Absence of the impurity-induced magnetic order in the electron-doped high-T_c cuprates Pr_0.86LaCe_0.14Cu _1-y(Zn, Ni)_yO₄

Zero-field muon-spin-relaxation measurements have been carried out in order to investigate the Zn- and Ni-substitution effects on the Cu-spin dynamics in the electron-doped Pr_0.86LaCe_0.14Cu_1-y(Zn, Ni)_yO_4+α-δ with y = 0, 0.01, 0.02, 0.05 and different values of the reduced oxygen content δ (δ ≦ 0.09). For the samples with y = 0 and very small δ values of δ < 0.01, a muon-spin precession due to the formation of a long-range antiferromagnetic order has been observed at low temperatures below ∼5 K. For the moderately oxygen-reduced samples of 0.01 ≦ δ ≦ 0.09, on the contrary, no muon-spin precession has been observed and the temperature dependence of the spectra is similar to one another regardless of the y value. That is, no impurity-induced slowing down of the Cu-spin fluctuations has been detected, which is very different from the results of the hole-doped high-T_c cuprates. The reason is discussed.