MAGNETIC STRUCTURE AND FIELD INDUCED PHASE TRANSITION IN La_<2-x>Sr_xCuO_4

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La_<2-x>Sr_xCuO_4 shows local minimum of T_c and a magnetic ordering around x-1/8. The relation between superconductivity and the magnetic order is strongly interested. In this project the magnetic order and field induced phase transition of the spin structure was investigated in La_<2-x>Sr_xCuO_4(x-1/8). Magnetic superlattice peaks have been observed in elastic neutron-diffraction measurements on orthorhombic La_<188> CuO_4 at reciprocal points of (l/2*epsilon1/2,0), and (1/2,1/2*epsilon, 0), in the tetragonal notation, suggesting a modulated antiferromagnetic order with the incommensurability epsilon-0.126 similar to the case of La_<1.48>Nd_<0.40>Sr_<0.12>CuO_4 observed by Tranquada et. al. in the low-temperature tetragonal (TLT) phase. It is revealed from La-NMR experiments on La_<2-x-y>Y_ySr_xCuO_4 that the direction of the ordered spin in the orthorhombic (OMT) phase is along (0, *1<minus-plus>delta*delta)_0 in the orthorhombic notation where delta<<1 both for antiferromagnetic La … More _<22-y>Y_yCuO_4 and for modulated magnetic ordering La_<2-x-y>Y_ySrxCuO4(x-1/8), on the other hand, in the TLT phase the direction of the ordered spin is along (*1, *1,0)_0. When the magnetic field is applied along (0,1,0)_0 in the OMT phase, spin-flop from (0,1*1<minus-plus>delta, *delta)_0 to (*1, <minus-plus>delta, 0,0)_0 occurs at a field of 6-12 T.In the case of La_<1.88>Sr_<0.12>CuO_4 where the instability of the OMT structure glows at lower temperatures below 15 K, the spin-flop to (*1, *1,0)_0 is observed with an elastic anomaly due to the structural change from the OMT to the ThT phase. The magnetic moment of ordered Cu-3d spin and volume percentage of the magnetic ordered state is estimated from the analysis of the resonance peak width of La-NMR of La_<1.88>Sr_<0.12>CuO_4 at 4.2 K to be mu_<cu> -0.32 mu_B and -100%, respectively. Elastic constants of the flux-line lattice is also measured and volume percentage of the superconducting state is estimated to be larger than 90%, therefore, superconductivity coexists with the magnetic order microscopically over the almost whole part of the Sr_<0.12>CuO_4 crystal. The superconducting fluctuation reveals to be very small compared with an optimally doped sample (chi-0.15) especially at lower temperatures. Less