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

La_<2-x>Sr_xCuO_4在x-1/8附近表现出T_c的局部极小值和磁有序。超导电性和磁序之间的关系引起了人们的强烈兴趣。本项目研究了La_ Sr_xCuO_4（x-1/8）体系的磁有序和场致自旋结构相变<2-x>。用弹性中子衍射方法测量了正交晶系La_ CuO_4的磁超晶格峰，<188>在四元数记法中，在倒数点（1/2* ε 1/2，0）和（1/2，1/2* ε 1，0）处观察到磁超晶格峰，表明它是一种调制的反铁磁有序，其不可逆性为ε-0.126，与Tranquada等人观察到的La_<1.48>Nd_<0.40>Sr_<0.12>CuO_4的情况相似。在低温四氢呋喃（TLT）相中。La_ Y_ySr_xCuO_4的La-NMR实验表明<2-x-y>，在正交晶系（OMT）相中，有序自旋的方向是沿着（0，*1 <minus-plus>delta*delta）_0，其中delta&lt;&lt;1，对于反铁磁La 关于我们 _<22-y>Y_yCuO_4和调制磁有序La_<2-x-y>Y_ySrxCuO_4（x-1/8），另一方面，在TLT相中，有序自旋的方向是沿着（*1，* 1，0）_0。当磁场沿着（0，1，0）_0方向施加时，在6-12 T的磁场下，发生从（0，1 *1 <minus-plus>delta，*delta）_0到（*1，<minus-plus>delta，0，0）_0的自旋翻转。在La_ Sr_ CuO_4的情况下<1.88><0.12>，OMT结构的不稳定性在低于15 K的低温下发光，观察到（*1，* 1，0）_0的自旋翻转，由于从OMT到ThT相的结构变化，观察到弹性异常。根据4.2 K下La-NMR共振峰宽的分析，估算了Cu-3d有序自旋的磁矩和磁有序态的体积百分比分别为μ<1.88>_<0.12><cu>（-0.32）μ_（B）和μ_（-100）%.测量了磁通线晶格的弹性常数，估计超导态的体积百分比大于90%，因此，在SrCuO_4晶体的几乎整个部分上，超导性与微观上的磁序共存<0.12>。与最佳掺杂样品（chi-0.15）相比，超导波动非常小，特别是在较低的温度下。少

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