NQR Studies on Magnetism and Superconductivity of Oxides Superconductors

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Nuclear quadrupole resonances (NQR) of ^<139>La and ^<63,65>Cu have been investigated in the wide range of x in hole-doped La_<2-x>M_xCuO_4 (M=Sr, Ba). The divergence of the nuclear relaxation rate of ^<139>La at T_c*-10K indicates the existence of the peculiar frustrated magnetic phase between the 3D-antiferro- magnetic and superconducting phases. The spin echo signals of ^<63,65>Cu are observed between 30-50KHz in zero external field. The spectra consist of the two component of the Cu-NQR lines for x<0.4, of which origin is still unclear. Relaxation rates, 1/T_1 and 1/T_2 are much enhanced with decreasing x. The temperature dependence of 1/T_1 for x>0.4 obeys the Korringa relation, T_1T=const., below 200K.We have confirmed the antiferromagntic order of Cu moments in the electron-doped Ln_<2-x>Ce_xCuO_4 (Ln =Nd, Pr, Sm). Unexpected observation of nuclear magnetic resonance (NMR) with vanishingly small quadrupole interactions in the Ce-doped samples suggests that the local electronic properties at Cu site in the electron-doped oxides differ from the hole-doped ones.