Incommensurate spin ordering and fluctuations in underdoped La2-xBaxCuO4

By using neutron scattering techniques, we have studied the incommensurate spin ordering as well as the low energy spin dynamics in single crystal underdoped La2-xBaxCuO4 with x similar to 0.095 and 0.08, which are high temperature superconductors with T-C similar to 27 and 29 K, respectively. Static two dimensional incommensurate magnetic order appears below T-N=39.5 +/- 0.3 K in La2-xBaxCuO4 (x=0.095) and a similar temperature for x=0.08 within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 T applied along the c axis in the x=0.095 sample. Such a magnetic field independent behavior is in marked contrast to the field induced enhancement of the staggered magnetization observed in the related La2-xSrxCuO4 system, which indicates that this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that the incommensurability delta is only weakly dependent on doping relative to La2-xSrxCuO4. Dispersive excitations in La2-xBaxCuO4 (x=0.095) at the same incommensurate wave vector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below T-C in a similar manner to that seen in the superconducting state of La2CuO4+y.