPR spectra, shows that the anisotropic linewidths \ensuremath{\Delta}${\mathit{H}}_{\mathit{i}}$ (i=x,y,z) gradually broaden at low temperatures, while the intensity I(T) shows antiferromagnetic behavior. EPR measurements on an ``aged'' ${\mathrm{Nd}}_{0.5}$${\mathrm{Y}}_{0.5}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ sample revealed that the ${\mathrm{Cu}}^{2+}$ EPR spectrum intensifies with time, a behavior probably related to oxygen ordering processes or to surface degradation effects. Analysis of the EPR resonance of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ 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. \textcopyright{} 1996 The American Physical Society.